V819 Herculis is a well-studied triple star system consisting of a ``wide'' pair with 5.5 year period, one component of which is a 2.2-day period eclipsing single-line spectroscopic binary. Differential astrometry measurements from the Palomar High-precision Astrometric Search for Exoplanet Systems (PHASES) are presented and used to determine a relative inclination between the short- and long-period orbits of 23.6 +- 4.9 degrees. This represents only the sixth unambiguous determination of the mutual inclination of orbits in a hierarchical triple system. This result is combined with those for the other five systems for analysis of the observed distribution of mutual inclinations in nearby triple systems. It is found that this distribution is different than that which one would expect from random orientations with statistical significance at the 94% level; implications for studying the spatial distribution of angular momentum in star forming regions is discussed.
We have identified over 2000 well resolved cluster halos, and also their associated bound subhalos, from the output of 1024^3 particle cosmological N-body simulation (of box size 320h^{-1}Mpc and softening length 3.2h^{-1}kpc). We present an algorithm to identify those halos still in the process of relaxing into dynamical equilibrium, and a detailed analysis of the integral and internal physical properties for all the halos in our sample. The majority are prolate, and tend to rotate around their minor principle axis. We find there to be no correlation between the spin and virial mass of the clusters halos and that the higher mass halos are less dynamically relaxed and have a lower concentration. Additionally, the orbital angular momentum of the substructure is typically well aligned with the rotational angular momentum of the `host' halo. There is also evidence of the transfer of angular momentum from subhalos to their host. Overall, we find that measured halo properties are often significantly influenced by the fraction of mass contained within substructure. Dimensionless properties do depend weakly on the ratio of halo mass (M_h) to our characteristic mass scale (M_* = 8x10^{14}h^{-1}M_sun). This lack of self-similarity is in the expected sense in that, for example, 'old halos' with M_h / M_* << 1 have less substructure than 'young halos' with M_h / M_* >> 1.
Early X-ray afterglows recently detected by {\it Swift} frequently show a phase of very shallow flux decay lasting from a few hundred seconds up to $\sim 10^4 $s, followed by a steeper, more familiar decay. We suggest that the flat early part of the light curve may be a combination of the decaying tail of the prompt emission and the delayed onset of the afterglow emission observed from viewing angles slightly outside the edge of the jet, as predicted previously. This would imply that a significant fraction of viewers have a very small external shock energy along their line of sight and a very high $\gamma $-ray to kinetic energy ratio. The early flat phase in the afterglow light curve implies, according to this or other interpretations, a very large $\gamma $-ray efficiency, typically $\gtrsim 90%$, which is very difficult to produce by internal shocks.
We study the global distribution and morphology of dark matter voids in a LCDM universe using density fields generated by N-body simulations. Voids are defined as isolated regions of the low-density excursion set specified via density thresholds, the density thresholds being quantified by the corresponding filling factors, i.e., the fraction of the total volume in the excursion set. Our work encompasses a systematic investigation of the void volume function, the volume fraction in voids, and the fitting of voids to corresponding ellipsoids and spheres. We emphasize the relevance of the percolation threshold to the void volume statistics of the density field both in the high redshift, Gaussian random field regime, as well as in the present epoch. By using measures such as the Inverse Porosity, we characterize the quality of ellipsoidal fits to voids, finding that such fits are a poor representation of the larger voids that dominate the volume of the void excursion set.
Silicates are an important component of interstellar dust and the structure of these grains -- amorphous versus crystalline -- is sensitive to the local physical conditions. We have studied the infrared spectra of a sample of ultra-luminous infrared galaxies. Here, we report the discovery of weak, narrow absorption features at 11, 16, 19, 23, and 28 microns, characteristic of crystalline silicates, superimposed on the broad absorption bands at 10 and 18 microns due to amorphous silicates in a subset of this sample. These features betray the presence of forsterite (Mg_2SiO_4), the magnesium-rich end member of the olivines. Previously, crystalline silicates have only been observed in circumstellar environments. The derived fraction of forsterite to amorphous silicates is typically 0.1 in these ULIRGs. This is much larger than the upper limit for this ratio in the interstellar medium of the Milky Way, 0.01. These results suggest that the timescale for injection of crystalline silicates into the ISM is short in a merger-driven starburst environment (e.g., as compared to the total time to dissipate the gas), pointing towards massive stars as a prominent source of crystalline silicates. Furthermore, amorphization due to cosmic rays, which is thought to be of prime importance for the local ISM, lags in vigorous starburst environments.
We present preliminary results from the first 3 months of the Swift BAT high galactic latitude survey in the 14--195 keV band. The survey reaches a flux of \~10^{-11} erg/cm^2/s and has ~2.7' (90% confidence) positional uncertainties for the faintest sources. This represents the most sensitive survey to date in this energy band. These data confirm the conjectures that a high energy selected AGN sample would have very different properties from those selected in other bands and represent a `true' sample of the AGN population. We have identified 86% of the 66 high-latitude sources. 12 are galactic type sources and 44 can be identified with previously known AGN. All but 5 of the AGN have archival X-ray spectra, enabling the estimation of line of sight column densities and other spectral properties. Both of the z > 0.11 objects are Blazars. The median redshift of the others (excluding radio-loud objects) is 0.012. We find that the column density distribution of these AGN is bimodal with 64% of the non-blazar sources having column densities N_H >= 10^{22} cm$^{-2}$. None of the sources with log L_X > 43.5 show high column densities and very few of the lower L_X sources have low column densities. Based on these data, we expect the final BAT catalog to have >200 AGN and reach fluxes of less than ~10^{-11} erg/cm^2/s over the entire sky.
We present infrared L-band (3-4 micron) nuclear spectra of a large sample of nearby ultraluminous infrared galaxies (ULIRGs).ULIRGs classified optically as non-Seyferts (LINERs, HII-regions, and unclassified) are our main targets. Using the 3.3 micron polycyclic aromatic hydrocarbon (PAH) emission and absorption features at 3.1 micron due to ice-covered dust and at 3.4 micron produced by bare carbonaceous dust, we search for signatures of powerful active galactic nuclei (AGNs) deeply buried along virtually all lines-of-sight. The 3.3 micron PAH emission, the signatures of starbursts, is detected in all but two non-Seyfert ULIRGs, but the estimated starburst magnitudes can account for only a small fraction of the infrared luminosities. Three LINER ULIRGs show spectra typical of almost pure buried AGNs, namely, strong absorption features with very small equivalent-width PAH emission. Besides these three sources, 14 LINER and 3 HII ULIRGs' nuclei show strong absorption features whose absolute optical depths suggest an energy source more centrally concentrated than the surrounding dust, such as a buried AGN. In total, 17 out of 27 (63%) LINER and 3 out of 13 (23%) HII ULIRGs' nuclei show some degree of evidence for powerful buried AGNs, suggesting that powerful buried AGNs may be more common in LINER ULIRGs than in HII ULIRGs. The evidence of AGNs is found in non-Seyfert ULIRGs with both warm and cool far-infrared colors. These spectra are compared with those of 15 ULIRGs' nuclei with optical Seyfert signatures taken for comparison.The overall spectral properties suggest that the total amount of dust around buried AGNs in non-Seyfert ULIRGs is systematically larger than that around AGNs in Seyfert 2 ULIRGs.
The low ionization state in parts of a sunspot may play an important role in its evolution and dynamical state. The cool magnetic interior region of the sunspot develops a substantial neutral atomic and molecular hydrogen osmotic pressure which can drive a wind outward from the umbra. Ambipolar diffusion against the magnetically pinned ionized plasma component can also distort the umbral magnetic field into a filamentary penumbral structure. This may be important for explaining the development of the sunspot penumbra and the Evershed flow. This fractionation process may also be important for the ``First Ionization Potential'' (FIP) effect seen in the solar wind. In support of this mechanism we find evidence for such ionization fractionization in UV observations of molecular hydrogen in a sunspot umbra and penumbra.
If an extended source, such as a galaxy, is gravitationally lensed by a massive object in the foreground, the lensing distorts the observed image. It is straightforward to simulate what the observed image would be for a particular lens and source combination. In practice, one observes the lensed image on the sky, but blurred by atmospheric and telescopic effects and also contaminated with noise. The question that then arises is, given this incomplete data, what combinations of lens mass distribution and source surface brightness profile could plausibly have produced this image? This is a classic example of an inverse problem, and the method for solving it is given by the framework of Bayesian inference. In this paper we demonstrate the application of Bayesian inference to the problem of gravitational lens reconstruction, and illustrate the use of Markov Chain Monte Carlo simulations which can be used when the analytical calculations become too difficult. Previous methods for performing gravitational lens inversion are seen in a new light, as special cases of the general approach presented in this paper. Thus, we are able to answer, at least in principle, lingering questions about the uncertainties in the reconstructed source and lens parameters, taking into account all of the data and any prior information we may have.
The ELVIS instrument was recently proposed by the authors for the Indian Chandrayaan-1 mission to the Moon and is presently under consideration by the Indian Space Research Organisation (ISRO). The scientific objective of ELVIS is to explore the electromagnetic environment of the moon. ELVIS samples the full three-dimensional (3D) electric field vector, E(x,t), up to 18 MHz, with selective Nyqvist frequency bandwidths down to 5 kHz, and one component of the magnetic field vector, B(x,t), from a few Hz up to 100 kHz.As a transient detector, ELVIS is capable of detecting pulses with a minimum pulse width of 5 ns. The instrument comprises three orthogonal electric dipole antennas, one magnetic search coil antenna and a four-channel digital sampling system, utilising flexible digital down conversion and filtering together with state-of-the-art onboard digital signal processing.
Using the Submillimeter Array we report the discovery of a compact low mass bipolar molecular outflow from L1014-IRS and confirm its association with the L1014 dense core at 200 pc. Consequently, L1014-IRS is the lowest luminosity (L \~0.09 Lsun) and perhaps the lowest mass source known to be driving a bipolar molecular outflow, which is one of the smallest known in size (~500 AU), mass (< 10^{-4} Msun), and energetics (e.g., force < 10^{-7} Msun km/s/yr).
We present the stellar mass-metallicity relation for 34 0.4<z<1 galaxies selected from CFRS and Marano fields, and compare it to those derived from three local samples of galaxies (NFGS, KISS and SDSS). Our metal abundance estimates account for extinction effects, as estimated from IR/optical ratios and Balmer line ratios. All three comparisons show that the intermediate mass galaxies at z~0.65 are more metal-deficient by 0.3 dex at a given M_K or stellar mass relative to z=0. We find no evidence that this discrepancy could be related to different methods used to derive mass and metallicity. Assuming a closed box model predicts a gas fraction converted into stars of 20-25% since z~0.65, if the gas fraction is 10-20% in present-day galaxies with intermediate masses. This result is in excellent agreement with previous findings that most of the decline of the cosmic star formation density is related to the population of intermediate mass galaxies, which is composed of 75% spirals today. We find no evidence for a change of the slope of the M_{\star}-Z relation from z~0.65 to z=0 within the intermediate mass range (10.5<log(M_{\star}) < 11.5).
We report the multiwavelength observations of one intermediate redshift (z=0.3884) galaxy in the Marano Field. These data include ISOCAM middle infrared, VLT/FORS2 spectroscopic and photometric data, associated with the ATCA 1.4 GHz radio and ROSAT PSPC X-ray observations from literature. The Spectral Energy Distribution obtained by VLT spectroscopy exhibits its early-type galaxy property, while, in the same time, it has obvious [OIII]5007 emission line. The diagnostic diagram from the optical emission line ratios shows its Seyfert galaxy property. Its infrared-radio relation follows the correlation of sources detected at 15 \mu and radio. It has a high X-ray luminosity of 1.26*10^{43} ergs/s, which is much higher than the general elliptical galaxies s with the similar B band luminosity, and is about 2 orders of magnitude higher than the derived value from the star forming tracer, the FIR luminosity. This means that the X-ray sources of this galaxy are not stellar components, but the AGN is the dominant component.
Pop III stars are the first stars in the universe. They do not contain metals and their formation and evolution may be different from that of stars of later generations. In fact, according to the theory of star formation, Pop III stars might have very massive components ($\sim 100 - 10000M_\odot$). In this paper, we compute the spherically symmetric gravitational collapse of these Pop III massive stars. We solve the general relativistic hydrodynamics and neutrino transfer equations simultaneously, treating neutrino reactions in detail. Unlike supermassive stars ($\gtrsim 10^5 M_\odot$), the stars of concern in this paper become opaque to neutrinos. The collapse is simulated until after an apparent horizon is formed. We confirm that the neutrino transfer plays a crucial role in the dynamics of gravitational collapse, and find also that the $\beta$-equilibration leads to a somewhat unfamiliar evolution of electron fraction. Contrary to the naive expectation, the neutrino spectrum does not become harder for more massive stars. This is mainly because the neutrino cooling is more efficient and the outer core is more massive as the stellar mass increases. Here the outer core is the outer part of the iron core falling supersonically. We also evaluate the flux of relic neutrino from Pop III massive stars. As expected, the detection of these neutrinos is difficult for the currently operating detectors. However, if ever observed, the spectrum will enable us to obtain the information on the formation history of Pop III stars. We investigate 18 models covering the mass range of $300 - 10^4 M_\odot$, making this study the most detailed numerical exploration of spherical gravitational collapse of Pop III massive stars. This will also serve as an important foundation for multi-dimensional investigations.
A multiwavelength study of the star forming regions associated with IRAS 19111+1048 and IRAS 19110+1045 has been carried out. These have been simultaneously mapped in two far infrared bands at lambda_eff ~ 130 and 200 micron with ~1' angular resolution using the TIFR 1-m balloon borne telescope. The radio emission from the ionised gas of these regions has been imaged at 1280, 610 and 325 MHz using the Giant Metrewave Radio Telescope, India. A total of 20 compact radio sources have been detected from the high resolution radio map of IRAS 19111+1048 at 1280 MHz. Assuming these sources to represent exciting zero age main sequence (ZAMS) stars, the initial mass function is found to be quite steep, with the power law index of 5.3+-0.5 for the mass range 14 < m/M_sun < 33. The spectral types of the ZAMS stars inferred independently from the radio and NIR measurements match very well for a good fraction of the radio sources having NIR counterparts. For IRAS 19110+1045 region, seven radio sources have been detected of which two are associated with deeply embedded 2MASS objects. Self consistent radiative transfer modelling aimed at extracting important physical and geometrical details of the two IRAS sources has been carried out. A uniform density distribution of dust and gas is implied for both the sources. The extents of ionised gas, number of ZAMS stars, presence of deeply embedded sources and lower value of L/M for the cloud, support the youth of IRAS 19110+1045 vis-a-vis its neighbour, IRAS 19111+1048, consistent with earlier studies.
We report the results of observations of the black hole binaries XTE J1550-564 and H 1743-322 in their quiescent state using the Chandra X-ray Observatory. Both sources are detected at their faintest level of X-ray emission ever observed with a 0.5-10 keV unabsorbed luminosity of 2 $\times$ 10$^{32}$ (d/5 kpc)$^2$ erg s$^{-1}$ for XTE J1550-564 and 9 $\times$ 10$^{31}$ (d/8 kpc)$^2$ erg s$^{-1}$ for H 1743-322. These luminosities are in the upper range compared to the faintest levels observed in other black hole systems, possibly related to residual accretion for these sources with frequent outbursts. For XTE J1550-564, the Chandra observations also constrain the X-ray spectrum as a fit with an absorbed power-law model yields a photon index of 2.25 $\pm$ 0.08, clearly indicating a softening of the X-ray spectrum at lower luminosities compared to the standard hard state. Similar softening at low luminosity is seen for several black hole transients with orbital periods less than 60 hours. Most of the current models of accreting black holes are able to reproduce such softening in quiescence. In contrast, we find that systems with orbital periods longer than 60 hours appear to have hard spectra in quiescence and their behaviour may be consistent with hardening in quiescence.
The formation of H2 and HD molecules on interstellar dust grains is studied using rate equation and master equation models. Rate equations are used in the analysis of laboratory experiments which examine the formation of molecular hydrogen on astrophysically relevant surfaces. However, under interstellar conditions, rate equations are not suitable for the calculation of reaction rates on dust-grain surfaces. Due to the low flux and the sub-micron size of the grains, the populations of H and D atoms on a single grain are likely to be small. In this case the reaction rates are dominated by fluctuations and should be calculated using stochastic methods. The rate of molecular hydrogen formation in interstellar clouds is evaluated using the master equation, taking into account the distribution of grain sizes.
We estimate the TeV gamma-ray fluxes expected from the population of young pulsars in terms of the self-consistent time dependent hadronic-leptonic model for the high energy processes inside the pulsar wind nebulae. This radiation model bases on the hypothesis of Arons and collaborators who postulate that leptons are accelerated inside the nebulae as a result of resonant scattering on heavy nuclei, which in turn are accelerated in the pulsar wind region or the pulsar inner magnetosphere. Our aim is to find out which nebulae on the nortehrn hemisphere are the best candidates for detection at energies above 60 GeV and 200 GeV by the next generation of low threshold Cherenkov telescopes.
(abridged)We use a sample of ~200,000 galaxies drawn from the Sloan Digital Sky Survey to study how clustering depends on properties such as stellar mass (M*), colour (g-r), 4000A break strength (D4000), concentration index (C), and stellar surface mass density (\mu_*). We find that more massive galaxies cluster more strongly than less massive galaxies, with the difference increasing above the characteristic stellar mass of the Schechter mass function. When divided by physical quantities, galaxies with redder colours, larger D4000, higher C and larger \mu_* cluster more strongly. The clustering differences are largest on small scales and for low mass galaxies. At fixed stellar mass,the dependences of clustering on colour and 4000A break strength are similar. Different results are obtained when galaxies are split by concentration or surface density. The dependence of w(r_p) on g-r and D4000 extends out to physical scales that are significantly larger than those of individual dark matter haloes (> 5 Mpc/h). This large-scale clustering dependence is not seen for the parameters C or \mu_*. On small scales (< 1 Mpc/h), the amplitude of the correlation function is constant for ``young'' galaxies with 1.1 < D4000< 1.5 and a steeply rising function of age for ``older'' galaxies with D4000>1.5. In contrast, the dependence of the amplitude of w(r_p) on concentration on scales less than 1 Mpc/h is strongest for disk-dominated galaxies with C<2.6. This demonstrates that different processes are required to explain environmental trends in the structure and in star formation history of galaxies.
(abridged) We present measurements of the pairwise velocity dispersion (PVD) for different classes of galaxies in the Sloan Digital Sky Survey. For a sample of about 200,000 galaxies, we study the dependence of the PVD on galaxy properties such as luminosity, stellar mass (M_*), colour (g-r), 4000A break strength (D4000), concentration index (C), and stellar surface mass density (\mu_*). The luminosity dependence of the PVD is in good agreement with the results of Jing & B\"orner (2004) for the 2dFGRS catalog. The value of \sigma_{12} measured at k=1 h/Mpc decreases as a function of increasing galaxy luminosity for galaxies fainter than L*, before increasing again for the most luminous galaxies in our sample. This behaviour is not reproduced using standard halo occupation distribution (HOD) models. Each of the galaxy subsamples selected according to luminosity or stellar mass is divided into two further subsamples according to colour, D4000, C and \mu_*. We find that galaxies with redder colours and higher D4000, C, and \mu_* values have larger PVDs on all scales and at all luminosities/stellar masses. The dependence of the PVD on parameters related to recent star formation(colour, D4000) is stronger than on parameters related to galaxy structure (C, \mu_*), especially on small scales and for faint galaxies. The reddest galaxies and galaxies with high surface mass densities and intermediate concentrations have the highest pairwise peculiar velocities, i.e. these move in the strongest gravitational fields. We conclude that the faint red population located in rich clusters is responsible for the high PVD values that are measured for low-luminosity galaxies on small scales.
We examine H-band number counts determined using new photometry over 0.30 sq.deg. to H=19, as well as H<14 counts from 2MASS. First, we examine 2MASS counts extracted for the 4000 sq.deg. APM survey area and find a deficiency of 25% at H=13, in line with previous results. In addition the |b|>20 counts display a relatively constant deficit in the counts of 15-20%. We investigate various possible causes for these results: In order to address the issue of the model normalisation, we examine faint number counts for the new faint photometry presented in this work and also for the LCIRS. In each case a zeropoint is chosen to match that of the 2MASS photometry at bright magnitudes. We find a large offset between 2MASS and the LCIRS data of 0.28+/-0.01 magnitudes. Applying a consistent zeropoint, the combined faint data is in good agreement with the homogeneous model prediction used previously. We examine possible effects arising from unexpected galaxy evolution and photometric errors and find no evidence for a significant contribution from either. However, incompleteness in the 2MASS catalogue and in the faint data may have a significant contribution. Addressing the contribution from large-scale structure, we estimate the cosmic variance in the bright counts over the APM survey area and for |b|>20 expected in a LCDM cosmology using 27 LCDM mock 2MASS catalogues. Accounting for the model normalisation uncertainty and taking an upper limit for the incompleteness, the APM survey area bright counts are in line with a rare fluctuation in the local galaxy distribution of 2.5 sigma. However, the |b|>20 counts represent a 4.0 sigma fluctuation, and imply a local hole which extends over the entire local galaxy distribution and is at odds with LCDM.
Planetary nebulae (PNe) and their central stars (CSs) are ideal tools to test
evolutionary theory: photospheric properties of their exciting stars give
stringent constraints for theoretical predictions of stellar evolution. The
nebular abundances display the star's photosphere at the time of the nebula's
ejection which allows to look back into the history of stellar evolution - but,
more importantly, they even provide a possibility to investigate on the
chemical evolution of our Galaxy because most of the nuclear processed material
goes back into the interstellar medium via PNe.
The recent developments in observation techniques and a new three-dimensional
photoionization code MOCASSIN enable us to analyze PNe properties precisely by
the construction of consistent models of PNe and CSs. In addition to PNe
imaging and spectroscopy, detailed information about the velocity field within
the PNe is a pre-requisite to employ de-projection techniques in modeling the
physical structureof the PNe.
We study the secular instability of magnetized differentially rotating radiative zones taking account of viscosity and magnetic and thermal diffusivities. The considered instability generalizes the well-known Goldreich-Schubert-Fricke instability for the case of a sufficiently strong magnetic field. In magnetized stars, instability can lead to a formation of non-spherical unstable zones where weak turbulence mixes the material between the surface and interiors. Such unstable zones can manifest themselves by a non-spherical distribution of abundance anormalies on the stellar surface.
Chameleon fields are scalar fields whose mass depends on the ambient matter density. We investigate the effects of these fields on the growth of density perturbations on sub-galactic scales and the formation of the first dark matter halos. Density perturbations on comoving scales $R < 1 {\rm pc}$ go non--linear and collapse to form structure much earlier than in standard $\Lambda$CDM cosmology. The resulting mini-halos are hence more dense and resilient to disruption. We therefore expect (provided that the density perturbations on these scales have not been erased by damping processes) that the dark matter distribution on small scales would be more clumpy in chameleon cosmology than in the $\Lambda$CDM model.
The best-sampled afterglow light curves are available for GRB 030329. A distinguishing feature of this event is the obvious rebrightening at around 1.6 days after the burst. Proposed explanations for the rebrightening mainly include the two-component jet model and the refreshed shock model, although a sudden density-jump in the circumburst environment is also a potential choice. Here we re-examine the optical afterglow of GRB 030329 numerically in light of the three models. In the density-jump model, no obvious rebrightening can be produced at the jump moment. Additionally, after the density jump, the predicted flux density decreases rapidly to a level that is significantly below observations. A simple density-jump model thus can be excluded. In the two-component jet model, although the observed late afterglow (after 1.6 days) can potentially be explained as emission from the wide-component, the emergence of this emission actually is too slow and it does not manifest as a rebrightening as previously expected. The energy-injection model seems to be the most preferred choice. By engaging a sequence of energy-injection events, it provides an acceptable fit to the rebrightening at $\sim 1.6$ d, as well as the whole observed light curve that extends to $\sim 80$ d. Further studies on these multiple energy-injection processes may provide a valuable insight into the nature of the central engines of gamma-ray bursts.
The gravitational wave background produced by magnetars is investigated. The statistical properties of these highly magnetized stars were derived by population synthesis methods and assumed to be also representative of extragalactic objects. The adopted ellipticity was calculated from relativistic models using different equations of state and different assumptions concerning the distribution of currents in the neutron star interior. The maximum amplitude occurs around 1.2 kHz, corresponding to $\Omega_{gw} \sim 10^{-9}$ for a type I superconducting neutron star model. The expected signal is a continuous background that could mask the cosmological contribution produced in the early stage of the Universe.
This article compares the distribution of Ks magnitude of Large Magellanic Cloud (LMC) asymptotic giant branch (AGB) stars obtained from the DENIS and 2MASS data with theoretical distributions. These have been constructed using up-to-date stellar evolution calculations for low and intermediate-mass stars, and in particular for thermally pulsing AGB stars. A fit of both the magnitude distribution of carbon- and oxygen-rich AGB stars allowed us to constrain the metallicity distribution across the LMC and its star formation rate (SFR). The LMC stellar population is found to be on average 5-6 Gyr old and is consistent with a mean metallicity corresponding to Z=0.006. These values may however be affected by systematic errors in the underlying stellar models, and by the limited exploration of the possible SFR histories. Our method should be, instead, particularly useful for detecting variations in the mean metallicity and SFR across the LMC disk. In fact, there are well defined regions where both the metallicity and the mean-age of the underlying stellar population span the whole range of grid parameters. The C/M ratio discussed in paper I is a tracer of the metallicity distribution if the underlying stellar population is older than about a few Gyr. A similar study across the Small Magellanic Cloud is given in paper III of this series.
In this paper the optical data of the ESO Deep-Public-Survey observed with the Wide Field Imager and reduced with the THELI pipeline (Erben et al. 2005) are presented. These images are publicly released to the community. Our main scientific goals with this survey are to study the high-redshift universe by optically pre-selecting high-redshift objects from imaging data and to use VLT instruments for follow-up spectroscopy as well as weak lensing applications. Here we present 61 fully reduced and stacked images. The astrometric and photometric calibrations are discussed and the properties of the images are compared to images released by the ESO Imaging Survey team.
We present in this paper new multiscale transforms on the sphere, namely the isotropic undecimated wavelet transform, the pyramidal wavelet transform, the ridgelet transform and the curvelet transform. All of these transforms can be inverted i.e. we can exactly reconstruct the original data from its coefficients in either representation. Several applications are described. We show how these transforms can be used in denoising and especially in a Combined Filtering Method, which uses both the wavelet and the curvelet transforms, thus benefiting from the advantages of both transforms. An application to component separation from multichannel data mapped to the sphere is also described in which we take advantage of moving to a wavelet representation.
We determine the first fossil group luminosity function based on spectroscopy of the member galaxies. The fossil group RX J1552.2+2013 has 36 confirmed members, it is at a mean redshift of 0.136 and has a velocity dispersion of 623 km/s (or 797 km/s if four emission lines galaxies in the outskirts of the velocity distribution are included). The luminosity function of RX J1552.2+2013, measured within the inner region of the system ~1/3 R_vir), in the range -23.5< M_i'<-17.5, is well fitted by a Schechter function with M*i'=-21.3 +/- 0.4 and alpha = -0.6 +/- 0.3 or a Gaussian function centered on M_i'= -20.0 +/- 0.4 and with sigma=1.29 +/- 0.24 i' mag. (H_0 = 70 km/s Mpc, Omega_M=0.3, Omega_Lambda=0.7. The luminosity function obtained from a photometric survey in g', r', i'-bands (and statistical background correction) confirms the spectroscopically determined results. There is a significant dip in the luminosity function at M_r'=-18 mag, as also observed for other clusters. RX~J1552.2+2013 is a rich, strongly red-galaxy dominated system, with at least 19 galaxies with magnitudes between m_3 and m_3 + 2, within a surveyed circular area of radius 625 kpc centered on the peak of the x-ray emission. Its mass, ~3.0 10^14 M_0, M/L of 507 M_sol/L_B_sol and L_X of 6.3 10^43 ergs/s (bolometric) are more representative of a fossil cluster than of a fossil group. The central object of RX J1552.2+2013 is a cD galaxy which may have accreted the more luminous ~L* former members of the group. Although dynamical friction and subsequent merging are probably the processes responsible for the lack of bright galaxies in the system, for the fainter members, there must be another mechanism in action (perhaps tidal disruption) to deplete the fossil group from intermediate-luminosity galaxies M_r' ~ -18.
We present numerical three-body experiments that include the effects of gravitational radiation reaction by using equations of motion that include the 2.5-order post-Newtonian force terms, which are the leading order terms of energy loss from gravitational waves. We simulate binary-single interactions and show that close approach cross sections for three 1 solar mass objects are unchanged from the purely Newtonian dynamics except for close approaches smaller than 1.0e-5 times the initial semimajor axis of the binary. We also present cross sections for mergers resulting from gravitational radiation during three-body encounters for a range of binary semimajor axes and mass ratios including those of interest for intermediate-mass black holes (IMBHs). Building on previous work, we simulate sequences of high-mass-ratio three-body encounters that include the effects of gravitational radiation. The simulations show that the binaries merge with extremely high eccentricity such that when the gravitational waves are detectable by LISA, most of the binaries will have eccentricities e > 0.9 though all will have circularized by the time they are detectable by LIGO. We also investigate the implications for the formation and growth of IMBHs and find that the inclusion of gravitational waves during the encounter results in roughly half as many black holes ejected from the host cluster for each black hole accreted onto the growing IMBH.
We present ultraviolet, optical, near-infrared, Spitzer mid-infrared, and radio images of 14 radio-selected objects in M 33. These objects are thought to represent the youngest phase of star cluster formation. We have detected the majority of cluster candidates in M 33 at all wavelengths. From the near-IR images, we derived ages 2-10 Myr, K_S-band extinctions (A_K_S) of 0-1 mag, and stellar masses of 10^3-10^4 M_solar. We have generated spectral energy distributions (SEDs) of each cluster from 0.1 micron to 160 microns. From these SEDs, we have modeled the dust emission around these star clusters to determine the dust masses (1-10^3 M_solar) and temperatures (40-90 K) of the clusters' local interstellar medium. Extinctions derived from the JHK_S, Halpha, and UV images are similar to within a factor of 2 or 3. These results suggest that eleven of the fourteen radio-selected objects are optically-visible young star clusters with a surrounding H II region, that two are background objects, possibly AGN, and that one is a Wolf-Rayet star with a surrounding H II region.
We present HST/NICMOS imaging of the H_2 2.12 \mu m emission in 5 fields in the Helix Nebula ranging in radial distance from 250-450" from the central star. The images reveal arcuate structures with their apexes pointing towards the central star. Comparison of these images with comparable resolution ground based images reveals that the molecular gas is more highly clumped than the ionized gas line tracers. From our images, we determine an average number density of knots in the molecular gas ranging from 162 knots/arcmin^2 in the denser regions to 18 knots/arcmin^2 in the lower density outer regions. Using this new number density, we estimate that the total number of knots in the Helix to be ~23,000 which is a factor of 6.5 larger than previous estimates. The total neutral gas mass in the Helix is 0.35 M_\odot assuming a mass of \~1.5x10^{-5} M_\odot for the individual knots. The H_2 intensity, 5-9x10^{-5} erg s^{-1} cm^{-2} sr^{-1}, remains relatively constant with projected distance from the central star suggesting a heating mechanism for the molecular gas that is distributed almost uniformly in the knots throughout the nebula. The temperature and H_2 2.12 \mu m intensity of the knots can be approximately explained by photodissociation regions (PDRs) in the individual knots; however, theoretical PDR models of PN under-predict the intensities of some knots by a factor of 10.
The Iron Sun formed on the collapsed core of a supernova and now acts as a magnetic plasma diffuser, as did the precursor star, separating ions by mass. This process covers the solar surface with lightweight elements and with lighter isotopes of each element. Running difference images expose rigid, iron-rich structures below the fluid photosphere made of lightweight elements. The energy source for the Sun and ordinary stars seems to be neutron-emission and neutron-decay, with partial fusion of the decay product, rather than simple fusion of hydrogen into helium or heavier elements. Neutron-emission from the solar core and neutron-decay generate about sixty five percent of solar luminosity and H-fusion generates about thirty-five percent. The upward flow of H ions maintains mass-separation in the Sun. Only about one percent of this neutron decay product survives its upward journey to depart as solar-wind hydrogen.
A null ray approaching a distant astronomical source appears to slow down, while a massive particle speeds up in accordance with Newtonian gravitation. The integration of these apparently incompatible aspects of motion in general relativity is due to the existence of a critical speed. Dynamics of particles moving faster than the critical speed could then be contrary to Newtonian expectations. Working within the framework of gravitoelectromagnetism, the implications of the existence of a critical speed are explored. The results are expected to be significant for high energy astrophysics.
The standard inflationary model presents a simple scenario within which the homogeneity, isotropy and flatness of the universe appear as natural outcomes and, in addition, fluctuations in the energy density are originated during the inflationary phase. These seminal density fluctuations give rise to fluctuations in the temperature of the Cosmic Microwave Background (CMB) at the decoupling surface. Afterward, the CMB photons propagate almost freely, with slight gravitational interactions with the evolving gravitational field present in the large scale structure (LSS) of the matter distribution and a low scattering rate with free electrons after the universe becomes reionized. These secondary effects slightly change the shape of the intensity and polarization angular power spectra (APS) of the radiation. The APS contain very valuable information on the parameters characterizing the background model of the universe and those parametrising the power spectra of both matter density perturbations and gravitational waves. In the last few years data from sensitive experiments have allowed a good determination of the shape of the APS, providing for the first time a model of the universe very close to spatially flat. In particular the WMAP first year data, together with other CMB data at higher resolution and other cosmological data sets, have made possible to determine the cosmological parameters with a precision of a few percent. The most striking aspect of the derived model of the universe is the unknown nature of most of its energy contents. This and other open problems in cosmology represent exciting challenges for the CMB community. The future ESA Planck mission will undoubtely shed some light on these remaining questions.
We have extracted a total of 1968 Mira variables from the OGLE-II data base in the Galactic bulge region. Among them, 1960 are associated with 2MASS sources, and 1541 are further identified with MSX point sources. Their photometric properties are compared with those of Mira variables in the Large and Small Magellanic Clouds. We have found that mass-losing stars with circumstellar matter are reddened such that the colour dependence of the absorption coefficient is similar to that of interstellar matter. We also discuss the structure of the bulge. The surface number density of the bulge Mira variables is well correlated with the 2.2-micron surface brightness obtained by the COBE satellite. Using this relation, the total number of Mira variables in the bulge is estimated to be about 600,000. The logP-K relation of the Mira variables gives their space distribution which supports the well-known asymmetry of the bar-like bulge.
We present structural parameters for 204 stellar clusters in the Small Magellanic Cloud derived from fitting King and Elson, Fall, & Freeman model profiles to the V-band surface brightness profiles as measured from the Magellanic Clouds Photometric Survey images. Both King and EFF profiles are satisfactory fits to the majority of the profiles although King profiles are generally slightly superior to the softened power-law profiles of Elson, Fall, and Freeman and provide statistically acceptable fits to ~90% of the sample. We find no correlation between the preferred model and cluster age. The only systematic deviation in the surface brightness profiles that we identify is a lack of a central concentration in a subsample of clusters, which we designate as "ring" clusters. In agreement with previous studies, we find that the clusters in the SMC are significantly more elliptical than those in the Milky Way. However, given the mean age difference and the rapid destruction of these systems, the comparison between SMC and MW should not directly be interpreted as either a difference in the initial cluster properties or their subsequent evolution. We find that cluster ellipticity correlates with cluster mass more strongly than with cluster age. We identify several other correlations (central surface brightness vs. local background density, core radius vs. tidal force, size vs. distance) that can be used to constrain models of cluster evolution in the SMC.
We analyze an oscillating universe model in brane world scenario. The oscillating universe cycles through a series of expansions and contractions and its energy density is dominated by dust matter at early-time expansion phase and by phantom dark energy at late-time expansion phase. We find that the period of the oscillating universe is not sensitive to the tension of the brane, but sensitive to the equation-of-state parameter $w$ of the phantom dark energy, and the ratio of the period to the current Hubble age approximately varies from 3 to 9 when the parameter $w$ changes from -1.4 to -1.1. The fraction of time that the oscillating universe spends in the coincidence state is also comparable to the period of the oscillating universe. This result indicates that the coincidence problem can be significantly ameliorated in the oscillating universe without singularity.
We present a parameter study of the possibility of tidally triggered disk
instability. Using a restricted N-body model which allows for a survey of an
extended parameter space, we show that a passing dwarf star with a mass between
0.1 and 1 M_sun can probably induce gravitational instabilities in the
pre-planetary solar disk for prograde passages with minimum separations below
80-170 AU for isothermal or adiabatic disks. Inclined and retrograde encounters
lead to similar results but require slightly closer passages. Such encounter
distances are quite likely in young moderately massive star clusters (Scally &
Clarke 2001; Bonnell et al. 2001). The induced gravitational instabilities may
lead to enhanced planetesimal formation in the outer regions of the
protoplanetary disk, and could therefore be relevant for the existence of
Uranus and Neptune, whose formation timescale of about 100 Myr (Wuchterl,
Guillot & Lissauer 2000) is inconsistent with the disk lifetimes of about a few
Myr according to observational data by Haisch, Lada & Lada (2001). The
relatively small gas/solid ratio in Uranus and Neptune can be matched if the
perturbing fly-by occurred after early gas depletion of the solar system, i.e.
when the solar system was older than about 5 Myr.
We also confirm earlier results by Heller (1993) that the observed 7 degree
tilt of the solar equatorial plane relative to the ecliptic plane could be the
consequence of such a close encounter.
The Swift Gamma-Ray Burst Explorer performed its first autonomous, X-ray follow-up to a newly detected GRB on 2005 January 17, within 193 seconds of the burst trigger by the Swift Burst Alert Telescope. While the burst was still in progress, the X-ray Telescope obtained a position and an image for an un-catalogued X-ray source; simultaneous with the gamma-ray observation. The XRT observed flux during the prompt emission was 1.1 x 10^{-8} ergs cm^{-2} s^{-1} in the 0.5-10 keV energy band. The emission in the X-ray band decreased by three orders of magnitude within 700 seconds, following the prompt emission. This is found to be consistent with the gamma-ray decay when extrapolated into the XRT energy band. During the following 6.3 hours, the XRT observed the afterglow in an automated sequence for an additional 947 seconds, until the burst became fully obscured by the Earth limb. A faint, extremely slowly decaying afterglow, alpha=-0.21$, was detected. Finally, a break in the lightcurve occurred and the flux decayed with alpha<-1.2$. The X-ray position triggered many follow-up observations: no optical afterglow could be confirmed, although a candidate was identified 3 arcsecs from the XRT position.
Here we show preliminary calculations of the cooling and contraction of a 2 MJ planet. These calculations, which are being extended to 1-10 MJ, differ from other published "cooling tracks" in that they include a core accretion-gas capture formation scenario, the leading theory for the formation of gas giant planets. We find that the initial post-accretionary intrinsic luminosity of the planet is ~3 times less than previously published models which use arbitrary initial conditions. These differences last a few tens of millions of years. Young giant planets are intrinsically fainter than has been previously appreciated. We also discuss how uncertainties in atmospheric chemistry and the duration of the formation time of giant planets lead to challenges in deriving planetary physical properties from comparison with tabulated model values.
We present high-resolution HST WFPC2 images of compact nebulosity surrounding the cool M-type hypergiants NML Cyg, VX Sgr and S Per. The powerful OH/IR source NML Cyg exhibits a peculiar bean-shaped asymmetric nebula that is coincident with the distribution of its H2O vapor masers. We show that NML Cyg's circumstellar envelope is likely shaped by photo-dissociation from the powerful, nearby association Cyg OB2 inside the Cygnus X superbubble. The OH/IR sources VX Sgr and S Per have marginally resolved envelopes. S Per's circumstellar nebula appears elongated in a NE/SW orientation similar to that for its OH and H2O masers, while VX Sgr is embedded in a spheroidal envelope. We find no evidence for circumstellar nebulosity around the intermediate-type hypergiants rho Cas, HR 8752, HR 5171a, nor the normal M-type supergiant mu Cep. We conclude that there is no evidence for high mass loss events prior to 500-1000 yrs ago for these four stars.
Binary radio pulsar system J0737-3039 provides an exceptional opportunity to
study innermost structure of pulsar magnetospheres due to very tight orbit,
favorable directions of pulsars' rotation and magnetic axes and extremely
fortuitous orientation of the orbit. In this system the millisecond pulsar A is
eclipsed once per orbit. During eclipse a clear modulation at the 2.77 s period
of pulsar B is seen, pointing unambiguously to magnetospheric origin of
eclipses. A simple geometric model, based on the idea that the radio pulses are
attenuated by synchrotron absorption on the closed magnetic field lines of
pulsar B, can successfully reproduces the eclipse light curves down to
intricate details. This detailed agreement confirms the dipolar structure of
the neutron star's magnetic field.
The model gives clear predictions for temporal evolution of eclipse profile
due to geodetic precession of pulsar B.
In addition, pulsar B shows orbital modulations of intensity, being
especially bright at two short orbital phases. We showed that these modulations
are due to distortion of pulsar B magnetosphere by pulsar A wind which produces
orbital phase-dependent changes of the direction along which radio waves are
emitted. Thus, pulsar B is intrinsically bright at all times but its radiation
beam misses the Earth at most orbital phases.
The Wide Angle Search for Planets prototype (WASP0) is a wide-field instrument used to search for extra-solar planets via the transit method. Here we present the results of a monitoring program which targeted a 9-degree field in Draco. WASP0 monitored 35000 field stars for two consecutive months. Analysis of the lightcurves resulted in the detection of 11 multi-transit candidates and 3 single-transit candidates, two of which we recommend for further follow-up. Monte-Carlo simulations matching the observing parameters estimate the expected number of transit candidates from this survey. A comparison of the expected number with the number of candidates detected is used to discuss limits on planetary companions to field stars.
Observation of the H2O megamaser galaxy IC 2560 with the Chandra Observatory reveals a complex spectrum composed of soft X-ray emission due to multi-temperature thermal plasma, and a hard continuum with strong emission lines. The continuum is most likely a Compton reflection (reprocessing) of primary emission that is completely absorbed at least up to 7 keV. The lines can be identified with fluorescence from Si, S and Fe in the lowest ionization stages. The equivalent widths of the Si and S lines are broadly compatible with those anticipated for reprocessing by optically thick cold plasma of Solar abundances, while the large equivalent width of the Fe line requires some overabundance of iron. A contribution to the line from a transmitted component cannot be ruled out, but the limits on the strength of the Compton shoulder make it less likely. From the bolometric luminosity of the nuclear region, we infer that the source radiates at 1 - 10% of its Eddington luminosity, for an adopted central mass of 3 million Solar masses. The overall spectrum is consistent with the hypotheses that the central engines powering the detected megamsers in accretion disks are obscured from direct view by the associated accretion disk material itself, and that there is a correlation between the occurrence of megamaser emission and Compton-thick absorption columns. For the 11 known galaxies with both column density measurements and maser emission believed to arise from accretion disks, eight AGN are Compton thick.
We study numerically the evolution of rotating cloud cores, from the collapse of a magnetically supercritical core to the formation of a protostar and the development of a protostellar disk during the main accretion phase. We find that the disk quickly becomes unstable to the development of a spiral structure similar to that observed recently in AB Aurigae. A continuous infall of matter from the protostellar envelope makes the protostellar disk unstable, leading to spiral arms and the formation of dense protostellar/protoplanetary clumps within them. The growing strength of spiral arms and ensuing redistribution of mass and angular momentum creates a strong centrifugal disbalance in the disk and triggers bursts of mass accretion during which the dense protostellar/protoplanetary clumps fall onto the central protostar. These episodes of clump infall may manifest themselves as episodes of vigorous accretion rate (\ge 10^{-4} M_sun/yr) as is observed in FU Orionis variables. Between these accretion bursts, the protostar is characterized by a low accretion rate (< 10^{-6} M_sun/yr). During the phase of episodic accretion, the mass of the protostellar disk remains less than or comparable to the mass of the protostar.
Accretion disc turbulence is investigated in the framework of the shearing box approximation. The turbulence is either driven by the magneto-rotational instability or, in the non-magnetic case, by an explicit and artificial forcing term in the momentum equation. Unlike the magnetic case, where most of the dissipation occurs in the disc corona, in the forced hydrodynamic case most of the dissipation occurs near the midplane. In the hydrodynamic case evidence is presented for the stochastic excitation of epicycles. When the vertical and radial epicyclic frequencies are different (modeling the properties around rotating black holes), the beat frequency between these two frequencies appear to show up as a peak in the temporal power spectrum in some cases. Finally, the full turbulent resistivity tensor is determined and it is found that, if the turbulence is driven by a forcing term, the signs of its off-diagonal components are such that this effect would not be capable of dynamo action by the shear--current effect.
It is generally regarded that the bulk of cosmic rays originate in the Galaxy
and that those below the 'knee' (the rapid steepening in the energy spectrum)
at a few PeV come from Galactic supernovae, the particles being accelerated by
the shocks in the supernova remnants. At higher energies, there are problems in
that conventional SNR - which surely constitute the bulk of the sources - have
a natural limit at a few tens of PeV (for iron nuclei). The question of the
origin of particles above this limit is thus an open one. Here we examine a
number of possibilities: a variety of supernovae and hypernovae, pulsars, a
Giant Galactic Halo and an Extragalactic origin.
A relevant property of any model is the extent to which it can provide the
lack of significant irregularity of the energy spectrum. Although it is
appreciated that spectral measurements are subject to systematic as well as
random errors we consider that contemporary data are good enough to allow at
least some progress in this field.
In the search for origin above PeV energies we conclude that shocks in the
Galactic Halo, whatever their source (Galactic wind, relativistic plasmoids -
'cannonballs', multiple shocks from supernovae etc.) are most likely, pulsrs
such as B0656+14 and hypernovae come a close second although such a suggestion
is not without its difficulties. What is most important is that trapping of
particles in the Halo is needed to reduce irregularities of the energy spectra
both below and above the 'knee' caused by the stochastic nature of supernova
explosions and other potential (discrete) Galactic sources.
We argue that precise experimental studies of spectral 'irregularities' will
provide considerable help in the search for cosmic ray origin.
A comparative study of various parametrizations of the dark energy equation of state is made. Astrophysical constraints from LSS, CMB and BBN are laid down to test the physical viability and cosmological compatibility of these parametrizations. A critical evaluation of the 4-index parametrizations reveals that Hannestad-M\"{o}rtsell as well as Lee parametrizations are simple and transparent in probing the evolution of the dark energy during the expansion history of the universe and they satisfy the LSS, CMB and BBN constraints on the dark energy density parameter for the best fit values.
We discuss a model for long Gamma-Ray-Bursts in which the central engine is associated with the conversion process of a metastable hadronic star into a star containing quark matter. We analyze the observational signatures of the model, i.e. the Supernova-GRB temporal connection and the existence of long quiescent times in the temporal structure of Gamma-Ray-Bursts.
This paper is the second in a series exploring the properties of 51 {\it optically} selected, single-nuclei merger remnants. Spectroscopic data have been obtained for a sub-sample of 38 mergers and combined with previously obtained infrared photometry to test whether mergers exhibit the same correlations as elliptical galaxies among parameters such as stellar luminosity and distribution, central stellar velocity dispersion ($\sigma$$_{\circ}$), and metallicity. Paramount to the study is to test whether mergers lie on the Fundamental Plane. Measurements of $\sigma$$_{\circ}$ have been made using the Ca triplet absorption line at 8500 {\AA} for all 38 mergers in the sub-sample. Additional measurements of $\sigma$$_{\circ}$ were made for two of the mergers in the sub-sample using the CO absorption line at 2.29 $\micron$. The results indicate that mergers show a strong correlation among the parameters of the Fundamental Plane but fail to show a strong correlation between $\sigma$$_{\circ}$ and metallicity (Mg$_{2}$). In contrast to earlier studies, the $\sigma$$_{\circ}$ of the mergers are consistent with objects which lie somewhere between intermediate-mass and luminous giant elliptical galaxies. However, the discrepancies with earlier studies appears to correlate with whether the Ca triplet or CO absorption lines are used to derive $\sigma$$_{\circ}$, with the latter almost always producing smaller values. Finally, the photometric and kinematic data are used to demonstrate for the first time that the central phase-space density of mergers are equivalent to elliptical galaxies. This resolves a long-standing criticism of the merger hypothesis.
The enigma source, RX J1856.5-3754, is one of the so-called dim thermal neutron stars. Two puzzles of RXJ1856.5-3754 exist: (1) the observational X-ray spectrum is completely featureless; (2) the UV-optical intensity is about seven times larger than that given by the continuation of the blackbody model yielded by the X-ray data. Both the puzzles would not exist anymore if RX J1856.5-3754 is a low mass bare strange quark star, which is in a propeller phase with a low accretion rate. A boundary layer of RX J1856.5-3754 is suggested and modelled, from which the UV-optical emission is radiated. Free-free absorption dominates the opacity of the boundary layer, which results in the opacity to be high in UV-optical but low in X-ray bands. The star's magnetic field, spin period, as well as the accretion rate are constrained by observations.
We have modeled nova light curves exceeding the Eddington luminosity. It has been suggested that a porous structure develops in nova envelopes during the super Eddington phase and the effective opacity is much reduced for such a porous atmosphere. Based on this reduced opacity model, we have calculated envelope structures and light curves of novae. The optically thick wind model is used to simulate nova winds. We find that the photospheric luminosity and the wind mass-loss rate increase inversely proportional to the reducing factor of opacities, but the wind velocity hardly changes. We also reproduce the optical light curve of V1974 Cygni (Nova Cygni 1992) in the super-Eddington phase, which lasts 13 days from the optical peak 1.7 mag above the Eddington luminosity.
The rapid succession of discovery of short--duration hard--spectrum GRBs has led to unprecedented insights into the energetics of the explosion and nature of the progenitors. Yet short of the detection of a smoking gun, like a burst of coincident gravitational radiation or a Li-Paczynski mini-supernova, it is unlikely that a definitive claim can be made for the progenitors. As was the case with long--duration soft--spectrum GRBs, however, the expectation is that a systematic study of the hosts and the locations of short GRBs could begin to yield fundamental clues about their nature. We present the first aggregate study of the host galaxies of short--duration hard--spectrum GRBs. In particular, we present the Gemini--North and Keck discovery spectra of the galaxies that hosted three short GRBs and a moderate--resolution (R~6000) spectrum of a fourth host. We find that these short--hard GRBs originate in a variety of low-redshift (z<1) environments that differ substantially from those of long--soft GRBs, both on individual galaxy scales and on galaxy--cluster scales. Specifically, three of the bursts are found to be associated with old and massive galaxies with no current (< 0.1 Msol/hr) or recent star formation. Two of these galaxies are located within a cluster environment. These observations support an origin from the merger of compact stellar remnants, such as double neutron stars of a neutron star--black hole binary. The fourth event, in contrast, occurred within a dwarf galaxy with a star formation rate exceeding 0.5 Msol/yr. Therefore, it appears that like supernovae of Type Ia, the progenitors of short--hard bursts are created in all galaxy types, suggesting a corresponding class with a wide distribution of delay times between formation and explosion.
We compare quiescent and flare X-ray spectra of the RS CVn binary Sigma Gem obtained with the Chandra and XMM-Newton grating spectrometers. We find that in addition to an overall 25% flux increase, which can be ascribed to variations in the system's quiescence activity over the 15 months that passed between the observations, there is a hot plasma component of kT_e > 3 keV that arises with the flare. The hot component is manifested primarily by emission from high charge states of Fe and by a vast continuum. The cooler (kT_e < 2 keV) plasma remains undisturbed during the flare. We find no significant variations in the relative abundances during the flare except for a slight decrease (<30%) of O and Ne.
Here we review the recent evidence for dark energy, dark matter and black holes as components of an expanding universe, for the vantage point of a non-expert; we speculate on a specific DM particle.
The history of the formation and evolution of the Milky Way Galaxy is found in the spatial distribution, kinematics, age and chemical abundance distributions of long-lived stars. From this fossil record one can in principle extract the star formation histories of different components, their chemical evolution, the stellar Initial Mass Function, the merging history -- what merged and when did it merge? -- and compare with theoretical models. Observations are driving models, and we live in exciting times.
The proper usage of Type Ia supernovae (SNe Ia) as distance indicators has revolutionized cosmology, and added a new dominant component to the energy density of the Universe, dark energy. Following the discovery and confirmation era, the currently ongoing SNe Ia surveys aim to determine the properties of the dark energy. ESSENCE is a five year ground-based supernova survey aimed at finding and characterizing 200 SNe Ia in the redshift domain z=[0.2-0.8]. The goal of the project is to put constraints on the equation of state parameter, w, of the dark energy with an accuracy of <10%. This paper presents these ongoing efforts in the context of the current developments in observational cosmology.
Statistical observations of the Epoch of Reionization using the 21 cm line of neutral hydrogen have the potential to revolutionize our understanding of structure formation and the first luminous objects. However, these observations are complicated by a host of strong foreground sources. Several foreground removal techniques have been proposed in the literature, and it has been assumed that these would be used in combination to reveal the Epoch of Reionization (EOR) signal. By studying the characteristic subtraction errors of the proposed foreground removal techniques, we identify an additional subtraction stage that can further reduce the EOR foreground contamination, and study the interactions between the foreground removal algorithms. This enables us to outline a comprehensive foreground removal strategy that incorporates all previously proposed subtraction techniques. Using this foreground removal framework and the characteristic subtraction errors, we discuss the complementarity of different foreground removal techniques and the implications for array design and the analysis of EOR data.
We present high-resolution near-infrared spectra, obtained with the NIRSPEC spectrograph on the W. M. Keck II Telescope, of a collection of hot, massive stars within the central 25 arcseconds of the Galactic center. We have identified a total of twenty-one emission-line stars, seven of which are new radial velocity detections with five of those being classified as He I emission-line stars for the first time. These stars fall into two categories based on their spectral properties: 1) those with narrow 2.112, 2.113 micron He I doublet absorption lines, and 2) those with broad 2.058 micron He I emission lines. These data have the highest spectral resolution ever obtained for these sources and, as a result, both components of the absorption doublet are separately resolved for the first time. We use these spectral features to measure radial velocities. The majority of the measured radial velocities have relative errors of 20 kms, smaller than those previously obtained with proper-motion or radial velocity measurements for similar stellar samples in the Galactic center. The radial velocities estimated from the He I absorption doublet are more robust than those previously estimated from the 2.058 micron emission line, since they do not suffer from confusion due to emission from the surrounding ISM. Using this velocity information, we agree that the stars are orbiting in a somewhat coherent manner but are not as defined into a disk or disks as previously thought. Finally, multi-epoch radial velocity measurements for IRS 16NE show a change in its velocity presumably due to an unseen stellar companion.
We have searched the four brightest objects in the Kuiper belt for the presence of satellites using the newly commissioned Keck Observatory Laser Guide Star Adaptive Optics system. Satellites are seen around three of the four objects: Pluto (whose satellite Charon is well-known), 2003 EL61, and 2003 UB313. The object 2005 FY9, the brightest Kuiper belt object after Pluto, does not have a satellite detectable within 0.4 arcseconds with a brightness of more than 0.5% of the primary. The presence of satellites to 3 of the 4 brightest Kuiper belt objects is inconsistent with the fraction of satellites in the Kuiper belt at large at the 99.1% confidence level, suggesting a different formation mechanism for these largest KBO satellites. The satellites of 2003 EL61 and 2003 UB313, with fractional brightnesses of 5% and 2% of their primaries, respectively, are significantly fainter relative to their primaries than other known Kuiper belt object satellites, again pointing to possible differences in their origin.
We consider the non-Gaussianity of the nonlinear density perturbations in a single field inflationary model when a scalar field couples nonminimally with the gravity. Gravity theories with a nonminimal coupling could be transformed into the Einstein gravity that has a canonical kinetic terms by a suitable conformal transformation. Then we have found that a nonlinear generalization of the gauge invariant quantity $\zeta_i$ is invariant under the conformal transformation. With the help of these conformal invariant property, we calculate the non-Gaussianity in the nonminimal coupled scalar field theory which is characterized by a nonlinear parameter $f_{NL}$.
Self-gravitating systems with nonlocal, long-range interactions are described by nonextensive statistics. Recently, Leubner demonstrated that the nonextensivity parameter $\kappa$ should be negative for self-gravitating, pressureless systems, such as dark matter halos. The equation for the spherically symmetric nonextensive dark matter halos has also been derived. Here we demonstrate that this equation is identical to the classical Lane-Emden equation describing the structure of self-gravitating polytropic spheres. This establishes an intimate connection between self-gravitating polytropes and nonextensive thermostatistics. Moreover, based on this fact and observational data, we put a stronger constraint on $\kappa$, namely $\kappa<-3.4$.
We have conducted phase-reference multi-epoch observations of the 22.2 GHz water masers using the VLBA and multi-frequency study of the continuum emission using the VLA towards the high-mass SFR AFGL 5142. The water maser emission comes from two elongated structures (indicated as Group I and Group II), with the measured proper motions aligned along the structures' elongation axes. Each group consists of two (blue- and red-shifted) clusters of features separated by a few hundreds and thousands of AU respectively for Group I and Group II. The maser features of Group II have both positions and velocities aligned along a direction close to the axis of the outflow traced by HCO+ and SiO emission on angular scales of tens of arcsec. We predict that the maser emission arises from dense, shocked molecular clumps displaced along the axis of the molecular outflow. The two maser clusters of Group I are oriented on the sky along a direction forming a large angle (> 60 degrees) with the axis of the jet/outflow traced by Group II maser features. We have detected a compact (8.4 and 22 GHz) continuum source that falls close to the centroid of Group I masers, indicating that the source ionizing the gas is also responsible for the excitation of the water masers. The kinematic analysis indicates that the Group I masers trace outflowing rather than rotating gas, discarding the Keplerian disk scenario proposed in a previous paper for Group I. Since the axis joining the two maser clusters of Group II does not cross the position of the continuum source, Group II masers might be excited by an (undetected) massive YSO, distinct from the one (pinpointed by the VLA continuum emission) responsible for the excitation of the Group I masers.
Theoretical Period-Age and Period-Age-Color relations for different chemical compositions have been recently derived using an updated homogeneous set of evolutionary and pulsational models. We apply these relations to Cepheids in the Magellanic Clouds to constrain the recent star formation history of these dwarf galaxies. Finally, we also compare the radial distribution of classical Cepheids with young clusters and star forming regions.
Morphological disturbances and gas kinematics of the SB0 galaxy NGC 7679=Arp 216 are investigated to get clues to the history of this highly composite object, where AGN and starburst signatures dominate each other in the X-ray and optical/IR regime, respectively. Perturbations of the ionized gas velocity field appear quite mild within 15'' (~5 kpc) from the center, so as it can be straightforwardly modeled as a circularly rotating disk. On the contrary, outside that radius, significant disturbances show up. In particular, the eastern distorted arm as well as the huge neutral hydrogen bridge connecting NGC 7679 with the nearby Seyfert spiral NGC 7682 unambiguously represent the vestige of a close encounter of the two objects dating back ~500 Myr ago. The relationship of such past event with the much more recent, centrally located starburst (not older than 20 Myr) cannot be easily established. Altogether, the classification of NGC 7679, turns out to be less extreme than that proposed in the past, being simply a (disturbed) galaxy where starburst and AGN activity cohexist with a starburst dominating the bolometric luminosity.
We report on a search for evidence of binarity in Far-Ultraviolet Spectroscopic Explorer (FUSE) observations of DAO white dwarfs. Spectra recorded by FUSE are built up from a number of separate exposures. Observation of changes in the position of photospheric heavy element absorption lines between exposures, with respect to the stationary interstellar medium lines, would reveal radial velocity changes - evidence of the presence of a binary system. This technique is successful in picking out all the white dwarfs already known to be binaries, which comprise 5 out of the sample of 16, but significant radial velocity shifts were found for only one additional star, Ton 320. This object is also known to have an infrared excess. DAOs can be separated broadly into low or normal mass objects. Low mass white dwarfs can be formed as a result of binary evolution, but it has been suggested that the lower mass DAOs evolve as single stars from the extended horizontal branch, and we find no evidence of binarity for 8 out of the 12 white dwarfs with relatively low mass. The existence of higher mass DAOs can also be explained if they are within binary systems, but of the four higher mass stars in the sample studied, PG 1210+533 and LB 2 do not exhibit significant radial velocity shifts, although there were only two exposures for the former object and the latter has an infrared excess.
Intermediate mass Pre-main sequence stars (1.5 $M_{\odot}< M < 5M_{\odot}$) cross the instability strip on their way to the main sequence. They are therefore expected to be pulsating in a similar way as the $\delta$ Scuti stars. In this contribution we present the status of the observational studies of pulsations in these stars with special emphasis on recent results from our group. The prospects for future investigations of these objects from the ground and from space are discussed.
We present a new version of the list of peculiar velocities of 1561 flat edge-on spirals from the RFGC catalogue. It differs from the previous version by 233 new data and 34 corrected data. A new regression was used for distances estimation based on the Tully-Fisher relationship in the "linear diameter - HI line width" variant. Moreover, we present velocities for 3 models of galaxies collective motion. They are a D-model (dipole, Hubble expansion + bulk motion with constant speed), DQ-model (a quadrupole terms are added) and DQO-model (DQ-model + octopole).
I review X-ray observations of accretion-powered millisecond pulsars and current theories for formation of their spectra and pulse profiles.
We report on the analysis of an X-ray grating spectrum of the Classical Nova V382 Vel (1999), obtained with the LETG+HRC-S instrument on board CHANDRA, which shows emission lines dominating over any continuum. Lines of Si, Mg, Ne, O, N, and C are identified, but no Fe lines are detected. The total luminosity in the lines is 4x10^27erg/s (corrected for N_H=1.2x10^21cm^-2). The lines have broad profiles with FWHM corresponding to a velocity 2900+-200km/s. Some structure is identified in the profiles, but for different elements we find different profile structures. While lines of O show a broadened Gaussian profile, those of Ne are double-peaked, suggesting a fragmented emitting plasma. Using the emission measure distribution we derive relative element abundances and find abundances of Ne and N that are significantly enhanced relative to that of O, while Fe is not over-abundant. The lack of any source emission longwards of 50A and the OVIII Ly_alpha/Ly_beta line ratio support previous values of the hydrogen column-density. We find weak continuum emission from the white dwarf, consistent with a black-body spectrum with an upper limit to the temperature of T=3x10^5K, assuming a source radius of 6000km. The upper limit for the integrated black-body luminosity is 2x10^36erg/s. The BeppoSAX and Chandra ACIS observations of V382 Vel show that the nova was bright and in the Super Soft phase as late as 1999 December 30. Our LETG observation obtained 6 weeks later, as well as all subsequent X-ray observations, showed a remarkable fading to a nearly pure emission line phase which suggests that nuclear burning on the white dwarf had turned off by February. In the absence of a photoionizing source the emission lines were formed in a collisionally ionized and excited expanding shell.
With currently available XMM-Newton EPIC pn observations spanned over about 3 years, we present a detailed spectral and temporal variability of the 0.2--10 keV X-ray emission from the X-ray bright BL Lac object PKS 2155-304. The spectral variability is examined with a model independent hardness ratio method. We find that the spectral evolution of the source follows the light curves well, indicating that the spectra harden when the fluxes increase. The plots of hardness ratios versus count rates show that the spectral changes are particularly significant during flares. The cross-correlation functions (CCFs) show that the light curves in the different energy bands are well correlated at different time lags. The CCF peaks (i.e., the maximum correlation coefficients) tend to become smaller with larger energy differences, and the variability in the different energy bands are more correlated for the flares than for the other cases. In most cases the higher energy band variations lead the lower energy band, but in two cases we observed the opposite behavior that the lower energy variability possibly leads the higher energy variability. The time lags increase with the energy differences between the two cross-correlated light curves. The maximum lag is found to be up to about one hour, in support with the findings obtained with previous low Earth orbit X-ray missions. We discuss our results in the context of the particle acceleration, cooling and light crossing timescales.
Numerical Simulation is an essential part of the design and optimisation of astronomical adaptive optics systems. Simulations of adaptive optics are computationally expensive and the problem scales rapidly with telescope aperture size, as the required spatial order of the correcting system increases. Practical realistic simulations of AO systems for extremely large telescopes are beyond the capabilities of all but the largest of modern parallel supercomputers. Here we describe a more cost effective approach through the use of hardware acceleration using field programmable gate arrays. By transferring key parts of the simulation into programmable logic, large increases in computational bandwidth can be expected. We show that the calculation of wavefront sensor image centroids can be accelerated by a factor of four by transferring the algorithm into hardware. Implementing more demanding parts of the adaptive optics simulation in hardware will lead to much greater performance improvements, of up to 1000 times.
We present the results of spectrophotometric observations of the old nova RR Pic performed in two spectral ranges, one centered in the Ha line and other covering Hb and Hg spectral lines. From the Hb radial velocity study we found a primary radial semi-amplitude of K1 = 37(1) km/s and a systemic velocity of g = 1.8(2) km/s. With this new values a mass diagram is constructed, constraining the possible mass intervals for the system. The possible orbital inclination range was restricted using the fact that RR Pic presents shallow eclipses. A secondary mass range below the limit of a main sequence star filling its Roche lobe indicate an evolved companion. We also calculated the Ha, Hb, Hg, HeI 6678 and HeII 4686 Doppler tomograms. The most conspicuous differences are found between the HeI and HeII tomograms, the former has a ring shape, while the second is filled at low velocities, suggesting that the low velocity emission is not coming from the accretion disk. Radial emissivity profiles for these lines were also derived.
Quantitative analyses of low-mass hydrogen-deficient (super-)giant stars - so-called extreme helium stars - to date face two major difficulties. First, theory fails to reproduce the observed helium lines in their entirety, wings and line cores. Second, a general mismatch exists for effective temperatures derived from ionization equilibria and from spectral energy distributions. Here, we demonstrate how the issue can be resolved using state-of-the-art non-LTE line-formation for these chemically peculiar objects. Two unique high-gravity B-type objects are discussed in detail, the pulsating variable V652 Her and the metal-poor star HD144941. In the first case atmospheric parameters from published LTE analyses are largely recovered, in the other a systematic offset is found. Hydrogen abundances are systematically smaller than previously reported, by up to a factor ~2. Extreme helium stars turn out to be important testbeds for non-LTE model atoms for helium. Improved non-LTE computations show that analyses assuming LTE or based on older non-LTE model atoms can predict equivalent widths, for the HeI 10830A transition in particular, in error by up to a factor ~3.
In this letter, deep radio observations of the quasar PKS 0743$-$67 are presented that reveal a central engine capable of driving jets with enormous kinetic luminosity, $Q>4.1 \times 10^{46}\mathrm{ergs/s}$. This result is significant because archival optical spectral data indicates that the accretion disk has a thermal luminosity, $L_{bol}>2\times 10^{47}\mathrm{ergs/s}$. Furthermore, estimates of the central black hole mass from line widths indicate that $L_{bol}/L_{Edd}\approx 1$. This suggests that neither a large $L_{bol}$ nor $L_{bol}/L_{Edd}$ suppresses jet power in quasars, despite claims that they do in the recent literature. Earlier studies have found $L_{bol}$ and $Q$ are correlated in blazars. However, by removing the BL-Lacs and leaving only the quasars in the sample, we found that $Q$ is very weakly correlated with $L_{bol}$ in the subsample.
We present in this paper optical and X-ray follow up observations for three X-ray selected objects extracted from the ROSAT North Ecliptic Pole survey which is a flux-limited, completely identified survey. All three objects have X-ray luminosities in the 10^44 erg/s regime and show narrow emission lines in their optical discovery spectra, typical of QSO2 type objects. Spectroscopic data for the three QSO2 candidates, obtained with the Telescopio Nazionale Galileo, confirm the widths of the Halpha or Hbeta emission lines are less than 750 km/s. On the other hand XMM-Newton data do not show any sign of obscuration as expected for this class of objects. The X-ray spectra of the three objects are all well fit by a single power law model with Gamma~1.7 with low energy absorption fixed to the Galactic value along the line of sight to each object. Most observational evidence supports the scenario where optical and X-ray obscurations are linked, contrary to our findings. We discuss the unanticipated results of these observations, and compute the space density in soft X-ray surveys of this possibly new class of objects. Their spatial density in the ROSAT NEP survey is 2.8{+2.7 / -1.5} 10^-8 h^3 Mpc^-3 in a Lambda-CDM model with h=0.7. Unobscured QSO\2 candidates could go unrecognized in current X-ray surveys where the low hydrogen column density is inferred by a hardness ratio rather than a more precise X-ray spectrum measurement.
We present the results of the first {\it XMM-Newton} observation of the
interacting type IIn supernova 1995N, performed in July 2003. We find that the
0.2--10.0 keV unabsorbed flux dropped at a value of $\simeq 1.8 \times
10^{-13}$ erg cm$^{-2}$ s$^{-1}$, almost one order of magnitude lower than that
of a previous {\it ASCA} observation of January 1998. From all the available
X-ray measurements, an interesting scenario emerges where the X-ray light
emission may be produced by a two-phase (clumpy/smooth) circumstellar medium.
The X-ray spectral analysis shows statistically significant evidence for the
presence of two distinct components, that can be modeled with emission from
optically thin, thermal plasmas at different temperatures. The exponent of the
ejecta density distribution inferred from these temperatures is $n\simeq 6.4$.
From the fluxes of the two spectral components we derive an estimate of the
mass loss rate of the supernova progenitor, ${\dot M} \sim 2 \times 10^{-4}
M_\odot {\rm yr}^{-1}$, at the upper end of the interval exhibited by red
super-giants. Coordinated optical and infrared observations allow us to
reconstruct the simultaneous infrared to X-ray flux distribution of SN 1995N.
We find that, at $\sim$ 9 years after explosion, the direct X-ray thermal
emission due to the wind/ejecta interaction is $\sim 5$ times larger than the
total reprocessed IR/optical flux.
CCD photometry on the intermediate-band vbyCaHbeta system is presented for the metal-deficient open cluster, NGC 2420. Restricting the data to probable single members of the cluster using the CMD and the photometric indices alone generates a sample of 106 stars at the cluster turnoff. The average E(b-y) = 0.03 +/- 0.003 (s.e.m.) or E(B-V) = 0.050 +/- 0.004 (s.e.m.), where the errors refer to internal errors alone. With this reddening, [Fe/H] is derived from both m1 and hk, using b-y and Hbeta as the temperature index. The agreement among the four approaches is reasonable, leading to a final weighted average of [Fe/H] = -0.37 +/- 0.05 (s.e.m.) for the cluster, on a scale where the Hyades has [Fe/H] = +0.12. When combined with the abundances from DDO photometry and from recalibrated low-resolution spectroscopy, the mean metallicity becomes [Fe/H] = -0.32 +/- 0.03. It is also demonstrated that the average cluster abundances based upon either DDO data or low-resolution spectroscopy are consistently reliable to 0.05 dex or better, contrary to published attempts to establish an open cluster metallicity scale using simplistic offset corrections among different surveys.
We report the discovery of a new planet candidate orbiting the subgiant star HD118203 with a period of P=6.1335 days. The best Keplerian solution yields an eccentricity e=0.31 and a minimum mass m2sin(i)=2.1MJup for the planet. This star has been observed with the ELODIE fiber-fed spectrograph as one of the targets in our planet-search programme biased toward high-metallicity stars, on-going since March 2004 at the Haute-Provence Observatory. An analysis of the spectroscopic line profiles using line bisectors revealed no correlation between the radial velocities and the line-bisector orientations, indicating that the periodic radial-velocity signal is best explained by the presence of a planet-mass companion. A linear trend is observed in the residuals around the orbital solution that could be explained by the presence of a second companion in a longer-period orbit. We also present here our orbital solution for another slightly evolved star in our metal-rich sample, HD149143, recently proposed to host a 4-d period Hot Jupiter by the N2K consortium. Our solution yields a period P=4.09 days, a marginally significant eccentricity e=0.08 and a planetary minimum mass of 1.36MJup. We checked that the shape of the spectral lines does not vary for this star as well.
We assess contribution to the X-ray (above 2 keV) luminosity of the Milky Way from different classes of low-mass binary systems and single stars. We begin by using the RXTE Slew Survey of the sky at |b|>10d to construct an X-ray luminosity function (XLF) of nearby X-ray sources in the range 10^30 erg/s<Lx<10^34 erg/s (where Lx is the luminosity over 2-10 keV), occupied by coronally active binaries (ABs) and cataclysmic variables (CVs). We then extend this XLF down to Lx~10^27.5 erg/s using the Rosat All-Sky Survey in soft X-rays and available information on the 0.1-10 keV spectra of typical sources. We find that the local cumulative X-ray (2-10 keV) emissivities (per unit stellar mass) of ABs and CVs are (2.0+/-0.8)x10^27 and (1.1+/-0.3)x10^27 erg/s/M_Sun, respectively. In addition to ABs and CVs, representing old stellar populations, young stars emit locally (1.5+/-0.4)x10^27 erg/s/M_Sun. We finally attach to the XLF of ABs and CVs a high luminosity branch (up to ~10^39 erg/s composed of neutron-star and black-hole low-mass X-ray binaries (LMXBs), derived in previous work. The combined XLF covers ~12 orders of magnitude in luminosity. The estimated combined contribution of ABs and CVs to the 2-10 keV luminosity of the Milky Way is ~2x10^38 erg/s, or ~3% of the integral luminosity of LMXBs (averaged over nearby galaxies). The XLF obtained in this work is used elsewhere (Revnivtsev et al.) to assess contribution of point sources to the Galactic ridge X-ray emission.
We analyze a map of the Galactic ridge X-ray emission (GRXE) constructed in
the 3-20 keV energy band from RXTE/PCA scan and slew observations. We show that
the GRXE intensity closely follows the Galactic near-infrared surface
brightness and thus traces the Galactic stellar mass distribution. The GRXE
consists of two spatial components which can be identified with the bulge/bar
and the disk of the Galaxy. The parameters of these components determined from
X-ray data are compatible with those derived from near-infrared data. The
inferred ratio of X-ray to near-infrared surface brightness I(3-20 keV) (1e-11
erg/s/cm2/deg2)/I_(3.5micron)(MJy/sr)=0.26+/-0.05, and the ratio of X-ray to
near-infrared luminosity L_(3-20 keV)/L_(3-4 micron)=(4.1+/-0.3)e-5. The
corresponding ratio of the 3-20 keV luminosity to the stellar mass is
L_x/M_Sun=
(3.5\pm0.5) 10^{27} erg/s, which agrees within the uncertainties with the
cumulative emissivity per unit stellar mass of point X-ray sources in the Solar
neighborhood, determined in an accompanying paper (Sazonov et al.). This
suggests that the bulk of the GRXE is composed of weak X-ray sources, mostly
cataclysmic variables and coronally active binaries. The fractional
contributions of these classes of sources to the total X-ray emissivity
determined from the Solar neighborhood data can also explain the GRXE energy
spectrum. Based on the luminosity function of local X-ray sources we predict
that in order to resolve 90% of the GRXE into discrete sources a sensitivity
limit of ~10^{-16} erg/s/cm2 (2--10 keV) will need to be reached in future
observations.
This paper deals with the study of the accretion of dark energy with equation of state $p=w\rho$ onto Kerr-Newman black holes. We have obtained that when $w>-1$ the mass and specific angular momentum increase, and that whereas the specific angular momentum increases up to a given plateau, the mass grows up unboundedly. On the regime where the dominant energy condition is violated our model predicts a steady decreasing of mass and angular momentum of black holes as phantom energy is being accreted. Masses and and angular momenta of all black holes tend to zero when one approaches the big rip. The results that cosmic censorship is violated and that the black hole size increases beyond the universe size itself are discussed in terms of considering the used models as approximations to a more general descriptions where the metric is time-dependent.
We determine the abundance of primordial black holes (PBHs) formed in the context of non-gaussian models with primordial density perturbations. We consider models with a renormalized $\chi^2$ probability distribution function parametrized by the number, $\nu$, of degrees of freedom. We show that if $\nu$ is not too large then the PBH abundance will be altered by several orders of magnitude with respect to the standard gaussian result obtained in the $\nu \to \infty$ limit. We also study the dependence of the spectral index constraints on the nature of the cosmological perturbations for a power-law primordial power spectrum.
Recent time-dependent, ideal-magnetohydrodynamic (ideal-MHD) simulations of polar magnetic burial in accreting neutron stars have demonstrated that stable, magnetically confined mountains form at the magnetic poles, emitting gravitational waves at $f_{*}$ (stellar spin frequency) and $2 f_{*}$. Global MHD oscillations of the mountain, whether natural or stochastically driven, act to modulate the gravitational wave signal, creating broad sidebands (full-width half-maximum $\sim 0.2f_*$) in the frequency spectrum around $f_{*}$ and $2 f_{*}$. The oscillations can enhance the signal-to-noise ratio achieved by a long-baseline interferometer with coherent matched filtering by up to 15 per cent, depending on where $f_*$ lies relative to the noise curve minimum. Coherent, multi-detector searches for continuous waves from nonaxisymmetric pulsars should be tailored accordingly.
We present the predictions for the photometric and emission line properties of galaxies present during the latter stages of reionization from z=8 to 6. These preliminary predictions are made from cosmological hydrodynamic simulations that include star formation and feedback, but not the effects of radiative transfer. We find significant numbers of galaxies that have stellar masses exceeding 10^8 Mo by z=8, with metallicities in the range of one-tenth solar. These galaxies are just beyond the reach of current near-infrared surveys, but should be found in large numbers by next-generation programs. The Lyman alpha luminosity function does not evolve much from z=6 to z=8, meaning that it should also be possible to detect these objects in significant numbers with upcoming narrow band surveys, unless the escape fraction of Ly-alpha evolves significantly between those epochs.
We describe our project to examine the evolution of distant disk galaxies, and present the results of our work based on the Tully-Fisher relation. Comparing matched cluster and field samples we find evidence that the cluster galaxies are on average 0.7+-0.2 mag brighter than those in the field. Considering the field sample alone we find a brightening with redshift amounting to 1.0+-0.5 mag by z=1, which is likely to be an upper limit, considering the selection effects. We also give brief details of the ESO Distant Clusters Survey (EDisCS) and describe our plans for expanding our studies using data from this survey.
The Gemini Deep Deep Survey, GDDS, produced several significant results relating to the evolution of galaxies. All of these results are consistent with the "downsizing" concept of galaxy formation and evolution, i.e., that the active periods of star formation moved progressively from very massive galaxies at high redshift to much lower mass galaxies at the present epoch. Spectra of massive red galaxies at z ~ 1.7 demonstrates that they contain old stellar populations and hence must have formed their stars in the first ~3 Gyr of cosmic history; indicators of star formation activity show that the star formation rate in the most massive galaxies was much higher at z = 2 than today, that the activity in intermediate mass galaxies peaked near z ~ 1.5, while, since z ~ 1 the activity is primarily confined to lower mass galaxies. The GDDS also uncovered a relatively high percentage of post-starburst galaxies at z ~ 1, a result that is anticipated given all the activity seen at higher redshifts. Measurements of the strengths of metal lines of a subsample of the GDDS and CFRS galaxies at z ~ 0.7 reveal that, at a given mass, they had lower metallicities than at present. The evolution in the mass-metallicity relation is consistent with a model in which star formation lasts longest in less massive galaxies, again an expected result in the downsizing scenario.
We discuss vortex-mediated mutual friction in the two-fluid model for superfluid neutron star cores. Our discussion is based on the general formalism developed by Carter and collaborators, which makes due distinction between transport velocity and momentum for each fluid. This is essential for an implementation of the so-called entrainment effect, whereby the flow of one fluid imparts momentum in the other and vice versa. The mutual friction follows by balancing the Magnus force that acts on the quantised neutron vortices with a resistive force due to the scattering of electrons off of the magnetic field with which each vortex core is endowed. We derive the form of the macroscopic mutual friction force which is relevant for a model based on smooth-averaging over a collection of vortices. We discuss the coefficients that enter the expression for this force, and the timescale on which the two interpenetrating fluids in a neutron star core are coupled. This discussion confirms that our new formulation accords well with previous work in this area.
We examined the properties of a sample of BATSE Gamma--Ray Bursts (GRBs) comprising events which have indications of association with a supernova (SN), some on the basis of indications of re--brightening in the optical afterglow light curve, but in most cases based only on the `loose' temporal and directional coincidence inferred from the cross correlation of catalogs. Despite of the large uncertainties in the latter selection method, the temporal and spectral analysis reveal three interesting statistical results when the sample is compared with that of all the BATSE GRBs: the GRBs tentatively associated with SNe are found to predominantly (in $\sim$ 80% of the cases) have single-peaked light curves, a softer spectrum (i.e. low energy power law index $\alpha \sim$ --1.5) and tend not to follow the Lag-Luminosity and Isotropic Energy--Peak Energy correlations. These three independent statistical properties point toward the existence of a significant number of under-luminous,GRB 980425-like events constituting -- at least from an observational point of view -- a tail or a separate class with respect to the whole of the BATSE GRB events. The unusually high percentage of SN Ibc among those identified by the catalog cross--correlation (factor $\sim 4$ higher than expected from SN catalog statistics) reinforces the non-randomness of (some of) the selected events.
The nature of Dark Energy is still very much a mystery, and the combination of a variety of experimental tests, sensitive to different potential Dark Energy properties, will help elucidate its origins. This white paper briefly surveys the array of theoretical approaches to the Dark Energy problem and their relation to experimental questions.
We compare the angular momentum extracted by a wind from a pre-main-sequence star to the torques arising from the interaction between the star and its Keplerian accretion disk. We find that the wind alone can counteract the spin-up torque from mass accretion, solving the mystery of why accreting pre-main-sequence stars are observed to spin at less than 10% of break-up speed, provided that the mass outflow rate in the stellar winds is ~10% of the accretion rate. We suggest that such massive winds will be driven by some fraction $\epsilon$ of the accretion power. For observationally constrained typical parameters of classical T-Tauri stars, $\epsilon$ needs to be between a few and a few tens of percent. In this scenario, efficient braking of the star will terminate simultaneously with accretion, as is usually assumed to explain the rotation velocities of stars in young clusters.
Using observations with the Rossi X-ray Timing Explorer, we examine the behavior of 2-10 Hz quasi-periodic oscillations (QPOs) during spectrally-hard dips in the x-ray light curve of GRS 1915+105 that are accompanied by infrared flares. Of the twelve light-curves examined, nine are beta-class and three are alpha-class following the scheme of Belloni et al. (2000). In most cases, the QPO frequency is most strongly correlated to the power law flux, which partially contradicts some earlier claims that the strongest correlation is between QPO frequency and blackbody flux. Seven beta-class curves are highly correlated to blackbody features. In several cases, the QPO evolution appears to decouple from the spectral evolution. We find that beta-class light-curves with strong correlations can be distinguished from those without by their ``trigger spike'' morphology. We also show that the origin and strength of the subsequent infrared flare may be causally linked to the variations in QPO frequency evolution and not solely tied to the onset of soft x-ray flaring behavior. We divide the twelve alpha- and beta-class light-curves into three groups based on the evolution of the QPO, the morphology of the trigger spike, and the infrared flare strength. An apparent crossover case leads us to conclude that these groups are not unique modes but represent part of a continuum of accretion behaviors. We believe the QPO behavior at the initiation of the hard dip can ultimately be used to determine the terminating x-ray behavior, and the following infrared flaring behavior.
XMM-Newton spectra of five red, 2MASS AGN, selected from a sample observed by Chandra to be relatively X-ray bright and to cover a range of hardness ratios, confirm the presence of substantial absorbing material in three sources with optical classifications ranging from Type 1 to Type 2. A flat (hard), power law continuum is observed in the other two. The combination of X-ray absorption and broad optical emission lines suggests either a small (nuclear) absorber or a favored viewing angle so as to cover the X-ray source but not the broad emission line region (BELR). A soft excess is detected in all three Type 1 sources. We speculate that this may arise in an extended region of ionised gas, perhaps linked with the polarised (scattered) optical light present in these sources. The spectral complexity revealed by XMM-Newton emphasizes the limitations of the low S/N \chandra data. The new results strengthen our earlier conclusions that the observed X-ray continua of red AGN are unusually hard at energies >2 keV. Their observed spectra are consistent with contributing significantly to the missing hard/absorbed population of the Cosmic X-ray Background (CXRB) although their intrinsic power law slopes are typical of broad-line (Type 1) AGN (Gamma ~1.7-1.9). This suggests that the missing X-ray-absorbed CXRB population may include Type 1 AGN/QSOs in addition to the Type 2 AGN generally assumed.
A review of observational evidence in favour of a metallicity dependence of WN and WC stars is presented. New near-IR studies of Milky Way, LMC and SMC early-type WN stars are presented, with weake winds amongst WN stars containing hydrogen. A metallicity dependence is supported for WN stars with hydrogen, with dM/dt propto Z^alpha with alpha approx 0.8 +/- 0.2 The influence of CNO content upon WN subtypes is discussed. Earlier WN spectral types are expected (and observed) at lower metallicity due to the abundance sensitivity of NIII-IV classification diagnostics. Recent physical and chemical results of WC stars in the Milky Way and LMC are discussed, suggesting a metallicity dependence of alpha approx 0.6 +/- 0.1. Earlier WC spectral types are predicted (and observed) in lower metallicity galaxies, due to the dependence of the CIII classification diagnostic on wind density. WO stars reveal lower wind velocities at lower metallicity, whilst the situation for WN and WC stars is unclear. Finally, the influence of a WR metallicity dependence upon the ionizing flux distributions and optical line luminosities is addressed, with particular regard to I Zw 18. Weaker winds at low metallicity would imply harder ionizing flux distributions and lower line luminosities, arguing for an substantially increased number of WR stars with respect to standard calibrations, exacerbating difficulties with single star evolutionary models at very low metallicity.
We use morphological measurements and the scatter of clusters about observed and simulated scaling relations to examine the impact of merging and core-related phenomena on the structure of galaxy clusters. All relations constructed from emission-weighted mean temperature and intracluster medium (ICM) mass, X-ray luminosity, isophotal size, or near-IR luminosity show a separation between cool core (CC) and non-cool core (NCC) clusters. We attribute this partially to a temperature bias in CC clusters, and partially to other cool core-related structural changes. We attempt to minimize CC/NCC separation in scaling relations by applying a uniform scale factor to CC cluster temperatures and determining the scale factor for each relation that minimizes the separation between CC and NCC populations, and by introducing central surface brightness as a third parameter in relations. The latter approach reduces scatter in relations more than temperature scaling. We compare the scatter within subsamples split by CC/NCC and morphological merger indicators. CC clusters and clusters with less substructure generally exhibit higher scatter about relations. The larger structural variations in CC clusters exit well outside the core, suggesting that a process more global than core radiative instability is at work. Simulations without cooling mechanisms also show no correlation between substructure and larger scatter about relations, indicating that any merger-related scatter increases are subtle. The results indicate cool core related phenomena, not merging processes, are the primary contributor to scatter in scaling relations. Our analysis does not appear to support the scenario in which clusters evolve cool cores over time unless they experience major mergers. (Abridged)
We measure the Petrosian structural properties of 33 brightest cluster galaxies (BCGs) at redshifts z<0.1 in X-ray selected clusters with a wide range of X-ray luminosities. We find that some BCGs show distinct signatures in their Petrosian profiles, likely to be due to cD haloes. We also find that BCGs in high X-ray luminosity clusters have shallower surface brightness profiles than those in low X-ray luminosity clusters. This suggests that the BCGs in high X-ray luminosity clusters have undergone up to twice as many equal-mass mergers in their past as those in low X-ray luminosity clusters. This is qualitatively consistent with the predictions of hierarchical structure formation.
We report ``infall asymmetry'' in the HCO$^+$ (1--0) and (3--2) lines toward NGC 1333, extended over $\sim 0.39 {\rm pc}^2$, a larger extent than has been reported be fore, for any star-forming region. The infall asymmetry extends over a major portion of the star-forming complex, and is not limited to a single protostar, or to a single dense core, or to a single spectral line. It seems likely that the infall asymmetry represents inward motions, and that these motions are physically associated with the complex. Both blue-asymmetric and red-asymmetric lines are seen, but in both the (3--2) and (1--0) lines of HCO$^+$ the vast majority of the asymmetric lines are blue, indicating inward motions. The (3--2) line, tracing denser gas, has the spectra with the strongest asymmetry and these spectra are associated with the protostars IRAS 4A and 4B, which most likely indicates a warm central source is affecting the line profiles. The (3--2) and (1--0) lines usually have the same sense of asymmetry in common positions, but their profiles differ significantly, and the (1--0) line appears to trace motions on much larger spatial scales than does the (3--2) line. Line profile models fit the spectra well, but do not strongly constrain their parameters. The mass accretion rate of the inward motions is of order 10$^{-4}$ M$_\odot$/yr, similar to the ratio of stellar mass to cluster age.
We describe an automated method for assigning the most likely physical parameters to the components of an eclipsing binary (EB), using only its photometric light curve and combined color. In traditional methods (e.g. WD and EBOP) one attempts to optimize a multi-parameter model over many iterations, so as to minimize the chi-squared value. We suggest an alternative method, where one selects pairs of coeval stars from a set of theoretical stellar models, and compares their simulated light curves and combined colors with the observations. This approach greatly reduces the EB parameter-space over which one needs to search, and allows one to determine the components' masses, radii and absolute magnitudes, without spectroscopic data. We have implemented this method in an automated program using published theoretical isochrones and limb-darkening coefficients. Since it is easy to automate, this method lends itself to systematic analyses of datasets consisting of photometric time series of large numbers of stars, such as those produced by OGLE, MACHO, TrES, HAT, and many others surveys.
Cosmologists today are confronted with the perplexing reality that the universe is currently accelerating in its expansion. Nevertheless, the nature of the fuel that drives today's cosmic acceleration is an open and tantalizing mystery. There exists the intriguing possibility that the acceleration is not the manifestation of yet another mysterious ingredient in the cosmic gas tank (dark energy), but rather our first real lack of understanding of gravity itself, and even possibly a signal that there might exist dimensions beyond that which we can currently observe. The braneworld model of Dvali, Gabadadze and Porrati (DGP) is a theory where gravity is altered at immense distances by the excruciatingly slow leakage of gravity off our three-dimensional Universe and, as a modified-gravity theory, has pioneered this line of investigation. I review the underlying structure of DGP gravity and those phenomenological developments relevant to cosmologists interested in a pedagogical treatment of this intriguing model.
We study the excitation of density and bending waves and the associated angular momentum transfer in gaseous disks with finite thickness by a rotating external potential. The disk is assumed to be isothermal in the vertical direction and has no self-gravity. The disk perturbations are decomposed into different modes, each characterized by the azimuthal index m and the vertical index n, which specifies the nodal number of the density perturbation along the disk normal direction. The n=0 modes correspond to the two-dimensional density waves previously studied by Goldreich & Tremaine and others. In a three-dimensional disk, waves can be excited at both Lindblad resonances (for modes with n=0,1,2...) and vertical resonances (for the $n\ge 1$ modes only). The torque on the disk is positive for waves excited at outer Linblad/vertical resonances and negative at inner Lindblad/vertical resonances. While the n=0 modes are evanescent around corotation, the $n\ge 1$ modes can propagate into the corotation region where they are damped and deposit their angular momenta. We have derived analytical expressions for the amplitudes of different wave modes excited at Lindblad and/or vertical resonances and the resulting torques on the disk. It is found that for $n\ge 1$, angular momentum transfer through vertical resonances is much more efficient than Lindblad resonances. This implies that in some situations (e.g., a circumstellar disk perturbed by a planet in an inclined orbit), vertical resonances may be the dominant channel of angular momentum transfer between the disk and the external potential. We have also derived new formulae for the angular momentum dissipation at corotation and studied wave excitations at disk boundaries.
We present deep Ks<21.5 (Vega) identifications, redshifts and stellar masses for most of the sources composing the bulk of the 24 micron background in the GOODS/CDFS. Our identified sample consists of 747 Spitzer/MIPS 24 micron objects, and includes ~94% of all the 24 micron sources in the GOODS-South field which have fluxes Snu(24)>83 microJy (the 80% completeness limit of the Spitzer/GTO 24 micron catalog). 36% of our galaxies have spectroscopic redshifts (mostly at z<1.5) and the remaining ones have photometric redshifts of very good quality, with a median of |dz|=|zspec-zphot|/(1+zspec)=0.02. We find that MIPS 24 micron galaxies span the redshift range z~0-4, and that a substantial fraction (28%) lie at high redshifts z>1.5. We determine the existence of a bump in the redshift distribution at z~1.9, indicating the presence of a significant population of galaxies with PAH emission at these redshifts. Massive (M>10^11 Msun) star-forming galaxies at redshifts 2<z<3 are characterized by very high star-formation rates (SFR>500 Msun/yr), and some of them are able to construct a mass of 10^10-10^11 Msun in a single burst lifetime (~0.01-0.1 Gyr). At lower redshifts z<2, massive star-forming galaxies are also present, but appear to be building their stars on long timescales, either quiescently or in multiple modest burst-like episodes. At redshifts z~1-2, the ability of the burst-like mode to produce entire galaxies in a single event is limited to some lower (M<7x10^10 Msun) mass systems, and it is basically negligible at z<1. Our results support a scenario where star-formation activity is differential with assembled stellar mass and redshift, and where the relative importance of the burst-like mode proceeds in a down-sizing way from high to low redshifts. (abridged)
We present the first sub-arcsecond radio observations of the nearby dwarf starburst galaxy NGC 3077 obtained with the MERLIN interferometer. We have detected two resolved sources which are coincident with the positions of two discrete X-ray sources detected by Chandra. One of the radio sources is associated with a supernova remnant and the observed radio flux is consistent with having a non-thermal origin. The age of the SNRs of about 760 years is between the average age of the SNRs detected in M82 and those detected in the Milky Way and the Large Magellanic Cloud. We use this detection to calculate a star formation rate (SFR) of 0.28 M_sun year-1 which is similar to the SFR calculated by using far infrared and millimeter observations but larger than the SFR given by optical recombination lines corrected for extinction. The other compact radio source detected by MERLIN which is coincident with the position of an X-ray binary, has the properties of an HII region with a flux density of about 747 microJy which corresponds to an ionizing flux of 6.8x10^50 s-1. A young massive stellar cluster with a mass of about 2x10^5 M_sun, detected by the Hubble Space Telescope could be the responsible for the production of the ionizing flux.
Through a combined optical and radio analysis, we have investigated the possible connection between the dynamical state of the merging cluster A3921 and its star formation properties, reaching the conclusion that the on-going merger is triggering a SF episode in the collision region.
From the Fourier analysis of the catalogue of ~200,000 variable candidates in the OGLE-II Galactic fields, we found a sample of about 230 short-period low-amplitude variable stars. From their position in the colour-magnitude diagram and the observed periods (multiple in most cases), we identify the stars as a mixture of Beta Cephei and Delta Scuti stars. Beta Cephei stars from this sample are located in the Galactic disk at distances from 3 to 6 kpc. Many of them shows large range of the excited periods, an indication of high metallicity. We estimate that even a half of the sample of 230 short-period variables we found, can be Beta Cephei stars. The periods alone, however, are rarely sufficient to distinguish between both types of pulsators. We point out, how this can be done observationally.
We use the algorithm of Cole et al. (2000) to generate merger trees for the
first star clusters in a Lambda CDM cosmology under an isotropic UV background
radiation field, parametrized by J_21. We have investigated the problem in two
ways: a global radiation background and local radiative feedback surrounding
the first star clusters.
Cooling in the first halos at high redshift is dominated by molecular
hydrogen, H_2 - we call these Generation 1 objects. At lower redshift and
higher virial temperature, T_vir > 10^4K, electron cooling dominates - we call
these generation 2. Radiation fields act to photo-dissociate H_2, but also
generate free electrons that can help to catalyse its production. At modest
radiation levels, J_{21}/(1+z)^3 ~ 10^{-12}-10^{-7}, the nett effect is to
enhance the formation of Generation 1 star-clusters. At higher fluxes the
heating from photo-ionisation dominates and halts their production. With a
realistic build-up of flux over time, the period of enhanced H_2 cooling is so
fleeting as to be barely discernable and the nett effect is move primordial
star cluster formation towards Generation 2 objects at lower redshift. A
similar effect is seen with local feedback. Provided that enough photons are
produced to maintain ionization of their host halo, they will suppress the
cooling in Generation 1 halos and boost the numbers of primordial star clusters
in Generation 2 halos. Significant suppression of Generation~1 halos occurs for
specific photon fluxes in excess of about 10^{43} ph s^{-1} Msun^{-1}.
We develop a model based on 3D mean-field MHD for the generation of large scale magnetic fields in fully convective objects like low-mass stars, brown dwarfs and possibly gaseous planets. The dynamo process is of alpha^2 type and thus differs from the shell-dynamo at work in more massive stars.The alpha^2 dynamo becomes supercritical for Rossby numbers Ro\la 10. It generates a large-scale, non-axisymmetric, steady field that is symmetric with respect to the equatorial plane. Saturation of the alpha^2-generated field at the equipartition yields strengths of several kGauss, in agreement with observations of active M dwarfs, and provides a qualitative explanation for the observed activity saturation in late M stars. For brown dwarfs with a conductive core, as occurs at the center of the most massive and oldest of these objects, we have also studied an alpha^2-Omega dynamo, i.e. the effect of differential rotation. In this case the field is predominantly toroidal, axisymmetric and oscillatory, like the solar field. The topology of the field in the fully convective objects exhibits a high order multipole character that differs from the aligned dipole field generated by the alpha-Omega dynamo. The strong reduction of the dipolar component due to the field non-axisymmetry should considerably reduce the Alfven radius and thus the efficiency of magnetic braking, providing an appealing explanation for the decreasing angular momentum loss rate observed in low-mass stars and brown dwarfs. This may have also implications for cataclysmic variables below the period gap. In spite of this large-scale field, the decreasing conductivity in the dominantly neutral atmosphere of these objects prevents the current generation necessary to support a chromosphere and thus activity. (Abridged)
We present some applications of our Synthetic Horizontal Branches (SHB) simulations, aimed to reproduce the peculiar period distributions of RR Lyrae belonging to the Galactic Globular Clusters M3 and M5. We show some evidence, supporting the importance of SHBs in obtaining parameters such as the mass distribution inside the instability strip.
The evolution of galaxies in groups may have important implications for the global evolution of star formation rate in the Universe, since many processes which operate in groups may suppress star formation, and the fraction of galaxies bound in groups at the present day is as high as ~60%. We present an analysis of our sample of 0.3<=z<=0.55 groups, selected from the CNOC2 redshift survey and supplemented with deep spectroscopy and HST ACS imaging. We find that these groups contain significantly more passive galaxies than the field, with excesses of S0, elliptical and passive spiral galaxy types. The morphological composition is closely matched to that of more massive irregular clusters at a similar epoch. Contrasting with galaxy samples in a variety of environments and epochs, we find that the fraction of passive galaxies (EW[OII]<5A), is strongly evolving in the group environment, with parallel evolution in the (global) field population, whilst little evolution is observed in cluster cores since z~1.
The cataclysmic variable ASAS J002511+1217.2 was discovered in outburst by the All-Sky Automated Survey in September 2004, and intensively monitored by AAVSO observers through the following two months. Both photometry and spectroscopy indicate that this is a very short-period system. Clearly defined superhumps with a period of 0.05687 +/- 0.00001 days (1-sigma) are present during the superoutburst, 5 to 18 days following the ASAS detection. We observe a change in superhump profile similar to the transition to ``late superhumps'' observed in other short-period systems; the superhump period appears to increase slightly for a time before returning to the original value, with the resulting superhump phase offset by approximately half a period. We detect variations with a period of 0.05666 +/- 0.00003 days (1-sigma) during the four-day quiescent phase between the end of the main outburst and the single echo outburst. Weak variations having the original superhump period reappear during the echo and its rapid decline. Time-resolved spectroscopy conducted nearly 30 days after detection and well into the decline yields an orbital period measurement of 82 +/- 5 minutes. Both narrow and broad components are present in the emission line spectra, indicating the presence of multiple emission regions. The weight of the observational evidence suggests that ASAS J002511+1217.2 is a WZ Sge-type dwarf nova, and we discuss how this system fits into the WZ classification scheme.
This study presents first results from an X-ray mini-survey carried out with XMM-Newton to investigate the diffuse Hot Ionized Medium in the halos of nine nearby star-forming edge-on spiral galaxies. Diffuse gaseous X-ray halos are detected in eight of our targets, covering a wide range of star formation rates from quiescent to starburst cases. For four edge-on spiral galaxies, namely NGC3044, NGC3221, NGC4634, and NGC5775, we present the first published high resolution/sensitivity detections of extended soft X-ray halos. EPIC X-ray contour maps overlaid onto Halpha imaging data reveals that in all cases the presence of X-ray halos is correlated with extraplanar Diffuse Ionized Gas. Moreover, these halos are also associated with non-thermal cosmic ray halos, as evidenced by radio continuum observations. Supplemental UV-data obtained with the OM-telescope at 210nm show Diffuse Ionized Gas to be well associated with UV emission originating in the underlying disk. Beside NGC891, NGC4634 is the second non-starburst galaxy with a diffuse soft X-ray halo (|z|<4kpc). In case of NGC3877, for which we also present the first high resolution X-ray imaging data, no halo emission is detectable. EPIC pn spectra (0.3-12keV) of the diffuse X-ray emission are extracted at different offset positions from the disk, giving evidence to a significant decrease of gas temperatures, electron densities, and gas masses with increasing distance to the plane. A comparison between dynamical and radiative cooling time scales implies that the outflow in all targets is likely to be sustained. We find very strong indications that spatially correlated multi-phase gaseous halos are created by star forming activity in the disk plane.
In this paper we analyze the evolution of actively star forming galaxies in the mid-infrared (MIR). This spectral region, characterized by continuum emission by hot dust and by the presence of strong emission features generally ascribed to polycyclic aromatic hydrocarbon (PAH) molecules, is the most strongly affected by the heating processes associated with star formation and/or active galactic nuclei (AGN). Following the detailed observational characterization of galaxies in the MIR by ISO, we have updated the modelling of this spectral region in our spectro-photometric model GRASIL (Silva et al. 1998). In the diffuse component we have updated the treatment of PAHs according to the model by Li & Draine (2001). As for the dense phase of the ISM associated with the star forming regions, the molecular clouds, we strongly decrease the abundance of PAHs as compared to that in the cirrus, basing on the observational evidences of the lack or weakness of PAH bands close to the newly formed stars, possibly due to the destruction of the molecules in strong UV fields. The robustness of the model is checked by fitting near infrared to radio broad band spectra and the corresponding detailed MIR spectra of a large sample of galaxies (Lu et al. 2003), at once. With this model, we have analyzed the larger sample of actively star forming galaxies by Dale et al. (2000). We show that the observed trends of galaxies in the ISO-IRAS-Radio color-color plots can be interpreted in terms of different evolutionary phases of star formation activity, and the consequent different dominance in the spectral energy distribution (SED) of the diffuse or dense phase of the ISM.
We present efficient algorithms for generating "infinite" streams of band-limited $1/f^alpha$ noise, by numerically filtering a white signal. In the $1/f^2$ case the algorithm is optimal, ie. only limited by the white generator, and is close to optimal in the general case ($0<\alpha<2$) . The properties and performances are evaluated in the context of the Planck simulation experiment. The software is available from this http URL
A recent article has employed the determination from groundbased images of high proper motions in the Orion Nebula Cluster to argue that JW 349, JW 355, and JW 451 are high velocity (38 km/s, 89 km/s, and 69 km/s, respectively) low mass runaway stars. We report on measurement of the proper motions of these stars using images made by the Hubble Space Telescope's WFPC2 imager and find that there is no evidence for motions above 6.2 km/s for JW 349 and 7.9 km/s for JW 355, while the motion of 5.5 km/s for JW 451 is only slightly larger than the measurement uncertainty of 3.9 km/s. We conclude that there is no observational support for these stars being high velocity runaway stars.
We present a stellar populations analysis of the first release of the CFHTLS (Canada-France-Hawai Telescope Legacy Survey) data. A detailed comparison between the Besan\c{c}on model of the Galaxy and the first data release of the CFHTLS-Deep survey is performed by implementing the MEGACAM photometric system in this model using stellar atmosphere model libraries. The reliability of the theoretical libraries to reproduce the observed colours in the MEGACAM system is investigated. The locations of various stellar species like subdwarfs, white dwarfs, late-type and brown dwarfs, binary systems are identified. The contamination of the stellar sample by quasars and compact galaxies is quantified using spectroscopic data from the VIMOS-VLT Deep Survey (VVDS) as a function of $i'$ magnitude and $r'-i'$ colour. A comparison between simulated counts using the standard IMF at low masses show that the number of very low mass dwarfs may have been underestimated in previous studies. These observations favour a power law IMF following $d(n)/dm \propto m^{-\alpha}$ with $\alpha=2.5$ for $m < $ 0.25 \Msun or $\alpha=3.0$ for $m < $ 0.2 \Msun for single stars. The resulting LF is in agreement with the local LF as measured from the 5 or 25 pc samples. It is in strong disagreement with the Zheng et al (2001) LF measured from deep HST data. We show that this discrepancy can be understood as an indication of a different IMF at low masses at early epochs of the Galaxy compared to the local thin disc IMF.
The INTEGRAL observatory has been performing a deep survey of the Galactic
central radian since 2003, with the goal of both extracting a catalog of
sources and gaining insight into the Galactic diffuse emission. This paper
concentrates on the estimation of the total point sources emission
contribution. It is now clear that unresolved point sources contribute to the
observed diffuse emission; the increasing sensitivity of instruments with time
has lead to a steady decrease in estimates of this ``diffuse emission''.
We have analysed the first year data obtained with the spectrometer and
imager SPI on board INTEGRAL. First, a catalog of 63 hard X-ray sources
detected, time-averaged, during our 2003 Galactic plane survey, is derived.
Second, after extracting the spectra of the sources detected by SPI, their
combined contribution is compared to the total (resolved and unresolved)
emission from the Galactic ridge. The data analysis is complex: it requires us
to split the total emission into several components, as discrete sources and
diffuse emission are superimposed in SPI data. The main result is that point
source emission dominates in the hard X-ray/soft $\gamma$-ray domain, and
contributes around 90 % of the total emission around 100 keV, while above 250
keV, diffuse electron-positron annihilation, through its three-photon
positronium continuum with a positronium fraction $\sim$ 0.97 and the 511 keV
electron-positron line, dominates over the sources.
We present archive ESO VLT photometric and spectroscopic data of the Ultraluminous X-ray source NGC 1313 X-2. The superb quality of the VLT images reveals that two distinct objects, with R magnitudes 23.7 and 23.6, are visible inside the Chandra error box. The two objects, separated by 0.75 arcsec, were unresolved in our previous ESO 3.6 m+EFOSC image. We show that both are stars in NGC 1313, the first a B0-O9 main sequence star of ~20 Msun, while the second a G supergiant of ~10 Msun. Irrespectively of which of the two objects the actual counterpart is, this implies that NGC 1313 X-2 is a high mass X-ray binary with a very massive donor.
We present new ISOCAM mid-infrared spectra of three starbursting nearby dwarf galaxies, NGC1569, IIZw40, NGC1140 and the 30Dor region of the LMC and explore the properties of the ISM in low-metallicity environments, also using additional sources from the literature. We analyse the various components of the ISM probed by the mid-infrared observations and compare them with other Galactic and extragalactic objects. The MIR spectra of the low-metallicity starburst sources are dominated by the [NeIII] and [SIV] lines, as well as a steeply rising dust continuum. PAH bands are generaly faint, both locally and averaged over the full galaxy, in stark contrast to dustier starburst galaxies, where the PAH features are very prominant and even dominate on global scales. The hardness of the modeled interstellar radiation fields for the dwarf galaxies increases as the presence of PAH band emission becomes less pronounced. The [NeIII]/[NeII] ratios averaged over the full galaxy are strikingly high, often >10. Thus, the hard radiation fields are pronounced and pervasive. We find a prominent correlation between the PAHs/VSGs and the [NeIII]/[NeII] ratios for a wide range of objects, including the low metallicity galaxies as well as Galactic HII regions and other metal-rich galaxies. This effect is consistent with the hardness of the interstellar radiation field playing a major role in the destruction of PAHs in the low metallicity ISM. We see a PAHs/VSGs and metallicity correlation, also found by Engelbracht et al. (2005) for a larger survey. Combined effects of metallicity and radiation field seem to be playing important roles in the observed behavior of PAHs in the low metallicity systems.
We present models for the chemistry in gas moving towards the ionization front of an HII region. When it is far from the ionization front, the gas is highly depleted of elements more massive than helium. However, as it approaches the ionization front, ices are destroyed and species formed on the grain surfaces are injected into the gas phase. Photodissociation removes gas phase molecular species as the gas flows towards the ionization front. We identify models for which the OH column densities are comparable to those measured in observations undertaken to study the magnetic fields in star forming regions and give results for the column densities of other species that should be abundant if the observed OH arises through a combination of the liberation of H2O from surfaces and photodissociation. They include CH3OH, H2CO, and H2S. Observations of these other species may help establish the nature of the OH spatial distribution in the clouds, which is important for the interpretation of the magnetic field results.
We study the dark matter problem with particular reference to a candidate particle within the Standard Model: the $H$ dibaryon. We consider as well a scenario which aims to connect the dark matter origin to the Baryon Asymmetry of the Universe, studying the examples of $H$ and of a BSM particle $X$. Strongly attractive color forces in the flavor singlet channel may lead to a tightly bound and compact $H$ dibaryon. We find that the observation of $\Lambda$ decays from doubly-strange hypernuclei puts a constraint on the $H$ wavefunction which is plausibly satisfied. In this case the $H$ is long-lived as we calculate. We also show that an $H$ or another compact, flavor singlet hadron is unlikely to bind to nuclei, so that experimental bounds on exotic isotopes do not exclude their existence. Remarkably, the $H$ appears to evade other experimental constraints as well, when account is taken of its expected compact spatial wavefunction. We check whether the $H$ is a viable DM candidate by considering DM experiments sensitive to light particles. Taking into account the DM interaction in the crust above underground detectors we find a window in the exclusion limits in the micro-barn, $m\lsi 2.4$ GeV, range which coincides with the range expected for the tightly bound $H$. Finally, we present a scenario in which DM carries (anti-)baryon number and which offers a mechanism to generate the baryon asymmetry observed in the Universe. If $\sigma^{annih} _{\bar{X}} < \sigma^{annih}_{X}$, the $\bar{X}$'s freeze out at a higher temperature and have a larger relic density than $X$'s. If $m_X \lsi 4.5 B_X$ GeV and the annihilation cross sections differ by 10% or more, this type of scenario naturally explains the observed $\Omega_{DM} \simeq 5 \Omega_b$. Two examples are given, the $H$ and an hypothetical BSM candidate $X$.
Using archival data of low-redshift (z < 0.01) Type Ia supernovae (SN Ia) and recent observations of high-redshift (0.16 < z <0.64; Matheson et al. 2005) SN Ia, we study the "uniformity'' of the spectroscopic properties of nearby and distant SN Ia. We find no difference in the measures we describe here. In this paper, we base our analysis solely on line-profile morphology, focusing on measurements of the velocity location of maximum absorption (vabs) and peak emission (vpeak). We find that the evolution of vabs and vpeak for our sample lines (Ca II 3945, Si II 6355, and S II 5454, 5640) is similar for both the low- and high-redshift samples. We find that vabs for the weak S II 5454, 5640 lines, and vpeak for S II 5454, can be used to identify fast-declining [dm15 > 1.7] SN Ia, which are also subluminous. In addition, we give the first direct evidence in two high-z SN Ia spectra of a double-absorption feature in Ca II 3945, an event also observed, though infrequently, in low-redshift SN Ia spectra (6/22 SN Ia in our local sample). We report for the first time the unambiguous and systematic intrinsic blueshift of peak emission of optical P-Cygni line profiles in Type Ia spectra, by as much as 8000 km/s. All the high-z SN Ia analyzed in this paper were discovered and followed up by the ESSENCE collaboration, and are now publicly available.
A revised near infrared classification scheme for T dwarfs is presented, based on and superseding prior schemes developed by Burgasser et al. and Geballe et al., and defined following the precepts of the MK Process. Drawing from two large spectroscopic libraries of T dwarfs identified largely in the Sloan Digital Sky Survey and the Two Micron All Sky Survey, nine primary spectral standards and five alternate standards spanning spectral types T0 to T8 are identified that match criteria of spectral character, brightness, absence of a resolved companion and accessibility from both northern and southern hemispheres. The classification of T dwarfs is formally made by the direct comparison of near infrared spectral data of equivalent resolution to the spectra of these standards. Alternately, we have redefined five key spectral indices measuring the strengths of the major H$_2$O and CH$_4$ bands in the 1-2.5 micron region that may be used as a proxy to direct spectral comparison. Two methods of determining T spectral type using these indices are outlined and yield equivalent results. These classifications are also equivalent to those from prior schemes, implying that no revision of existing spectral type trends is required. The one-dimensional scheme presented here provides a first step toward the observational characterization of the lowest luminosity brown dwarfs currently known. Future extensions to incorporate spectral variations arising from differences in photospheric dust content, gravity and metallicity are briefly discussed. A compendium of all currently known T dwarfs with updated classifications is presented.
It is already known that about 10% of central stars of PNe are very short-period binaries (hours to days), which are detected through photometric variations. These must have been formed through common-envelope interactions in initially wide binaries, accompanied by ejection of the envelope and its subsequent photoionization as a PN. Radial-velocity observations by ourselves and others are now suggesting that an even larger fraction of planetary nuclei may be spectroscopic binaries, making the total binary fraction very large. However, we have not as yet been able to rule out the possibility that the apparent velocity changes are actually due to stellar-wind variations. Pending follow-up spectroscopic observations with large telescopes, it presently appears plausible that binary-star ejection is the major formation channel for planetary nebulae.
We obtained spectra, covering the CaII H and K region, for 49 exoplanet host (EH) stars, observable from the southern hemisphere. We measured the chromospheric activity index, Rhk. We compiled previously published values of this index for the observed objects as well as the remaining EH stars in an effort to better smooth temporal variations and derive a more representative value of the average chromospheric activity for each object. We used the average index to obtain ages for the group of EH stars. In addition we applied other methods, such as: Isochrone, lithium abundance, metallicity and transverse velocity dispersions, to compare with the chromospheric results. The kinematic method is a less reliable age estimator because EH stars lie red-ward of Parenago's discontinuity in the transverse velocity dispersion vs dereddened B-V diagram. The chromospheric and isochrone techniques give median ages of 5.2 and 7.4 Gyr, respectively, with a dispersion of 4 Gyr. The median age of F and G EH stars derived by the isochrone technique is 1--2 Gyr older than that of identical spectral type nearby stars not known to be associated with planets. However, the dispersion in both cases is large, about 2--4 Gyr. We searched for correlations between the chromospheric and isochrone ages and Lir/L* (the excess over the stellar luminosity) and the metallicity of the EH stars. No clear tendency is found in the first case, whereas the metallicy dispersion seems to slightly increase with age.
Current astrophysical research suggests that the most persistently luminous objects in the Universe are powered by the flow of matter through accretion disks onto black holes. Accretion disk systems are observed to emit copious radiation across the electromagnetic spectrum, each energy band providing access to rather distinct regimes of physical conditions and geometric scale. X-ray emission probes the innermost regions of the accretion disk, where relativistic effects prevail. While this has been known for decades, it also has been acknowledged that inferring physical conditions in the relativistic regime from the behavior of the X-ray continuum is problematic and not satisfactorily constraining. With the discovery in the 1990s of iron X-ray lines bearing signatures of relativistic distortion came the hope that such emission would more firmly constrain models of disk accretion near black holes, as well as provide observational criteria by which to test general relativity in the strong field limit. Here we provide an introduction to this phenomenon. While the presentation is intended to be primarily tutorial in nature, we aim also to acquaint the reader with trends in current research. To achieve these ends, we present the basic applications of general relativity that pertain to X-ray spectroscopic observations of black hole accretion disk systems, focusing on the Schwarzschild and Kerr solutions to the Einstein field equations. To this we add treatments of the fundamental concepts associated with the theoretical and modeling aspects of accretion disks, as well as relevant topics from observational and theoretical X-ray spectroscopy.
Using all available archival data from the Rossi X-ray Timing Explorer (RXTE), we follow the frequency of the kilo-Hz QPOs in three low luminosity neutron star low mass X-ray binaries; namely 4U 1636-536, 4U 1608-522, and 4U1735-44. Following earlier work, we focus our analysis on the lower kilo-Hz QPO, for which we study the dependency of its quality factor (Q) amplitude as a function of frequency over a range covering from 500 Hz to 1000 Hz. As previously found for 4U 1636-536, we show that the quality factor of the lower kilo-Hz increases with frequency up to a maximum frequency around 800 Hz, beyond which an abrupt drop of its coherence is observed down to a limiting frequency where the QPO disappears completely. Simultaneously the amplitude of the QPOs is almost constant below the peak frequency and starts to decrease smoothly afterwards. The peak frequency is 850 Hz, 820 Hz, 740 Hz whereas the limiting frequency is 920 Hz, 900 Hz and 830 Hz for 4U 1636-536, 4U 1608-522 and 4U 1735-44 respectively. A ceiling of the lower QPO frequencies is also seen clearly in a frequency versus count rate diagram for all sources. This behavior is reproducible within an object and between objects. We suggest here that the drop of coherence of the lower QPO may be a geometry-related effect, which could be related to the last stable circular orbit.
Interacting Binaries consist of a variety of stellar objects in different stages of evolution and those containing accreting compact objects still represent a major challenge to our understanding of not only close binary evolution but also of the chemical evolution of the Galaxy. These end-points of binary star evolution are ideal laboratories for the study of accretion and outflow processes, and provide insight on matter under extreme physical conditions. One of the key-questions of fundamental relevance is the nature of SNIa progenitors. The study of accreting compact binary systems relies on observations over the entire electromagnetic spectrum and we outline here those unresolved questions for which access to the ultraviolet range is vital, as they cannot be addressed by observations in any other spectral region.
It has long been known that there are two classes of gamma-ray bursts (GRBs), mainly distinguished by their durations. The breakthrough in our understanding of long-duration GRBs (those lasting more than ~2 s), which ultimately linked them with energetic Type Ic supernovae, came from the discovery of their long-lived X-ray and optical afterglows, when precise and rapid localizations of the sources could finally be obtained. X-ray localizations have recently become available for short (duration <2 s) GRBs, which have evaded optical detection for more than 30 years. Here we report the first discovery of transient optical emission (R-band magnitude ~23) associated with a short burst; GRB 050709. The optical afterglow was localized with subarcsecond accuracy, and lies in the outskirts of a blue dwarf galaxy. The optical and X-ray afterglow properties 34 h after the GRB are reminiscent of the afterglows of long GRBs, which are attributable to synchrotron emission from ultrarelativistic ejecta. We did not, however, detect a supernova, as found in most nearby long GRB afterglows, which suggests a different origin for the short GRBs.
We have measured the weak lensing signal as a function of restframe luminosity for a sample of `isolated' galaxies. These results are based on four-band photometry from the Red-Sequence Cluster Survey, enabling us to determine photometric redshifts for a large number of galaxies. We select a secure sample of lenses with photometric redshifts 0.2<z<0.4 and study the relation between the virial mass and baryonic contents. In addition, we discuss the implications of the derived photometric redshift distribution for published cosmic shear studies. The virial masses are derived from a fit to the observed lensing signal. For a galaxy with a fiducial luminosity of 10^10 h^-2 L_Bsun we obtain a mass M_vir=9.9^{+1.5}_{-1.3}\times 10^11 h^-1 M_sun. The virial mass as a function of luminosity is consistent with a power-law ~L^1.5, with similar slopes for the three filters considered here. These findings are in excellent agreement with results from the Sloan Digital Sky Survey and semi-analytic models of galaxy formation. We measure the fraction of mass in stars and the baryon fraction in galaxies by comparing the virial mass-to-light ratio to predicted stellar mass-to-light ratios. We find that star formation is inefficient in converting baryons into stars, with late-type galaxies converting ~33% and early-type galaxies converting only ~14% of baryons into stars. Our results imply that the progenitors of early-type galaxies must have low stellar mass fractions, suggestive of a high formation redshift.
The first arcsecond localization of a short gamma-ray burst, GRB 050509B, has enabled detailed studies of a short burst environment. We here report on studies of the environment of GRB 050509B using the Swift X-ray Telescope (XRT). The XRT error circle of the burst overlaps with an elliptical galaxy in the cluster of galaxies ZwCl 1234.0+02916. Based on the measured X-ray flux of the cluster we estimate that the probability for a chance superposition of GRB 050509B and a cluster at least as X-ray bright as this cluster is $< 2\times 10^{-3}$, presenting the first strong case of a short burst located in a cluster of galaxies. We also consider the case for GRB 050509B being located behind ZwCl 1234.0+02916 and gravitationally lensed. From the velocity dispersion of the elliptical galaxy and the temperature of hot intracluster gas, we model the mass distribution in the elliptical galaxy and the cluster, and calculate the gravitational lensing magnification within the XRT error circle. We find that, if GRB050509B would be positioned significantly behind the cluster, it is most likely magnified by a factor less than two, but that the burst could be strongly lensed if it is positioned within 2 arcsec of the center of the bright elliptical galaxy. Further mapping of arcsecond size short burst error boxes is a new promising route to determine the spatial distribution of old stars throughout the Universe.
Based on new calculations we reconfirm the low and high density limits on the forbidden fine structure line ratio [O II] I(3729)/I(3726): lim_{N_ e} --> 0} = 1.5 and lim_{N_ e} --> \infty} = 0.35. Employing [O II] collision strengths calculated using the Breit-Pauli R-matrix method we rule out any significant deviation due to relativistic effects from these canonical values. The present results are in substantial agreement with older calculations by Pradhan (1976) and validate the extensive observational analysis of gaseous nebulae by Copetti and Writzel (2002) and Wang et al (2004) that reach the same conclusions. The present theoretical results and the recent observational analyses differ significantly from the calculations by MacLaughlin and Bell (1998) and Keenan et al (1999). The new maxwellian averaged effective collision strengths are presented for the 10 transitions among the first 5 levels to enable computations of [O II] line ratios.
We use a simplified version of the halo model with a power law power spectrum to study scale dependence in galaxy bias at the very large scales relevant to baryon oscillations. In addition to providing a useful pedagogical explanation of the scale dependence of galaxy bias, the model provides an analytic tool for studying how changes in the Halo Occupation Distribution (HOD) impact the scale dependence of galaxy bias on scales between 10 and 1000 Mpc/h, which is useful for interpreting the results of complex N-body simulations. We find that changing the mean number of galaxies per halo of a given mass will change the scale dependence of the bias, but that changing the way the galaxies are distributed within the halo has a smaller effect on the scale dependence of bias at large scales. We use the model to explain the decay in amplitude of the baryon oscillations as k increases, and generalize the model to make predictions about scale dependent galaxy bias when redshift space distortions are introduced.
We report the discovery of PM J13420-3415, a faint (V=17) white dwarf with a very high proper motion mu = 2.55 arcsec/yr. The star was found in the southern sky extension of the SUPERBLINK proper motion survey. A red spectrum shows the classical signature of a DA white dwarf, with a weak H-alpha line in absorption as the only prominent feature. The star is also found to have a large radial velocity V_rad = +212+/-15 km/s. At the adopted distance of 18 pc, the star has a very large space motion of 313 km/s relative to the Sun. An integration of the space motion shows that the star is on a nearly polar Galactic orbit, and is thus an unambiguous member of the Galactic halo. However, with an estimated effective temperature 5,000K < T_eff < 5,500K, the white dwarf appears to be much younger than expected for a denizen of the halo. The apparent paradox can be explained if the white dwarf is the relatively young (~2 Gyr) remnant of a longer-lived (10-14 Gyr) main sequence star, in which case the object is predicted to be a low-mass white dwarf with M ~ 0.45 M_sun.
We address the correlations of black hole (BH) mass with four different host-galaxy properties from eleven existing data sets. To guide theoretical understanding, we first try to quantify the tightness of the intrinsic correlations. Given the estimated measurement errors, we evaluate the probability distribution of the residual variance in excess of that expected from the measurement errors. Our central result is that the current data sets do not allow definite conclusions regarding the quality of the true correlations because the obtained probability distributions for the residual variance overlap for most quantities. We then consider which of the relations offer the best inferences of BH mass when there is no direct measurement available. As with the residual variances, we find that the probability distribution of expected uncertainty in inferred BH masses overlaps significantly for most of the relations. Photometric methods would then be preferred because the data are easier to obtain, as long as bulge-disk decomposition or detailed modeling of the photometric profile (as in \citet{graham:01}) do not present problems. Determining which correlation offers the best inferences requires reducing the uncertainty in the expected error in the inferred BH masses (the ``error on the error''). This uncertainty is currently limited by uncertainty in the residual variance for all of the relations.
Pulsating white dwarfs, especially DBVs, can be used as laboratories to study elusive particles such as plasmon neutrinos and axions. In the degenerate interiors of DBVs, plasmon decay is the dominant neutrino producing process. We can measure the neutrino luminosity using asteroseismology and constrain plasmon neutrino rates. In the same way, we can measure any additional loss of energy due to other weakly interacting particles, such as axions. Depending upon their (theoretically largely unconstrained) mass, axions could be a significant source of energy loss for DAVs as well. We are looking at what the uncertainties in the observables are, and what mass and temperature range minimizes them.
There is a fairly tight correlation between the pulsation periods and effective temperatures of ZZ Ceti stars (cooler stars have longer periods). This seems to fit the theoretical picture, where driving occurs in the partial ionization zone, which lies deeper and deeper within the star as it cools. It is reasonable to assume that the pulsation periods should be related to the thermal timescale in the region where driving occurs. As that region sinks further down below the surface, that thermal timescale increases. Assuming this connection, the pulsation periods could provide an additional way to determine effective temperatures, independent of spectroscopy. We explore this idea and find that in practice, things are not so simple.
We demonstrate that there are systematic scale errors in the [Fe/H] values
determined by the Hamburg/ESO Survey (and by inference by the HK Survey in the
past) for certain extremely metal poor highly C-enhanced giants. The
consequences of these scale errors are that a) the fraction of carbon stars at
extremely low metallicities has been overestimated in several papers in the
recent literature b) the number of extremely metal poor stars known is somewhat
lower than has been quoted in the recent literature c) the yield for extremely
metal poor stars by the HES Survey is somewhat lower than is stated in the
recent literature. A preliminary estimate for the frequency of Carbon stars
among the giants in the HES sample with -4 < [Fe/H] < -2.0 dex is 7.4 +-2.9%;
adding an estimate for the C-enhanced giants with [C/Fe] > 1.0 dex without
detectable C2 bands raises the fraction to 14 +-4$%.
We rely on the results of an extensive set of homogeneous detailed abundance
analyses of stars expected to have [Fe/H] < -3.0 dex selected from the HES to
establish these claims. We have found that the Fe-metallicity of the cooler
(Teff < 5200K) C-stars as derived from spectra taken with HIRES at Keck are a
factor of ~10 higher than those obtained via the algorithm used by the HES
project to analyze the moderate resolution follow-up spectra, which is
identical to that used until very recently by the HK Survey. This error in
Fe-abundance estimate for C-stars arises from a lowering of the emitted flux in
the continuum bandpasses of the KP (3933 A line of CaII) and particularly the
HP2 (Hdelta) indices used to estimate [Fe/H] due to absorption from strong
molecular bands.
We aim to settle the debate regarding the fraction of the Local Group's peculiar velocity that is induced by structures beyond the Great Attractor by calculating the dipole anisotropy of the largest, all-sky, truly X-ray selected cluster sample compiled to date. The sample is the combination of the REFLEX catalog in the southern hemisphere, the eBCS sample in the north, and the CIZA survey in the Galactic plane. The composite REFLEX+eBCS+CIZA sample overcomes many of the problems inherent to previous galaxy and cluster catalogs which limited their effectiveness in determining the origin of the Local Group's motion. From the dipole anisotropy present in the cluster distribution we determine that 44% of the Local Group's peculiar velocity is due to infall into the GA region, while 56% is in the form of a large-scale flow induced by more distant overdensities between 130 and 180 h^-1 Mpc away. In agreement with previous analyses, we find that the Shapley supercluster is the single overdensity most responsible for the increase in the dipole amplitude beyond 130 h^-1 Mpc, generating 30.4% of the large-scale contribution. We find that numerous groupings and loose associations of clusters at roughly the same distance as the Shapley region induce a significant acceleration on the Local Group. These include the well known Horologium-Reticulum concentration, as well as newly noted associations centered on Abell 3667 and Abell 3391 and a string of CIZA clusters near C1410 which may trace an extension of the Shapley complex into the Zone of Avoidance. We also note that the large-scale anisotropy measured in the cluster distribution near 150 h^-1 Mpc may be due to an observed underdensity of clusters in the northern hemisphere near this distance. (abridged)
Why does the Zel'dovich approximation (ZA) work well for a gravitational collapse in the universe? This problem is examined by focussing the dependence on the dimensionality of the collapse. The ZA is known to be exact for a one-dimensional collapse. We show that the ZA is more accurate in the order of three-, two-, and one-dimensional collapses. Furthermore, using models for spheroidal collapse, we show that the ZA remains accurate in any collapse, which becomes progressively lower-dimensional with the passage of time. That is, the ZA is accurate because the essence of the gravitational collapse is incorporated in the ZA.
We investigate the properties of objects in the Subaru Deep Field (SDF), using public catalogs constructed from images in several optical passbands. Using a small subset of objects most likely to be stars, we construct a stellar locus in three-dimensional color space. We then compare the position of all objects relative to this locus to create larger samples of stars in the SDF with rough spectral types. The number counts of stars defined in this way are consistent with those of current models of the Galaxy.
Two nearby stars, HD 128311 and HD 82943, are believed to host pairs of Jupiter-like planets involved in a strong first order 2:1 mean motion resonance. In this work we reanalyze available radial velocity (RV) measurements and demonstrate that it is also possible to explain the observed RV variations of the parents stars as being induced by a pair of Trojan planets. We show that these Trojan configurations reside in extended zones of stability in which such systems may easily survive in spite of large masses of the planets, large eccentricities and nonzero mutual inclinations of their orbits.
The final chapter in the long-standing mystery of the gamma-ray bursts (GRBs) centres on the origin of the short-hard class, suspected on theoretical grounds to result from the coalescence of neutron star or black hole binary systems. Numerous searches for the afterglows of short-hard bursts have been made, galvanized by the revolution in our understanding of long-duration GRBs that followed the discovery in 1997 of their broadband (X-ray, optical, and radio) afterglow emission. Here we present the discovery of the X-ray afterglow of a short-hard burst whose accurate position allows us to unambiguously associate it with a star-forming galaxy at redshift z=0.160, and whose optical lightcurve definitively excludes a supernova association. Together with results from three other recent short-hard bursts, this suggests that short-hard bursts release much less energy than the long-duration GRBs. Models requiring young stellar populations, such as magnetars and collapsars, are ruled out, while coalescing degenerate binaries remain the most promising progenitor candidates.
We report on the changes of the Sun's subsurface stratification inferred from helioseismology data. Using SOHO/MDI (SOlar and Heliospheric Observatory/Michelson Doppler Imager) data for the last 9 years and, more precisely, the temporal variation of f-mode frequencies, we have computed the variation of the radius of subsurface layers of the Sun by applying helioseismic inversions. We have found a variability of the ``helioseismic'' radius in antiphase with the solar activity, with the strongest variations of the stratification being just below the surface around 0.995$R_{\odot}$. Besides, the radius of the deeper layers of the Sun, between 0.975$R_{\odot}$ and 0.99$R_{\odot}$ changes in phase with the 11-year cycle.
An analysis of the high-energy emission from IGR J16393-4643 (=AX J1639.0-4642) is presented using data from INTEGRAL and XMM-Newton. The source is persistent in the 20-40 keV band at an average flux of 5.1x10^-11 ergs/cm2/s, with variations in intensity by at least an order of magnitude. A pulse period of 912.0+/-0.1 s was discovered in the ISGRI and EPIC light curves. The source spectrum is a strongly-absorbed (nH=(2.5+/-0.2)x10^23 atoms/cm2) power law that features a high-energy cutoff above 10 keV. Two iron emission lines at 6.4 and 7.1 keV, an iron absorption edge >7.1 keV, and a soft excess emission of 7x10^-15 ergs/cm2/s between 0.5-2 keV, are detected in the EPIC spectrum. The shape of the spectrum does not change with the pulse. Its persistence, pulsation, and spectrum place IGR J16393-4643 among the class of heavily-absorbed HMXBs. The improved position from EPIC is R.A. (J2000)=16:39:05.4 and Dec.=-46:42:12 (4" uncertainty) which is compatible with that of 2MASS J16390535-4642137.
(Abridged.) We present one-zone chemical evolution models for two dwarf starburst galaxies, NGC 1705 and NGC 1569. Using information about the past star formation history and initial mass function of the systems previously obtained from Hubble Space Telescope colour-magnitude diagrams, we identify possible scenarios of chemical enrichment and development of galactic winds. In order not to overestimate the current metallicity of the interstellar gas inferred from H II region spectroscopy, we suggest that the winds efficiently remove from the galaxies the metal-rich ejecta of dying stars. Conversely, requiring the final mass of neutral gas to match the value inferred from 21-cm observations implies a relatively low efficiency of interstellar medium entrainment in the outflow, thus confirming previous findings that the winds driving the evolution of typical starbursts are differential. These conclusions could be different only if the galaxies accrete huge fractions of unprocessed gas at late times. By assuming standard stellar yields we obtain a good fit to the observed nitrogen to oxygen ratio of NGC 1569, while the mean N/O ratio in NGC 1705 is overestimated by the models. Reducing the extent of hot bottom burning in low-metallicity intermediate-mass stars does not suffice to solve the problem. Localized self-pollution from stars more massive than 60 MSun in NGC 1705 and/or funneling of larger fractions of nitrogen through its winds are then left to explain the discrepancy between model predictions and observations. Inspection of the log(N/O) vs. log(O/H)+12 diagram for a sample of dwarf irregular and blue compact dwarf galaxies in the literature favours the latter hypothesis.
A thermal radiative component is likely to accompany the first stages of the prompt emission of Gamma-ray bursts (GRB's) and X-ray flashes (XRF's). We analyze the effect of such a component on the observable spectrum, assuming that the observable effects are due to a dissipation process occurring below or near the thermal photosphere. We consider both the internal shock model and a 'slow heating' model as possible dissipation mechanisms. For comparable energy densities in the thermal and the leptonic component, the dominant emission mechanism is Compton scattering. This leads to a nearly flat energy spectrum (\nu F_\nu \propto \nu^0) above the thermal peak at ~10-100 keV and below 10-100 MeV, for a wide range of optical depths 0.03 <~ \tau_{\gamma e} <~ 100, regardless of the details of the dissipation mechanism or the strength of the magnetic field. At lower energies steep slopes are expected, while above 100 MeV the spectrum depends on the details of the dissipation process. For higher values of the optical depth, a Wien peak is formed at 100 keV - 1 MeV, and no higher energy component exists. For any value of \tau_{\gamma e}, the number of pairs produced does not exceed the baryon related electrons by a factor larger than a few. We conclude that dissipation near the thermal photosphere can naturally explain both the steep slopes observed at low energies and a flat spectrum above 10 keV, thus providing an alternative scenario to the optically thin synchrotron - SSC model.
Special features of magnetohydrodynamic waves linear dynamics in smooth shear flows are studied. Quantitative asymptotic and numerical analysis are performed for wide range of system parameters when basic flow has constant shear of velocity and uniform magnetic field is parallel to the basic flow. The special features consist of magnetohydrodynamic wave mutual transformation and over-reflection phenomena. The transformation takes place for arbitrary shear rates and involves all magnetohydrodynamic wave modes. While the over-reflection occurs only for slow magnetosonic and Alfv\'en waves at high shear rates. Studied phenomena should be decisive in the elaboration of the self-sustaining model of magnetohydrodynamic turbulence in the shear flows.
Properties of circularly polarized waves are studied in the pulsar magnetosphere plasma. It is shown that some observational characteristics of the circular polarization observed in the pulsar radio emission can be qualitatively explained in the framework of the model based on anomalous Doppler resonance. Performed analysis provides that if the difference between Lorentz factors of electrons and positrons is relatively high, one of the circularly polarized waves becomes super-luminal and therefore can not be generated by cyclotron instability. We suggest that this case corresponds to the pulsars with the domination of one hand of circular polarization through the whole averaged pulse profile at all observed frequencies. For intermediate values of the difference between Lorentz factors both circularly polarized waves are generated, but the waves of one handness are much more effectively generated for high frequencies, whereas generation of another handness dominates for low frequencies. This should correspond to the pulsars with strong frequency dependence of the degree of circular polarization. The case of relatively small difference between Lorentz factors corresponds to the pulsars with sign reversal of the circular polarization in the centre of averaged pulse profiles.
I briefly highlight the salient properties of modified-inertia formulations of MOND, contrasting them with those of modified-gravity formulations, which describe practically all theories propounded to date. Future data (e.g. the establishment of the Pioneer anomaly as a new physics phenomenon) may prefer one of these broad classes of theories over the other. I also outline some possible starting ideas for modified inertia.
Observations of fluctuations in the redshifted 21 cm radiation from neautral
hydrogen (HI) are perceived to be an important future probe of the universe at
high redshifts. Under the assimption that at redshifts z less than 6
(Post-Reionization Era), the HI traces the underlying dark matter with a
possible bias, we investigate the possibility of using observations of
redshifted 21 cm radiation to detect the bispectrum arising from non-linear
gravitational clustering and from non-linear bias. We find that the expected
signal is ~ 0.1 mJy at 325 MHz (z=3.4) for the small baselines at the GMRT, the
strength being a few times larger at higher frequencies 610 MHz (z=1.3).
Further, the magnitude of the signal from the bispectrum is predicted to be
comparable to that from the power spectrum, allowing a detection of both in
roughly the same integration time. The HI signal is found to be uncorrelated
beyond frequency separations of 1.3 MHz whereas the continuum sources of
continuum are expected to be correlated across much larger frequencies.
This signature can in principle be used to distinguish the HI signal from the
contamination. We also consider the possibility of using observations of the
bispectrum to determine the linear and quadratic bias parameters of the HI at
high redshifts, this having possible implications for theories of galaxy
formation.
Among the 160 known exoplanets, mainly detected in large radial-velocity surveys, only 8 have a characterization of their actual mass and radius thanks to the two complementary methods of detection: radial velocities and photometric transit. We started in March 2004 an exoplanet-search programme biased toward high-metallicity stars which are more frequently host extra-solar planets. This survey aims to detect close-in giant planets, which are most likely to transit their host star. For this programme, high-precision radial velocities are measured with the ELODIE fiber-fed spectrograph on the 1.93-m telescope, and high-precision photometry is obtained with the CCD Camera on the 1.20-m telescope, both at the Haute-Provence Observatory. We report here the discovery of a new transiting hot Jupiter orbiting the star HD189733. The planetary nature of this object is confirmed by the observation of both the spectroscopic and photometric transits. The exoplanet HD189733b, with an orbital period of 2.219 days, has one of the shortest orbital periods detected by radial velocities, and presents the largest photometric depth in the light curve (~ 3%) observed to date. We estimate for the planet a mass of 1.15 +- 0.04 Mjup and a radius of 1.26 +- 0.03 RJup. Considering that HD189733 has the same visual magnitude as the well known exoplanet host star HD209458, further ground-based and space-based follow-up observations are very promising and will permit a characterization of the atmosphere and exosphere of this giant exoplanet.
With future wide and deep cosmological sky surveys, a large number of
gravitationally lensed, multiply imaged systems will be found. In addition to
multiply imaged galaxies and quasars, sources will include transient events
like supernovae and gamma ray bursts in which case very accurate time delay
measurements are possible. Also, large numbers of systems with several lensed
sources behind a single lens will be observed.
In this paper, we study different possibilities of using future strong
lensing data to probe lens matter distributions and to determine cosmological
parameters. We find that - in addition to giving very narrow constraints on the
slope of lens density profiles - strong lensing can constrain the Hubble
parameter and the matter density of the universe independently of other
cosmological probes.
We present the results of a study which uses the 3CRR sample of radio-loud active galactic nuclei to investigate the evolution of the black-hole:spheroid mass ratio in the most massive early-type galaxies from 0<z<2. Radio-loud unification is exploited to obtain virial (line-width) black-hole mass estimates from the 3CRR quasars, and stellar mass estimates from the 3CRR radio galaxies, thereby providing black-hole and stellar mass estimates for a single population of early-type galaxies. At low redshift (z<1) the 3CRR sample is consistent with a black-hole:spheroid mass ratio of M_bh/M_sph ~0.002, in good agreement with that observed locally for quiescent galaxies of similar stellar mass (M_sph ~5x10^11 M_sun). However, over the redshift interval 0<z<2 the 3CRR black-hole:spheroid mass ratio is found to evolve as M_bh/M_sph \propto (1+z)^{2.07\pm0.76}, reaching M_bh/M_sph ~0.008 by redshift z~2. This evolution is found to be inconsistent with the local black hole:spheroid mass ratio remaining constant at a moderately significant level (98%). If confirmed, the detection of evolution in the 3CRR black-hole:spheroid mass ratio further strengthens the evidence that, at least for massive early-type galaxies, the growth of the central supermassive black hole may be completed before that of the host spheroid.
Motivated by the recent report of a high Li6 ``plateau'' extending to low metallicities in Galactic halo stars, we study the energetics of an early production of Li6 through the interaction of energetic particles (EP) with the interstellar medium. We explore then the energetic potential of various candidate sources of pre-galactic EP and we show that, in general, they fail to satisfy the observational and theoretical requirements. Succesfull candidates are supernova explosions of abnormally low metal yield and the supermassive black hole in the Galactic center (provided that it was formed early on and that it was then radiating much more efficiently than today). Assuming Li6 is indeed pre-galactic, we study in a self-consistent way the galactic evolution of the light isotopes Li7, Li6 and Be. We find that the existence of a Li6 plateau is hard to justify, unless a fine-tuned and metallicity-dependent depletion mechanism of Li6 in stellar envelopes is invoked. The depletion of Li6 should be different, both in magnitude and in its metallicity dependence, than the depletion required to explain current observations of Li (mostly Li7) in halo stars. If the recently reported Li6 ``plateau'' is confirmed, our analysis suggests important implications for our understanding of the production, evolution and stellar depletion of the Li isotopes.
In the past years a wealth of observations allowed to unravel the structural
properties of the Dark Matter Halos around spirals. First, their rotation
curves follow an Universal profile (URC) that can be described in terms of an
exponential thin stellar disk and a dark halo with a constant density core,
whose relative importance increases with galaxy luminosity. Careful studies of
individual objects reveal that dark halos have a core, whose size $r_0$
correlates with the central density $\rho_0$.
These properties are in serious discrepancy with the cuspy density
distribution predicted by N-body simulations in collisionless $\Lambda$CDM
Cosmology.
We report on a novel search for radio gravitational lenses. Using the Very Large Array, we imaged ten candidates with both dual redshifts in Sloan Digital Sky Survey spectra and 1.4 GHz radio flux >2 mJy in the FIRST survey. The VLA maps show that in each case the radio emission is associated with the foreground galaxy rather than being lensed emission from the background galaxy, although at least four of our targets are strong lenses at optical wavelengths. These SDSS dual-redshift systems do not have lensed radio emission at the sensitivity of current radio surveys.
We report the detection of the HI line at 21 cm from the circumstellar shell around the AGB star X Her using the position-switching technique with the Nancay Radio Telescope. At the star position the line shows 2 components: (i) a broad one (FWHM ~ 13 km/s) centered at -72.2 km/s, and (ii) a narrow one (FWHM \~ 4 km/s) centered at ~ -70.6 km/s. Our map shows that the source associated to the broad component is asymmetric with material flowing preferentially towards the North-East. This source extends to ~ 10 arcmin. (~ 0.4 pc) from the star in that direction. On the other hand, the narrow component is detected only at the star position and indicates material flowing away from the observer. The total mass of atomic hydrogen is ~ 6.5 10^{-3} solar mass which, within a factor 2, agrees with the estimate obtained from IRAS data at 60 microns.
Numerical computation of the time evolution of the mass transfer rate in a close binary can be and, in particular, has been a computational challenge. Using a simple physical model to calculate the mass transfer rate, we show that for a simple explicit iteration scheme the mass transfer rate is numerically unstable unless the time steps are sufficiently small. In general, more sophisticated explicit algorithms do not provide any significant improvement since this instability is a direct result of time discretization. For a typical binary evolution, computation of the mass transfer rate as a smooth function of time limits the maximum tolerable time step and thereby sets the minimum total computational effort required for an evolutionary computation. By methods of ``Controlling Chaos'' it can be shown that a specific implicit iteration scheme, based on Newton's method, is the most promising solution for the problem.
A comprehensive analysis of 355 high-quality WSRT HI 21-cm line maps of nearby galaxies shows that the properties and incident rate of Damped Lyman-alpha (DLA) absorption systems observed in the spectra of high redshift QSOs are in good agreement with DLAs originating in gas disks of galaxies like those in the z~0 population. Comparison of low-z DLA statistics with the HI incidence rate and column density distribution f(N) for the local galaxy sample shows no evidence for evolution in the integral "cross section density" below z~1.5, implying that there is no need for a hidden population of galaxies or HI clouds to contribute significantly to the DLA cross section. Compared with z~4, our data indicates evolution of a factor of two in the comoving density along a line of sight. We find that dN/dz(z=0)=0.045 +/- 0.006. The idea that the local galaxy population can explain the DLAs is further strengthened by comparing the properties of DLAs and DLA galaxies with the expectations based on our analysis of local galaxies. The distribution of luminosities of DLA host galaxies, and of impact parameters between QSOs and the centres of DLA galaxies, are in good agreement with what is expected from local galaxies. Approximately 87% of low z DLA galaxies are expected to be fainter than L* and 37 per cent have impact parameters less than 1'' at z=0.5. The analysis shows that some host galaxies with very low impact parameters and low luminosities are expected to be missed in optical follow up surveys. The well-known metallicity-luminosity relation in galaxies, in combination with metallicity gradients in galaxy disks, cause the expected median metallicity of low redshift DLAs to be low (~1/7 solar), which is also in good agreement with observations of low z DLAs. (Abridged)
The present work explores the theoretical effects of rotation in calculating the period ratios of double-mode radial pulsating stars with special emphasis on high-amplitude delta Scuti stars (HADS). Diagrams showing these period ratios vs. periods of the fundamental radial mode have been employed as a good tracer of non-solar metallicities and are known as Petersen diagrams (PD).In this paper we consider the effect of moderate rotation on both evolutionary models and oscillation frequencies and we show that such effects cannot be completely neglected as it has been done until now. In particular it is found that even for low-to-moderate rotational velocities (15-50 km/s), differences in period ratios of some hundredths can be found. The main consequence is therefore the confusion scenario generated when trying to fit the metallicity of a given star using this diagram without a previous knowledge of its rotational velocity.
We use deep wide-field V-band imaging obtained with the Wide Field Camera at the prime focus of the Issac Newton Telescope to study the spatial and luminosity distribution of galaxies in three low redshift (0.04<z<0.2) clusters: Abell 119, Abell 2443 and Abell 2218. The absolute magnitude limits probed in these clusters are M_{V} - 5logh_{0.7} = -13.3, -15.4 and -16.7mag respectively. The galaxy population, at all luminosities, along the line-of-sight to the clusters can be described by the linear combination of a King profile and a constant surface density of field galaxies. We find that, for these three clusters, the core radius is invariant with intrinsic luminosity of the cluster population to the above limits and thus there is no evidence for luminosity segregation in these clusters. The exception is the brightest galaxies in A2218 which exhibit a more compact spatial distribution. We find the total projected luminosity distribution (within 1Mpc of the cluster centre) can be well represented by a single Schechter function with moderately flat faint-end slopes: alpha=-1.22 (A119), alpha=-1.11 (A2443) and alpha=-1.14 (A2218). We perform a geometric deprojection of the cluster galaxy population and confirm that no `statistically significant' evidence of a change in the shape of the luminosity distribution with cluster-centric radius exists. Again, the exception being A2218 which exhibits a core region with a flatter faint-end slope.
We have analyzed a homogeneous set of observations of eighty-one transneptunian objects obtained with the NIC2 camera on the Hubble Space Telescope with the goal of identifying partially resolved binaries. Using PSF-fitting we have identified six likely binaries in addition to the three new binaries already found in this data set. We find that 11% of transneptunian objects are binaries at separation and brightness limits of the NIC2 camera. The identification of these new binaries significantly increases the known lower limit to the binary fraction among transneptunian objects. The origin of such a high fraction of binaries remains to be determined. Most interestingly, detectable binaries appear to be about four times more common among the cold classical disk than in the dynamically excited populations.
We present the first results of our ongoing chemical study of carbon stars in the Local Group of galaxies. We used spectra obtained with UVES at the 8.2 m Kueyen-VLT telescope and a new grid of spherical model atmospheres for cool carbon-rich stars which include polyatomic opacities, to perform a full chemical analysis of one carbon star, BMB-B~30, in the Small Magellanic Cloud (SMC) and two, IGI95-C1 and IGI95-C3, in the Sagittarius Dwarf Spheroidal (Sgr dSph) galaxy. Our main goal is to test the dependence on the stellar metallicity of the s-process nucleosynthesis and mixing mechanism occurring in AGB stars. For these three stars, we find important s-element enhancements with respect to the mean metallicity ([M/H]), namely [s/M]$\approx$+1.0, similar to the figure found in galactic AGB stars of similar metallicity. The abundance ratios derived between elements belonging to the first and second s-process abundance peaks, corresponding to nuclei with a magic number of neutrons N=50 (88Sr, 89Y, 90Zr) and N=82 (138Ba, 139La, 140Ce, 141Pr), agree remarkably well with the theoretical predictions of low mass (M $<3$~M$\_\odot$) metal-poor AGB nucleosynthesis models where the main source of neutrons is the $^{13}$C$(\alpha,n)^{16}$O reaction activated duringthe long interpulse phase, in a small pocket located within the He-rich intershell. The derived C/O and $^{12}$C/$^{13}$C ratios are, however, more difficult to reconcile with theoretical expectations. Possible explanations, like the extrinsic origin of the composition of these carbon stars or the operation of a non-standard mixing process during the AGB phase (such as the {\it cool bottom process}), are discussed on the basis of the collected observational constraints.
The multiplicity of early-type stars is still not well established. The derived binary fraction is different for individual star forming regions, suggesting a connection with the age and the environment conditions. The few studies that have investigated this connection do not provide conclusive results. To fill in this gap, we started the first detailed adaptive-optic-assisted imaging survey of early-type field stars to derive their multiplicity in a homogeneous way. The sample has been extracted from the Hipparcos Catalog and consists of 341 BA-type stars within ~300 pc from the Sun. We report the current status of the survey and describe a Monte-Carlo simulation that estimates the completeness of our companion detection.
We present the results of a survey of the Coma Berenices open star cluster (Melotte 111), undertaken using proper motions from the USNO-B1.0 and photometry from the 2MASS Point Source catalogues. We have identified 60 new candidate members with masses in the range 1.007<M<$0.269M_solar. For each we have estimated a membership probability by extracting control clusters from the proper motion vector diagram. All 60 are found to have greater than 60 per cent probability of being clusters more than doubling the number of known cluster members. The new luminosity function for the cluster peaks at bright magnitudes, but is rising at K~12, indicating that it is likely lower mass members may exist. The mass function also supports this hypothesis.
I will review the latest developments in understanding the high-energy emission of rotation-powered pulsars and magnetically-powered Anomalous X-ray Pulsars (AXPs) and Soft Gamma-Ray Repeaters (SGRs). These fields have been extremely active in the last few years, both observationally and theoretically, driven partly by new X-ray data from Chandra, XMM-Newton and RXTE. At the same time, the Parkes Multibeam Survey has discovered over 700 new radio pulsars, some of them young and coincident with EGRET sources, and others having magnetar-strength magnetic fields. These new observations are raising important questions about neutron star birth and evolution, as well as the properties of their high-energy emission.
A number of rotation-powered millisecond pulsars are powerful sources of X-ray emission. We present predictions for the spectral characteristics of these sources at gamma-ray energies, using a model for acceleration and pair cascades on open magnetic field lines above the polar caps. Since these pulsars have low surface magnetic fields, the majority do not produce sufficient pairs to completely screen the accelerating electric field allowing particle acceleration to high altitude. The resulting emission above 1 GeV comes from curvature radiation by primary electrons with radiation-reaction-limited Lorentz factors. The spectra are very hard power-laws with exponential cutoffs between 1 and 50 GeV, and the spectral power peaks near the cutoff energy. Millisecond pulsars are thus ideal targets for air-Cherenkov detectors that are able to reach energy thresholds below 50 GeV.
We present our work on the halo evolution of sulfur, based on observations of the S I lines around 9220 A for ten stars for which the S abundance was obtained previously from much weaker S I lines at 8694 A. We cannot confirm the rise and the high [S/Fe] abundances for low [Fe/H], as claimed in the literature from analysis of the 8694 A lines. The reasons for claims of an increase in [S/Fe] with decreasing [Fe/H] are probably twofold: uncertainties in the measurements of the weak 8694 A lines, and systematic errors in metallicity determinations from Fe I lines. The near-infrared sulfur triplet at 9212.9, 9228.1, and 9237.5 A are preferred for an abundance analysis of sulfur for metal-poor stars. Our work was presented in full by Ryde & Lambert (2004).
The divergence of the momentum density field of the large scale structure generates a secondary anisotropy contribution to the Cosmic Microwave Background (CMB). While the effect is best described as a non-linear extension to the well-known integrated Sachs-Wolfe effect, due to mathematical coincidences, the anisotropy contribution is also described as the lensing of the dipole seen in the rest-frame of a moving mass. Given the closeness, there is a remote possibility that local concentrations of mass in the form of the Great Attractor and the Shapley concentration generate large angular scale fluctuations in CMB and could potentially be responsible, at least partly, for some of the low-multipole anomalies in WMAP data. While the local anisotropy contribution peaks at low multipoles, for reasonable models of the mass and velocity distributions associated with local super structures we find that the amplitude of temperature anisotropies is at most at a level of 10$^{-2}$ $\mu$K and is substantially smaller than primordial fluctuations. It is extremely unlikely that the momentum density of local mass concentrations is responsible for any of the large angular scale anomalies in WMAP data.
BAT99-129 is a rare, short-period eclipsing Wolf-Rayet binary in the Large Magellanic Cloud. We present here medium-resolution NTT/EMMI spectra that allow us to disentangle the spectra of the two components and find the orbital parameters of the binary. We also present VLT/FORS1 spectra of this binary taken during the secondary eclipse, i.e. when the companion star passes in front of the Wolf-Rayet star. With these data we are able to extract, for the first time in absolute units for a WR+O binary, the sizes of the line emitting regions.
Expected mean angular diameters and amplitudes of angular diameter variations are estimated for all monoperiodic Classical Cepheids brighter than <V> = 8.0mag. The catalog is intended to help selecting best Cepheid targets for interferometric observations.
Recent search for multiperiodicity in LMC Cepheids (Moskalik, Kolaczkowski & Mizerski 2004) has led to discovery of periodic modulation of amplitudes and phases in many of the first/second overtone (FO/SO) double mode pulsators. We discuss observational characteristics and possible mechanisms responsible for this behaviour.
We report optical and near-infrared broad band observations of the short-duration GRB 050724 host galaxy, used to construct its spectral energy distribution (SED). Unlike the hosts of long-duration gamma-ray bursts (GRBs), which show younger stellar population, the SED of the GRB 050724 host galaxy is optimally fitted with a synthetic elliptical galaxy template based on an evolved stellar population (age ~2.6 Gyr). The SED of the host is difficult to reproduce with non-evolving metallicity templates. In contrast, if the host galaxy metallicity enrichment is considered, the synthetic templates fit satisfactorily the observed SED. The internal host extinction is low (Av ~< 0.4 mag) so it cannot explain the faintness of the afterglow. The host galaxy is more massive (5x10^10 Msun) and luminous (~1.1 L*) than most of the long-duration GRB hosts. A statistical comparison based on ages of short- and long-duration GRB host galaxies strongly suggests that short-duration GRB hosts contain, on average, older progenitors. These findings support a different origin for short- and long-duration GRBs.
By means of a detailed chemical evolution model, we follow the evolution of barium and europium in four Local Group Dwarf Spheroidal Galaxies, in order to set constraints on the nucleosynthesis of these elements and on the evolution of this type of galaxies compared with the Milky Way. The model, which is able to reproduce several observed abundance ratios and the present day total mass and gas mass content of these galaxies, adopts up to date nucleosynthesis and takes into account the role played by supernovae of different types (II, Ia) allowing us to follow in detail the evolution of several chemical elements (H, D, He, C, N, O, Mg, Si, S, Ca, Fe, Ba and Eu). By assuming that barium is a neutron capture element produced in low mass AGB stars by s-process but also in massive stars (in the mass range 10 - 30 $M_{\odot}$) by r-process, during the explosive event of supernovae of type II, and that europium is a pure r-process element synthesized in massive stars also in the range of masses 10 - 30 $M_{\odot}$, we are able to reproduce the observed [Ba/Fe] and [Eu/Fe] as functions of [Fe/H] in all four galaxies studied. We confirm also the important role played by the very low star formation efficiencies ($\nu$ = 0.005 - 0.5 Gyr$^{-1}$) and by the intense galactic winds (6-13 times the star formation rate) in the evolution of these galaxies. These low star formation efficiencies (compared to the one for the Milky Way disc) adopted for the Dwarf Spheroidal Galaxies are the main reason for the differences between the trends of [Ba/Fe] and [Eu/Fe] predicted and observed in these galaxies and in the metal-poor stars of our Galaxy. Finally, we provide predictions for Sagittarius galaxy for which data of only two stars are available.
SHARC II, 350-micron continuum and archival HST J-H band maps are presented of NGC 3656, the brightest of our sample of six elliptical galaxies for which resolved CO gas disks have recently been detected with 7''-spatial-resolution, interferometry mapping. These gas disks confirm the conclusions of earlier results showing optical dust lanes and unresolved CO that implied the common existence of molecular gas in ellipticals and the disk-like structure of this gas. The presented SHARC II mapping results provide the best to date resolved FIR-submm extent of NGC 3656 and of any elliptical galaxy > 40 Mpc, showing that dust of 29 K exists out to at least 1.8 kpc in this galaxy. These new data are used in conjunction with the archival HST maps and other published data to determine dust properties and associations with galactic structures, including dominant heating sources such as nuclear-activity, star-formation or diffuse-stellar radiation.
We present a study of compact star clusters in the nearby pair of interacting galaxies NGC 5194/95 (M51), based on multifilter Hubble Space Telescope WFPC2 archival images. We have detected ~400 isolated. Our requirement that clusters be detected based only on their morphology results in the selection of relatively isolated objects, and we estimate that we are missing the majority by a factor 4-6) of <10 Myr clusters due to crowding. Hence we focus on the cluster population older than 10 Myr. An age distribution shows a broad peak between 100-500 Myr, which is consistent with the crossing times of NGC 5195 through the NGC 5194 disk estimated in both single and multiple-passage dynamical models. We estimate that the peak contains approximately 2.2-2.5 times more clusters than expected from a constant rate of cluster formation over this time interval. We estimate the effective radii of our sample clusters and find a median value of 3-4 pc. Additionally, we see correlations of increasing) cluster size with cluster mass (with a best fit slope of 0.14\pm0.03) at the 4sigma level, and with cluster age (0.06\pm0.02) at the 3sigma level. Finally, we report for the first time the discovery of faint, extended star clusters in the companion, NGC 5195, an SB0 galaxy. These have red [(V-I)>1.0] colors, effective radii >7 pc, and are scattered over the disk of NGC 5195. Our results indicate that NGC 5195 is therfore currently the third known barred lenticular galaxy to have formed so-called "faint fuzzy" star clusters. abridged)
We present the analysis of the deepest near-UV image obtained with HST using the WFPC2(F300W) as part of the parallel observations of the Ultra Deep Field campaign. The U-band 10sigma limiting magnitude measured over 0.2 arcsec square is m(AB)=27.5 which is 0.5 magnitudes deeper than that in the HDF-North. We matched the U-band catalog with those in the ACS images taken during the GOODS observations of the CDF-South and obtained photometric-z for 306 matched objects. We find that the UV-selected galaxies span all the major morphological types at 0.2<z_phot<1.2. However, disks are more common at lower redshifts, 0.2<z_phot<0.8. Higher-z objects (0.7<z_phot<1.2) are on average bluer than lower-z and have spectral type typical of starbursts. Their morphologies are compact, peculiar or low surface brightness galaxies. The average half-light radius (rest-frame 1200--1800 A) of the UV-selected galaxies at 0.66<z_ phot<1.5 is 0.26 +- 0.01 arcsec (2.07 +- 0.08 kpc). The UV-selected galaxies are on average fainter (M_B=-18.43+-0.13) than Lyman Break Galaxies (M_B=-23+-1). Our sample includes early-type galaxies that are presumably massive and forming stars only in their cores, as well as starburst-type systems that are more similar to the LBGs, although much less luminous. This implies that even the starbursts in our sample are either much less massive than LBGs or are forming stars at a much lower rate or both. The low surface brightness galaxies have no overlap with the LBGs and form an interesting new class of their own.
N-body simulations find a universal structure for the halos which result from the nonlinear growth of Gaussian-random-noise density fluctuations in the CDM universe. This talk summarized our attempts to derive and explain this universal structure by analytical approximation and simplified models. As an example, we show here that a 1D spherical infall model involving a fluid approximation derived from the Boltzmann equation can explain not only the halo density profile but its phase-space density profile, as well.
The masses, radii and oscillation periods of RR Lyrae variable stars and the counterpart parameters of helium atoms undergoing single-level transitions between Rydberg states are shown to obey discrete self-similar scaling relations. The agreement between observed period distributions for the two classes of systems appears to be unique.
We report progress in the calibration of a method to determine cool dwarf star metallicities using molecular band strength indices. The molecular band index to metallicity relation can be calibrated using chemical abundances calculated from atomic line equivalent width measurements in high resolution spectra. Building on previous work, we have measured Fe and Ti abundances in 32 additional M and K dwarf stars to extend the range of temperature and metallicity covered. A test of our analysis method using warm star - cool star binaries shows we can calculate reliable abundances for stars warmer than 3500 K. We have used abundance measurements for warmer binary or cluster companions to estimate abundances in 6 additional cool dwarfs. Adding stars measured in our previous work and others from the literature provides 76 stars with Fe abundance and CaH2 and TiO5 index measurements. The CaH2 molecular index is directly correlated with temperature. TiO5 depends on temperature and metallicity. Metallicity can be estimated to within plus or minus 0.3 dex within the bounds of our calibration, which extends from roughly [Fe/H] = +0.05 to -1.0 with a limited extension to -1.5.
We measure the build-up of the stellar mass of galaxies from z=6 to z=1. Using 15 band multicolour imaging data in the NICMOS Ultra Deep Field we derive photometric redshifts and masses for 796 galaxies down to H_AB=26.5. The derived evolution of the global stellar mass density of galaxies is consistent with previous star formation rate density measurements over the observed range of redshifts. Ongoing research in the CFHTLS Deep Fields confirms this result at lower redshifts. Further, if the sample is split by morphological type, a substantial increase is seen in the number of bulge dominated galaxies relative to disk-dominated galaxies since z=1.
We present empirical calibrations that estimate stellar metallicity,
effective temperature and surface gravity as a function of Lick/IDS indices.
These calibrations have been derived from a training set of 261 stars for which
(1) high-precision measurements of [Fe/H], T_eff and log g have been made using
spectral-synthesis analysis of HIRES spectra, and (2) Lick indices have also
been measured. Our [Fe/H] calibration, which has precision 0.07 dex, has
identified a number of bright (V < 9) metal-rich stars which are now being
screened for hot Jupiter-type planets. Using the Yonsei-Yale stellar models, we
show that the calibrations provide distance estimates accurate to 20% for
nearby stars.
This paper outlines the second tier of the screening of planet-search targets
by the N2K Consortium, a project designed to identify the stars most likely to
harbor extrasolar planets. Discoveries by the N2K Consortium include the
transiting hot Saturn HD 149026 b (Sato et al. 2005, astro-ph/0507009) and HD
88133 b (Fischer et al. 2005). See Ammons et al. (2005, In Press) for a
description of the first tier of N2K metallicity screening, calibrations using
broadband photometry.
In this talk I present a short review of primordial magnetic helicity effects on Cosmic Microwave Background (CMB) temperature and polarization anisotropies. These effects allow us to test for cosmological magnetic helicity, however, very accurate CMB fluctuation data is required. This scheme for magnetic helicity detection is valid only for a cosmological magnetic field with a present amplitude larger than $10^{-9}-10^{-10}$ Gauss.
Near infrared spectroscopy and photometry of the Wolf-Rayet Star WR 143 (HD 195177) were obtained in the $JHK$ photometric bands. High resolution spectra observed in the J and H bands exhibit narrow 1.083-micron He I line and the H I Pa Beta and the Brackett series lines in emission superposed on the broad emission line spectrum of the Wolf-Rayet star, giving strong indications of the presence of a companion. From the narrow emission lines observed, the companion is identified to be an early-type Be star. The photometric magnitudes exhibit variations in the JHK bands which are probably due to the variability of the companion star. The flux density distribution is too steep for a Wolf-Rayet atmosphere. This is identified to be mainly due to the increasing contribution from the early-type companion star towards shorter wavelengths.
Recent observations of nearby star forming regions have offered evidence that young brown dwarfs undergo a period of mass accretion analogous to the T Tauri phase observed in young stars. Brown dwarf analogs to stellar protostars, however, have yet to be definitively observed. These young, accreting objects would shed light on the nature of the dominant brown dwarf formation process, as well as provide ideal laboratories to investigate the dependence of the accretion mechanism on protostellar mass. Recent near infrared surveys have identified candidate proto-brown dwarfs and characterized low mass protostars in nearby star forming regions. These techniques allow near infrared spectra to diagnose the effective temperature, accretion luminosity, magnetic field strength and rotation velocity of young low mass stars across the stellar/substellar boundary. The lowest mass proto-brown dwarfs (M < 40 M_Jup), however, will prove challenging to observe given current near IR observational capabilities.
It is well known that the simple criterion proposed originally by Polyachenko and Shukhman (1981) for the onset of the radial orbit instability, although being generally a useful tool, faces significant exceptions both on the side of mildly anisotropic systems (with some that can be proved to be unstable) and on the side of strongly anisotropic models (with some that can be shown to be stable). In this paper we address two issues: Are there processes of collisionless collapse that can lead to equilibria of the exceptional type? What is the intrinsic structural property that is responsible for the sometimes noted exceptional stability behavior? To clarify these issues, we have performed a series of simulations of collisionless collapse that start from homogeneous, highly symmetrized, cold initial conditions and, because of such special conditions, are characterized by very little mixing. For these runs, the end-states can be associated with large values of the global pressure anisotropy parameter up to 2K_r/K_T \approx 2.75. The highly anisotropic equilibrium states thus constructed show no significant traces of radial anisotropy in their central region, with a very sharp transition to a radially anisotropic envelope occurring well inside the half-mass radius (around 0.2 r_M). To check whether the existence of such almost perfectly isotropic "nucleus" might be responsible for the apparent suppression of the radial orbit instability, we could not resort to equilibrium models with the above characteristics and with analytically available distribution function; instead, we studied and confirmed the stability of configurations with those characteristics by initializing N-body approximate equilibria (with given density and pressure anisotropy profiles) with the help of the Jeans equations.
We present observations of the Type Ia supernovae (SNe) 1999M, 1999N, 1999Q,
1999S, and 1999U, at redshift z~0.5. They were discovered in early 1999 with
the 4.0~m Blanco telescope at Cerro Tololo Inter-American Observatory by the
High-z Supernova Search Team (HZT) and subsequently followed with many
ground-based telescopes. SNe 1999Q and 1999U were also observed with the Hubble
Space Telescope. We computed luminosity distances to the new SNe using two
methods, and added them to the high-z Hubble diagram that the HZT has been
constructing since 1995.
The new distance moduli confirm the results of previous work. At z~0.5,
luminosity distances are larger than those expected for an empty universe,
implying that a ``Cosmological Constant,'' or another form of ``dark energy,''
has been increasing the expansion rate of the Universe during the last few
billion years.
We have analyzed the 9.7 and ``18'' micron interstellar silicate absorption features along the line of sight toward four heavily extincted galactic WC-type Wolf-Rayet (WR) stars. We construct two interstellar extinction curves from 1.25 to 25 micron using near-IR extinction measurements from the literature along with the silicate profiles of WR 98a (representing the local ISM) and GCS 3 (representing the Galactic Center). We have investigated the mineralogy of the interstellar silicates by comparing extinction profiles for amorphous silicates with olivine and pyroxene stochiometry to the 9.7 and ``18'' micron absorption features in the WR 98a spectrum. In this analysis, we have considered solid and porous spheres and a continuous distribution of ellipsoids. While it is not possible to simultaneously provide a perfect match to both profiles, we find the best match requires a mixture of these two types of compounds. We also consider iron oxides, aluminosilicates and silicate carbide (SiC) as grain components. Iron oxides cannot be accommodated in the observed spectrum, while the amount of Si in SiC is limited to <4%. Finally, we discuss the cosmic elemental abundance constraints on the silicate mineralogy, grain shape and porosity.
We have obtained accurate circular spectropolarimetric observations for a sample of Vega-like and Herbig Ae/Be stars with FORS1 at the VLT in an attempt to detect their magnetic fields. No magnetic field could be diagnosed in any Vega-like star. The most accurate determination of a magnetic field, at 2.6 sigma level, has been performed for the Vega-like star iota Cen for which we have measured <B_z> = -77+-30 G. In the prototype of Vega-like stars, the star beta Pictoris, which shows conspicuous signs of chromospheric activity, a longitudinal magnetic field is measured only at ~1.5 sigma level. We diagnose for the first time a longitudinal magnetic field at a level larger than 3 sigma for the two Herbig Ae stars HD31648 and HD144432 and we confirm the previous detection of a magnetic field in a third Herbig Ae star, HD139614. Finally, we discuss the discovery of distinctive Zeeman features in the unusual Herbig Ae star HD190073 where the Ca II doublet displays several components in both H and K lines. From the measurement of circular polarization in all Balmer lines from H_beta to H_8 we obtain <B_z> = +26+-34 G. However, using for the measurement of circular polarization only the Ca II H and K lines, we are able to diagnose a longitudinal magnetic field at 2.8 sigma level, <B_z> = +84+-30 G.
To properly understand the physics of upper main sequence stars it is particularly important to identify the origin of their magnetic fields. Recently, we confirmed that magnetic fields appear in Ap stars of mass below 3 M_sun only if they have already completed at least approximately 30% of their main-sequence lifetime. The absence of stars with strong magnetic fields close to the ZAMS might be seen as an argument against the fossil field theories. Here we present the results of our recent magnetic survey with FORS1 at the VLT in polarimetric mode of a sample of A, B and Herbig Ae stars with previously undetected magnetic fields and briefly discuss their significance for our understanding of the origin of the magnetic fields in intermediate mass stars.
The Solar Mass Ejection Imager (SMEI) views nearly every point on the sky once every 102 minutes and can detect point sources as faint as R~10th magnitude. Therefore, SMEI can detect or provide upper limits for the optical afterglow from gamma-ray bursts in the tens of minutes after the burst when different shocked regions may emit optically. Here we provide upper limits for 58 bursts between 2003 February and 2005 April.
The WMAP Q, V, and W band radial profiles of temperature deviation of the CMB were constructed for a sample of 31 randomly selected nearby clusters of galaxies in directions of |b| > 30 deg. The profiles were compared in detail with the expected CMB SZ effect caused by these clusters, with the hot gas properties of each cluster obtained direct from X-ray observations, and with the WMAP point spread function fully taken into consideration. While the WMAP profiles of some clusters do exhibit the SZE, the phenomenon is also noted to be weak or absent from other clusters. Reliable conclusions can be drawn from the combined (co-added) datasets of all 31 clusters, because (a) any remaining systematic uncertainties are low, and (b) the data are extremely clean (i.e. free from foreground contaminants). Both (a) and (b) are facts which we established by examining hundreds of random fields. The verdict from the 31 co-added cluster fields is that the observed SZE only accounts for about 1/4 of the expected decrement. The discrepancy represents too much extra flux for optically thin intracluster thermal emission to be the cause. Radio sources (discrete or halo) are also excluded by WMAP filter ratio analysis. The fact that there is at best only partial SZE silohuette by the nearby clusters suggests an origin of the CMB unrelated to the Big Bang. Either substantial fraction of the radiation may have nearby associations, or WMAP's ability to fathom degree scale CMB structures is questionable.
Abundances of about 18 elements including the heavy elements Y and Zr are determined from Hubble Space Telescope Space Telescope Imaging Spectrograph ultraviolet spectra of seven extreme helium stars (EHes): LSE 78, BD+10 2179, V1920 Cyg, HD 124448, PV Tel, LS IV -1 2, and FQ Aqr. New optical spectra of the three stars -- BD+10 2179, V1920 Cyg, and HD 124448 were analysed. The abundance analyses is done using LTE line formation and LTE model atmospheres especially constructed for these EHe stars. The stellar parameters derived from an EHe's UV spectrum are in satisfactory agreement with those derived from its optical spectrum. Adopted abundances for the seven EHes are from a combination of the UV and optical analyses. Published results for an additional ten EHes provide abundances obtained in a nearly uniform manner for a total of 17 EHes, the largest sample on record. The initial metallicity of an EHe is indicated by the abundance of elements from Al to Ni; Fe is adopted to be the representative of initial metallicity. Iron abundances range from approximately solar to about one-hundredth of solar. Clues to EHe evolution are contained within the H, He, C, N, O, Y, and Zr abundances. Two novel results are (i) the O abundance for some stars is close to the predicted initial abundance yet the N abundance indicates almost complete conversion of initial C, N, and O to N by the CNO-cycles; (ii) three of the seven stars with UV spectra show a strong enhancement of Y and Zr attributable to an s-process. The observed compositions are discussed in light of expectations from accretion of a He white dwarf by a CO white dwarf.
Several authors[1] have justifiably questioned if the Black Hole Candidates (BHCs) have ``hard surface'' why Type I X-ray bursts are not seen from them. It is pointed out that a ``physical surface'' need not always be ``hard'' and instead could be ``gaseous'' in case the compact object is sufficiently hot. Even if a ``hard surface'' would be there, presence of strong strong intrinsic magnetic field could inhibit Type I X-ray burst from a compact object as is the case for Her X-1. Thus, non-occurrence of Type I bursts actually rules out those alternatives of BHs which are either non-magnetized or COLD and, hence, is no evidence for existence of Event Horizons (EHs). On the other hand, from the first principle, we again show that the BHCs being uncharged and having finite masses cannot be BHs, because uncharged BHs have a unique mass M=0. Thus the previous results[2,3] that the so-called BHCs are actually extremely HOT, ultramagnetized, Magnetospheric Eternally Collapsing Objects (MECOs) ($www.phys.uni-sofia.bg/~astro.html$) rather than anything else get reconfirmed by non-occurrence of Type I X-ray bursts in BHCs.
The work is devoted to the analysis of the surface photometric observations of two total lunar eclipses in 2004. The lunar surface relative brightness distribution inside the umbra was used to retrieve the vertical distribution of aerosol extinction of the solar radiation expanding by a tangent path and its dependence on the location at the limb of the Earth. The upper altitude of troposphere aerosol layer was estimated for different latitude zones. The correlation between additional aerosol extinction in the upper troposphere and cyclones was investigated.
We consider several ways to test for topology directly in harmonic space by comparing the measured a_lm with the expected correlation matrices. Two tests are of a frequentist nature while we compute the Bayesian evidence as the third test. Using correlation matrices for cubic and slab-space tori, we study how these tests behave as a function of the minimal scale probed and as a function of the size of the universe. We also apply them to different first-year WMAP CMB maps and confirm that the universe is compatible with being infinitely big for the cases considered. We argue that there is an information theoretical limit (given by the Kullback-Leibler divergence) on the size of the topologies that can be detected.
We analyze the dust attenuation properties of a volume-limited, optically-selected sample of normal star forming galaxies in nearby clusters as observed by GALEX. The internal attenuation is estimated using three independent indicators, namely: the ratio of the total infrared to far-ultraviolet emission, the ultraviolet spectral slope beta and the Balmer decrement. We confirm that normal galaxies follow a L_TIR/L_FUV-beta relation offset from the one observed for starburst galaxies. This offset is found to weakly correlate with the birthrate parameter, thus with the galaxy star formation history. We study the correlations of dust attenuation with other global properties, such as the metallicity, dynamical mass, ionized gas attenuation, Halpha emission and mass surface density. Metal-rich, massive galaxies are, as expected, more heavily extinguished in the UV than are small systems. For the same gas metallicity normal galaxies have lower L_TIR/L_FUV ratio than starbursts, in agreement with the difference observed in the L_TIR/L_FUV-beta relation. Unexpectedly we find however that normal star forming galaxies follow exactly the same relationship between metallicity and ultraviolet spectral slope beta determined for starbursts, complicating our understanding of dust properties. This result might indicate a different dust geometry between normal galaxies and starbursts, but it could also be due to aperture effects eventually present in the IUE starbursts dataset. The present multiwavelength study allows us to provide some empirical relations from which the total infrared to far ultraviolet ratio (L_TIR/L_FUV) can be estimated when far infrared data are absent.
Detailed quantitative spectroscopy of Type Ia supernovae (SNe~Ia) provides
crucial information needed to minimize systematic effects in both ongoing SNe
Ia observational programs such as the Nearby Supernova Factory, ESSENCE, and
the SuperNova Legacy Survey (SNLS) and in proposed JDEM missions such as SNAP,
JEDI, and DESTINY.
Quantitative spectroscopy is mandatory to quantify and understand the
observational strategy of comparing ``like versus like''. It allows us to
explore evolutionary effects, from variations in progenitor metallicity to
variations in progenitor age, to variations in dust with cosmological epoch. It
also allows us to interpret and quantify the effects of asphericity, as well as
different amounts of mixing in the thermonuclear explosion.
The observed hard X-ray (HXR) flux spectrum $I(\epsilon)$ from solar flares is a combination of primary bremsstrahlung photons $I_P(\epsilon)$ with a spectrally modified component from photospheric Compton backscatter of downward primary emission. The latter can be significant, distorting or hiding the true features of the primary spectrum which are key diagnostics for acceleration and propagation of high energy electrons and of their energy budget. For the first time in solar physics, we use a Green's function approach to the backscatter spectral deconvolution problem, constructing a Green's matrix including photoelectric absorption. This approach allows spectrum-independent extraction of the primary spectrum for several HXR flares observed by the {\it Ramaty High Energy Solar Spectroscopic Imager} (RHESSI). We show that the observed and primary spectra differ very substantially for flares with hard spectra close to the disk centre. We show in particular that the energy dependent photon spectral index $\gamma (\epsilon)=-d \log I/d \log \epsilon$ is very different for $I_P(\epsilon)$ and for $I(\epsilon)$ and that inferred mean source electron spectra ${\bar F}(E)$ differ greatly. Even for a forward fitting of a parametric ${\bar F}(E)$ to the data, a clear low-energy cutoff required to fit $I(\epsilon)$ essentially disappears when the fit is to $I_P(\epsilon)$ - i.e. when albedo correction is included. The self-consistent correction for backscattered photons is thus shown to be crucial in determining the energy spectra of flare accelerated electrons, and hence their total number and energy.
We derive individual distances to six Cepheids in the young populous star cluster NGC1866 in the Large Magellanic Cloud employing the near-IR surface brightness technique. With six stars available at the exact same distance we can directly measure the intrinsic uncertainty of the method. We find a standard deviation of 0.11 mag, two to three times larger than the error estimates and more in line with the estimates from Bayesian statistical analysis by Barnes et al. (2005). Using all six distance estimates we determine an unweighted mean cluster distance of 18.30+-0.05. The observations indicate that NGC1866 is close to be at the same distance as the main body of the LMC. If we use the stronger dependence of the p-factor on the period as suggested by Gieren et al. (2005) we find a distance of 18.50+-0.05 (internal error) and the PL relations for Galactic and MC Cepheids are in very good agreement.
A number of recent technical developments, including the Hipparcos satellite, the Hubble Space Telescope fine guidance sensors and long base line near-IR interferometry has made it possible to employ several largely geometrical methods to determine direct distances to RR Lyrae stars and Cepheids. The distance scale now rests on a much firmer basis and the significant differences between the distances based on RR Lyrae stars (short) and Cepheids (long) to the LMC have been largely eliminated. The effects of metalicity on the RR Lyrae period-luminosity (PL) relation in the K-band as well as on the Cepheid PL relation appears to be the main remaining issues but even here empirical results are beginning to show convergence. I review here some of these recent developments seen from the perspective of the near-IR surface brightness method.
Some performances of the present CODALEMA experiment, set up to analyse radio-detected Extensive Air Shower (EAS) events, are presented. Characteristics of the EAS electric field distribution sampled on a 600~m long axis are discussed.
As a part of an ongoing effort to characterise the young stellar populations in the Large Magellanic Cloud, we present HST-WFPC2 broad and narrow band imaging of two fields with recent star formation activity in the Tarantula region. A population of objects with Halpha and/or Balmer continuum excess was identified. On account of the intense Halpha emission (equivalent widths up to several tens of Angstroms), its correlation with the Balmer continuum excess and the stars' location on the HR diagram, we interpret them as low mass (~1-2 Mo) Pre-Main Sequence stars. In this framework, the data show that coeval high and low mass stars have significantly different spatial distributions, implying that star formation processes for different ranges of stellar masses are rather different and/or require different initial conditions. We find that the overall slope of the mass function of the young population is somewhat steeper than the classical Salpeter value and that the star formation density of this young component is 0.2-0.4 Mo/yr/kpc2, i.e. intermediate between the value for an active spiral disk and that of a starburst region. The uncertainties associated with the determination of the slope of the mass function and the star formation density are thoroughly discussed.
We constrain the possibility of a non-trivial refractive index in free space corresponding to an energy-dependent velocity of light: c(E) \simeq c_0 (1 - E/M), where M is a mass scale that might represent effect of quantum-gravitational space-time foam, using the arrival times of sharp features observed in the intensities of radiation with different energies from a large sample of gamma-ray bursters (GRBs) with known redshifts. We use wavelet techniques to identify genuine features, which we confirm in simulations with artificial added noise. Using the weighted averages of the time-lags calculated using correlated features in all the GRB light curves, we find a systematic tendency for more energetic photons to arrive earlier. However, there is a very strong correlation between the parameters characterizing an intrinsic time-lag at the source and a distance-dependent propagation effect. Moreover, the significance of the earlier arrival times is less evident for a subsample of more robust spectral structures. Allowing for intrinsic stochastic time-lags in these features, we establish a statistically robust lower limit: M > 0.9x10^{16} GeV on the scale of violation of Lorentz invariance.
The galaxy cluster RXJ 0152.7-1357 is emitting X-rays at the high rate of 148 counts $ks^{-1}$. It would be one of the most luminous X-ray clusters known if it is at its redshift distance of z = .8325. It is conspicuously elongated, however, toward the bright, X-ray active galaxy NGC 720 about 14 arcmin away. At the same distance on the other side of NGC 720, and almost perfectly aligned, is an X-ray BSO of 5.8 cts/ks. It is reported here that the redshift of this quasar is z = .8312.
We present Near-Infrared (NIR) $J$ and $K_s$-band observations for 181 RR Lyrae stars in the Galactic Globular Cluster $\omega$ Cen. The comparison between predicted and empirical slopes of NIR Period-Luminosity (PL) relations indicates a very good agreement. Cluster distance estimates based on NIR PL relations agree quite well with recent determinations based on different standard candles, giving a true mean distance modulus $\mu = 13.71\pm0.05$, and $d=5.52\pm0.13$ kpc.
The results of 3D modelling of the flow structure in the classical symbiotic
system Z~Andromedae are presented. Outbursts in systems of this type occur when
the accretion rate exceeds the upper limit of the steady burning range.
Therefore, in order to realize the transition from a quiescent to an active
state it is necessary to find a mechanism able to sufficiently increase the
accretion rate on a time scale typical to the duration of outburst development.
Our calculations have confirmed the transition mechanism from quiescence to
outburst in classic symbiotic systems suggested earlier on the basis of 2D
calculations (Bisikalo et al, 2002). The analysis of our results have shown
that for wind velocity of 20 km/s an accretion disc forms in the system. The
accretion rate for the solution with the disc is ~22.5-25% of the mass loss
rate of the donor, that is, ~4.5-5*10^(-8)Msun/yr for Z And. This value is in
agreement with the steady burning range for white dwarf masses typically
accepted for this system. When the wind velocity increases from 20 to 30 km/s
the accretion disc is destroyed and the matter of the disc falls onto the
accretor's surface. This process is followed by an approximately twofold
accretion rate jump. The resulting accretion rate growth is sufficient for
passing the upper limit of the steady burning range, thereby bringing the
system into an active state. The time during which the accretion rate is above
the steady burning value is in a very good agreement with observations.
The analysis of the results presented here allows us to conclude that small
variations in the donor's wind velocity can lead to the transition from the
disc accretion to the wind accretion and, as a consequence, to the transition
from quiescent to active state in classic symbiotic stars.
Observations of the H66a recombination line from the ionized gas in the cluster of newly formed massive stars, G10.6-0.4, show that most of the continuum emission derives from the dense gas in an ionized accretion flow that forms an ionized disk or torus around a group of stars in the center of the cluster. The inward motion observed in the accretion flow suggests that despite the equivalent luminosity and ionizing radiation of several O stars, neither radiation pressure nor thermal pressure has reversed the accretion flow. The observations indicate why the radiation pressure of the stars and the thermal pressure of the HII region are not effective in reversing the accretion flow. The observed rate of the accretion flow, 0.001 solar masses/yr, is sufficient to form massive stars within the time scale imposed by their short main sequence lifetimes. A simple model of disk accretion relates quenched HII regions, trapped hypercompact HII regions, and photo-evaporating disks in an evolutionary sequence.
The outer atmosphere of the M supergiant Betelgeuse is puzzling. Published observations of different kinds have shed light on different aspects of the atmosphere, but no unified picture has emerged. They have shown, for example, evidence of a water envelope (MOLsphere) that in some studies is found to be optically thick in the mid-infrared. In this paper, we present high-resolution, mid-infrared spectra of Betelgeuse recorded with the TEXES spectrograph. The spectra clearly show absorption features of water vapor and OH. We show that a spectrum based on a spherical, hydrostatic model photosphere with T_eff = 3600 K, an effective temperature often assumed for Betelgeuse, fails to model the observed lines. Furthermore, we show that published MOLspheres scenarios are unable to explain our data. However, we are able to model the observed spectrum reasonably well by adopting a cooler outer photospheric structure corresponding to T_mod = 3250 K. The success of this model may indicate the observed mid-infrared lines are formed in cool photospheric surface regions. Given the uncertainties of the temperature structure and the likely presence of inhomogeneities, we cannot rule out the possibility that our spectrum could be mostly photospheric, albeit non-classical. Our data put new, strong constraints on atmospheric models of Betelgeuse and we conclude that continued investigation requires consideration of non-classical model photospheres as well as possible effects of a MOLsphere. We show that the mid-infrared water-vapor features have great diagnostic value for the environments of K and M (super-) giant star atmospheres.
In an undulant universe, cosmic expansion is characterized by alternating periods of acceleration and deceleration. We examine cosmologies in which the dark-energy equation of state varies periodically with the number of e-foldings of the scale factor of the universe, and use observations to constrain the frequency of oscillation. We find a tension between a forceful response to the cosmic coincidence problem and the standard treatment of structure formation.
Interferometers from the ground and space will be able to resolve the two
images in a microlensing event. This will at least partially lift the inherent
degeneracy between physical parameters in microlensing events. To increase the
signal-to-noise ratio, intrinsically bright events with large magnifications
will be preferentially selected as targets. These events may be influenced by
finite source size effects both photometrically and astrometrically. Using
observed finite source size events as examples, we show that the fringe
visibility can be affected by 5% - 10%, and the closure phase by a few degrees:
readily detectable by ground and space interferometers. Such detections will
offer unique information about the lens-source trajectory relative to the
baseline of the interferometers. Combined with photometric finite source size
effects, interferometry offers a way to measure the angular sizes of the source
and the Einstein radius accurately.
Limb-darkening changes the visibility by a small amount compared with a
source with uniform surface brightness, marginally detectable with ground-based
instruments.
We discuss the implications of our results for the plans to make
interferometric observations of future microlensing events.
We summarize the main results obtained recently by our group on the identification and study of very high-z galaxies (z>7) using lensing clusters as natural gravitational telescopes. A description of our pilot survey with ISAAC/VLT is presented, aimed at the spectroscopic confirmation of z>7 candidate galaxies photometrically selected from deep near-IR, HST and optical ground-based imaging. The first results issued from this survey are discussed, in particular the global photometric properties of our high-z candidates, and the implications for the global star formation rate at very high-z.
We present here an overview of PulsarSpectrum, a program that simulates the gamma ray emission from pulsars. This simulator reproduces not only the basic features of the observed gamma ray pulsars, but it can also simulate more detailed effects related to pulsar timing. It is a very useful tool to understand the GLAST capabilities in the pulsar science.
Aims: We want to investigate the growth of bar instability in stellar disks
embedded in a suitable dark matter halo evolving in a fully consistent
cosmological framework.
Methods: We perform seven cosmological simulations to emphasise the role of
both the disk-to-halo mass ratio and of the Toomre parameter, Q, on the
evolution of the disk.We also compare our fully cosmological cases with
corresponding isolated simulations where the same halo, is extracted from the
cosmological scenario and evolved in physical coordinates.
Results: A long living bar, lasting about 10 Gyr, appears in all our
simulations. In particular, disks expected to be stable according to classical
criteria, form indeed weak bars. We argue that such a result is due to the
dynamical properties of our cosmological halo which is far from stability and
isotropy, typical of the classical halos used in literature; it is dynamically
active, endowed of substructures and infall.
Conclusions: At least for mild self-gravitating disks, the study of the bar
instability using isolated isotropic halos, in gravitational equilibrium, can
lead to misleading results. Furthermore, the cosmological framework is needed
for quantitatively investigating such an instability.
The halo masses of nearby individual elliptical galaxies can be estimated by using the kinematics of their stars, planetary nebulae, and globular clusters -- ideally in combination. With currently improving coverage of galaxies of ordinary luminosities and morphologies, systematic trends may be identified. Bright, boxy ellipticals show strong signatures of dark matter, while faint, disky ones typically do not. The former result is problematic for the MOND theory of gravity, and the latter is a challenge to explain in the LCDM paradigm of galaxy formation.
We have performed a comprehensive parameter study of the collapse of
rotating, strongly magnetized stellar cores in axisymmetry to determine their
gravitational wave signature based on the Einstein quadrupole formula. We use a
Newtonian explicit magnetohydrodynamic Eulerian code based on the relaxing-TVD
method for the solution of the ideal MHD equations, and apply the
constraint-transport method to guarantee a divergence--free evolution of the
magnetic field. We neglect effects due to neutrino transport and employ a
simplified equation of state. The pre--collapse initial models are polytropes
in rotational equilibrium with a prescribed degree of differential rotation and
rotational energy (~ 1 % of the gravitational energy). The initial magnetic
fields are purely poloidal the field strength ranging from 10^10 G to 10^13 G.
The evolution of the core, whose collapse is initiated by reducing the gas
pressure by a prescribed amount, is followed until a few ten milliseconds past
core bounce.
The initial magnetic fields are amplified mainly by the differential rotation
of the core giving rise to a strong toroidal field component. The poloidal
field component grows by compression during collapse, but does not change
significantly after core bounce if (abbreviated)
We present results from a 112 ks long look by XMM-Newton at the ultraluminous X-ray source (ULX) Holmberg II X-1, long thought to be the one of best candidates for the missing class of intermediate mass black holes (IMBHs). Our data comprises the first high quality XMM-Newton/RGS spectrum of an ULX, and an XMM-Newton/EPIC spectrum with unprecedented signal-to-noise. A detailed timing analysis shows that any variability on time-scales of minutes to hours is very weak (< few per cent fractional rms), though larger amplitude variations on much shorter time-scales could be hidden by photon counting statistics. This result suggests that if Ho II X-1 harbours an IMBH, then we are observing this source in a highly unusual and atypical state when compared with the known variability behaviour of other accreting systems of large mass. Moreover unlike Galactic X-ray binaries, our spectral analysis indicates the possible presence of an optically-thick low temperature corona. Taken together our timing and spectral analysis suggests that the compact companion is most-likely a high luminosity analogue of black hole binary systems similar to GRS 1915+105, the Galactic microquasar, harbouring a compact object of mass no greater than 100 solar masses.
We present deep near-infrared Js, H, and Ks band imaging of a field around MS1054-03, a massive cluster at z=0.83. The observations were carried out with ISAAC at the ESO VLT as part of the Faint InfraRed Extragalactic Survey (FIRES). The total integration time amounts to 25.9h in Js, 24.4h in H, and 26.5h in Ks, divided nearly equally between four pointings covering 5.5'x5.3'. The 3-sigma total limiting AB magnitudes for point sources from the shallowest to deepest pointing are Js=26.0-26.2, H=25.5-25.8, and Ks=25.3-25.7. The effective spatial resolution of the coadded images has FWHM=0.48", 0.46", and 0.52" in Js, H, and Ks. We complemented the ISAAC data with deep optical imaging using existing HST WFPC2 mosaics in the F606W and F814W filters and new U, B and V band data from VLT FORS1. We constructed a Ks-band limited multicolour source catalogue to Ks(total,AB)=25 (about 5-sigma for point sources). The catalogue contains 1858 objects, of which 1663 have eight-band photometry. We describe the observations, data reduction, source detection and photometric measurements method. We present the number counts, colour distributions, and photometric redshifts z_ph of the catalogue sources. We find that our counts at the faint end 22<Ks(AB)<25, with slope dlog(N)/dm=0.20, lie at the flatter end of published counts in other deep fields and are consistent with those we derived in the HDF-South, the other FIRES field. Spectroscopic redshifts z_sp are available for about 330 sources in the MS1054-03 field; comparison between the z_ph and z_sp shows very good agreement, with <|z_sp-z_ph|/(1+z_sp)>=0.078. The MS1054-03 field observations complement our HDF-South data set with nearly five times larger area at about 0.7 brighter magnitudes. [ABRIDGED]
More than 30 million of high-energy muons collected with the MACRO detector at the underground Gran Sasso Laboratory have been used to search for flux variations of different natures. Two kinds of studies were carried out: search for periodic variations and for the occurrence of clusters of events. Different analysis methods, including Lomb-Scargle spectral analysis and Scan Test statistics have been applied to the data.
The energy spectrum of ultra-high energy cosmic rays (UHECR) is usually calculated for sources with identical properties. Assuming that all sources can accelerate UHECR protons to the same extremely high maximal energy E_max > 10^{20} eV and have the steeply falling injection spectrum 1/E^{2.7}, one can reproduce the measured cosmic ray flux above E > 10^{18} eV. We show that relaxing the assumption of identical sources and using a power-law distribution of their maximal energy allows one to explain the observed UHECR spectrum with the injection 1/E^2 predicted by Fermi shock acceleration.
We have examined a probable soft X-ray transient source in the M31 bulge at R.A.=0:42:41.814 +/- 0.08", Dec. = 41:16:35.86 +/- 0.07". On the three occasions we observed the source, its spectrum was soft (kT_{in} ~1 keV). The brightest detection of the source was 2004 July 17 with a 0.3-7 keV luminosity of ~5 X 10^{37} erg/s. The only previous detection of the source was in 1979 by the Einstein observatory. The multiple detections over 25 years suggest the duty cycle of the source is in the range 0.02-0.06. Coordinated HST/ACS imaging before, during, and after the outburst revealed no variable optical source within the position errors of the X-ray source. The optical data place a firm upper limit on the brightness of the counterpart of the X-ray outburst of B>24.7, suggesting the binary has a period <5.2 days. The X-ray spectrum and lack of bright stars at the source location indicate the source was a soft transient event occurring in a low-mass X-ray binary, making this source a good black hole candidate in M31.
Gamma-Ray Bursts (GRBs) fall into two classes: short-hard and long-soft bursts. The latter are now known to have X-ray and optical afterglows, to occur at cosmological distances in star-forming galaxies, and to be associated with the explosion of massive stars. In contrast, the distance scale, the energy scale, and the progenitors of short bursts have remained a mystery. Here we report the discovery of a short-hard burst whose accurate localization has led to follow-up observations that have identified the X-ray afterglow and (for the first time) the optical afterglow of a short-hard burst. These, in turn, have led to identification of the host galaxy of the burst as a late-type galaxy at z=0.16 showing that at least some short-hard bursts occur at cosmological distances in the outskirts of galaxies, and are likely to be due to the merging of compact binaries.
We extend our earlier work on cluster cores with distinct radio bubbles, adding more active bubbles, i.e. those with Ghz radio emission, to our sample, and also investigating ``ghost bubbles,'' i.e. those without GHz radio emission. We have determined k, which is the ratio of the total particle energy to that of the electrons radiating between 10 MHz and 10 GHz. Constraints on the ages of the active bubbles confirm that the ratio of the energy factor, k, to the volume filling factor, f lies within the range 1 < k/f < 1000. In the assumption that there is pressure equilibrium between the radio-emitting plasma and the surrounding thermal X-ray gas, none of the radio lobes has equipartition between the relativistic particles and the magnetic field. A Monte-Carlo simulation of the data led to the conclusion that there are not enough bubbles present in the current sample to be able to determine the shape of the population. An analysis of the ghost bubbles in our sample showed that on the whole they have higher upper limits on k/f than the active bubbles, especially when compared to those in the same cluster. A study of the Brightest 55 cluster sample shows that 17, possibly 20, clusters required some form of heating as they have a short central cooling time, t_cool < 3 Gyr, and a large central temperature drop, T_centre/T_outer< 1/2. Of these between 12 (70 per cent) and 15 (75 per cent), contain bubbles. This indicates that the duty cycle of bubbles is large in such clusters and that they can play a major role in the heating process.
The discovery of long-lasting (~100 s) X-ray flares following short gamma-ray bursts initially called into question whether they were truly classical short-hard bursts. Opinion over the last few years has coalesced around the view that the short-hard bursts arise from the merger of pairs of neutron stars, or a neutron star merging with a stellar-mass black hole. The natural timescales associated with these processes, however, essentially preclude an X-ray flare lasting ~100 s. Here we show that an interaction between the GRB outflow and a non-compact stellar companion at a distance of ~a light-minute provides a natural explanation for the flares. In the model, the burst is triggered by the collapse of a neutron star after accreting matter from the companion. This is reminiscent of type Ia supernovae, where there is a wide distribution of delay times between formation and explosion, leading to an association with both star-forming galaxies and old ellipticals.
(abbreviated abstract) We present the long term spectral evolution of the Galactic black hole candidate Cygnus X-1 in the X-rays and at 15GHz using ~200 pointed observations taken between early 1999 and late 2004 with RXTE and the Ryle radio telescope. The X-ray spectra are remarkably well described by a simple broken power law spectrum with an exponential cutoff. Physically motivated Comptonization models, e.g., compTT or eqpair, can reproduce this simplicity. Broken power law models reveal a significant linear correlation between the photon index of the lower energy power law and the hardening of the power law at approximately 10keV. Comptonization models show that the bolometric flux of a soft excess (e.g., disk component) is strongly correlated with the compactness ratio of the Comptonizing medium. We also find that the fraction of the time spent in low radio emission/soft X-ray spectral states has increased from ~10% in 1996-2000 to ~34% since early 2000. Radio flares typically occur during state transitions and failed state transitions. There is also a strong correlation between the 10-50keV X-ray flux and the radio luminosity of the source. We demonstrate that rather than there being distinctly separated states, in contrast to the timing properties the spectrum of Cyg X-1 shows variations between extremes of properties, with clear cut examples of spectra at every intermediate point in the observed spectral correlations.