We show that an interaction between dark matter and dark energy generically results in an effective dark energy equation of state of w<-1. This arises because the interaction alters the redshift-dependence of the matter density. An observer who fits the data treating the dark matter as non-interacting will infer an effective dark energy fluid with w<-1. We argue that the model is consistent with all current observations, the tightest constraint coming from estimates of the matter density at different redshifts. Comparing the luminosity and angular-diameter distance relations with LambdaCDM and phantom models, we find that the three models are degenerate within current uncertainties but likely distinguishable by the next generation of dark energy experiments.
We report on the first dedicated monitoring campaign of spectroscopic variability in young brown dwarfs. High-resolution optical spectra of six targets in nearby star-forming regions were obtained over 11 nights between 2005 January-March on the Magellan 6.5m telescope. We find significant variability in Halpha and a number of other emission lines related to accretion and outflow processes on a variety of timescales ranging from hours to weeks to years. The most dramatic changes are seen for 2M1207, 2M1101 and ChaI-ISO217. We observe possible accretion rate changes by about an order of magnitude in two of these objects, over timescales of weeks (2M1207) or hours (2M1101). The accretion 'burst' seen in 2M1101 could be due to a 'clumpy' flow. We also see indications for changes in the outflow rate in at least three objects. In one case (ISO217), there appears to be a ~1-hour time lag between outflow and accretion variations, consistent with a scenario in which the wind originates from the inner disk edge. Our variability study supports a close to edge-on inclination for the brown dwarf LS-RCrA 1. The fact that all targets in our sample show variations in accretion and/or outflow indicators suggests that studies of young brown dwarf properties should be based either on large samples or time series. As an example, we demonstrate that the large scatter in the recently found accretion rate vs. mass relationship can be explained primarily with variability. The observed profile variations imply asymmetric accretion flows in brown dwarfs, which, in turn, is evidence for magnetic funneling by large-scale fields. We show that accreting sub-stellar objects may harbor magnetic fields with ~kG strength (abridged).
Using Infrared Array Camera (IRAC) images at 3.6, 4.5, 5.8, and 8 microns from the GLIMPSE Legacy science program on the Spitzer Space Telescope, we searched for infrared counterparts to the 95 known supernova remnants that are located within galactic longitudes 65>|l|>10 degrees and latitudes |b|<1 degree. Eighteen infrared counterparts were detected. Many other supernova remnants could have significant infrared emission but are in portions of the Milky Way too confused to allow separation from bright HII regions and pervasive mid-infrared emission from atomic and molecular clouds along the line of sight. Infrared emission from supernova remnants originates from synchrotron emission, shock-heated dust, atomic fine-structure lines, and molecular lines. The detected remnants are G11.2-0.3, Kes 69, G22.7-0.2, 3C 391, W 44, 3C 396, 3C 397, W 49B, G54.4-0.3, Kes 17, Kes 20A, RCW 103, G344.7-0.1, G346.6-0.2, CTB 37A, G348.5-0.0, and G349.7+0.2. The infrared colors suggest emission from molecular lines (9 remnants), fine-structure lines (3), and PAH (4), or a combination; some remnants feature multiple colors in different regions. None of the remnants are dominated by synchrotron radiation at mid-infrared wavelengths. The IRAC-detected sample emphasizes remnants interacting with relatively dense gas, for which most of the shock cooling occurs through molecular or ionic lines in the mid-infrared.
We present observations made with the newly commissioned Keck laser-guide star adaptive optics system of 6 objects in M31 that are alleged in multiple recent studies to be young globular clusters (GCs); all are supposed to have ages less than 5 Gyr. The resulting FWHM of the PSF core in our images is ~70 mas. The four youngest of these objects are asterisms; they are with certainty not young GCs in M31. Based on their morphology, the two oldest are GCs in M31. While the M31 GCs with ages 5 -- 8 Gyr appear to be mostly genuine, it appears that many of the alleged very young GCs in M31 are spurious identifications. This problem will be even more severe in studies of the GC systems of more distant spiral galaxies now underway, for which imaging at the spatial resolution of our observations in M31 may not be adequate to detect sample contamination by asterisms.
We have analyzed an XMM-Newton observation of the cluster Abell 2589. Apart from a low-level asymmetry in the central region, the cluster appears very relaxed and does not show presence of a central AGN. We derived constraints for the radial temperature, density and, assuming hydrostatic equilibrium, mass profiles. We find that the best fit to the dark matter profile is given by the Sersic-like profile proposed by Navarro et al. (2004). The NFW model does not provide a good fit. We also tested whether the central stellar component could affect the profile through the adiabatic contraction model but were unable to distinguish it from a simple "dark matter + stars" modeling.
Exoplanetary transits produce a double-horned color signature that is distinct from both binaries and blends and can thus be used to separate exoplanets from false positives in transit searches. Color photometry with precision sufficient to detect this signal in transits of HD 209458 is available in the literature. Analysis of these observations reveals that, while the signature does exhibit the expected shape, it is significantly stronger than PHOENIX atmospheric models predict.
The recombination of hydrogen and helium at z~1000-7000 gives unavoidable distortions to the Cosmic Microwave Background (CMB) spectrum. We present a detailed calculation of the line intensities arising from the Ly-alpha (2p-1s) and two-photon (2s-1s) transitions for the recombination of hydrogen, as well as the corresponding lines from helium. We give an approximate formula for the strength of the main recombination line distortion on the CMB in different cosmologies, this peak occurring at about 170 microns. We also find a previously undescribed long wavelength peak (which we call the pre-recombination peak) from the lines of the 2p-1s transitions, which are formed before significant recombination of the corresponding atoms occurred. Detailed calculations of the two-photon emission line shapes are presented here for the first time. The frequencies of the photons emitted from the two-photon transition have a wide spectrum and this causes the location of the peak of the two-photon line of hydrogen to be located almost at the same wavelength as the main Ly-alpha peak. The helium lines also give distortions at similar wavelengths, so that the combined distortion has a complex shape. The detection of this distortion would provide direct supporting evidence that the Universe was indeed once a plasma. Moreover, the distortions are a sensitive probe of physics during the time of recombination. Although the spectral distortion is overwhelmed by dust emission from the Galaxy, and is maximum at wavelengths roughly where the cosmic far-infrared background peaks, it may be able to tailor an experiment to detect its non-trivial shape.
Recent observations of the ground state transition of HDO at 464 GHz towards the protoplanetary disk of DM Tau have detected the presence of water vapor in the regions just above the outer disk midplane (Ceccarelli et al 2005). In the absence of non-thermal desorption processes, water should be almost entirely frozen onto the grain mantles and HDO undetectable. In this Letter we present a chemical model that explores the possibility that the icy mantles are photo-desorbed by FUV (6eV < h nu < 13.6eV) photons. We show that the average Interstellar FUV field is enough to create a layer of water vapor above the disk midplane over the entire disk. Assuming a photo-desorption yield of 10^{-3}, the water abundance in this layer is predicted to be ~ 3 x 10^{-7} and the average H2O column density is ~ 1.6x 10^{15} cm^{-2}. The predictions are very weakly dependent on the details of the model, like the incident FUV radiation field, and the gas density in the disk. Based on this model, we predict a gaseous HDO/H2O ratio in DM Tau of ~1%. In addition, we predict the ground state transition of water at 557 GHz to be undetectable with ODIN and/or HSO-HIFI.
Very high-quality spectra of 24 metal-poor halo dwarfs and subgiants have been acquired with ESO's VLT/UVES for the purpose of determining Li isotopic abundances. The derived 1D, non-LTE 7Li abundances from the LiI 670.8nm line reveal a pronounced dependence on metallicity but with negligible scatter around this trend. Very good agreement is found between the abundances from the LiI 670.8nm line and the LiI 610.4nm line. The estimated primordial 7Li abundance is $7Li/H = 1.1-1.5 x 10^-10, which is a factor of three to four lower than predicted from standard Big Bang nucleosynthesis with the baryon density inferred from the cosmic microwave background. Interestingly, 6Li is detected in nine of our 24 stars at the >2sigma significance level. Our observations suggest the existence of a 6Li plateau at the level of log 6Li = 0.8; however, taking into account predictions for 6Li destruction during the pre-main sequence evolution tilts the plateau such that the 6Li abundances apparently increase with metallicity. Our most noteworthy result is the detection of 6Li in the very metal-poor star LP815-43. Such a high 6Li abundance during these early Galactic epochs is very difficult to achieve by Galactic cosmic ray spallation and alpha-fusion reactions. It is concluded that both Li isotopes have a pre-Galactic origin. Possible 6Li production channels include proto-galactic shocks and late-decaying or annihilating supersymmetric particles during the era of Big Bang nucleosynthesis. The presence of 6Li limits the possible degree of stellar 7Li depletion and thus sharpens the discrepancy with standard Big Bang nucleosynthesis.
We model the constraints set on the evolution of dusty starburst galaxies by the current deep extragalactic surveys performed in the far-infrared with $\it Spitzer$, and at radio wavelengths with the VLA. Our models fit the number counts in all the available spectral bands well, and also provide a reasonably close match to the redshift distribution of the $\it Spitzer$ detections. We find: 1.) dusty starburst galaxies with infrared burst phases triggered by galactic interactions at redshift $z \sim 1-2$ are good candidates to fit the $\it Spitzer$ results at $24\mu m$, $70 \mu m$ and $160 \mu m$, assuming plausible strengths for the PAH features for the infrared luminous sources. An Arp220-like spectral energy distribution (SED) for Ultraluminous Infrared Galaxies (ULIGs) of $L_{\rm ir}>10^{12} L_{\odot}$ and one like that of M82 for Luminous Infrared Galaxies (LIGs) of $L_{\rm ir}\sim 10^{11-12} L_{\odot}$ give a successful fit to the $\it Spitzer$ $24\mu m$ and $\rm ISOCAM 15\mu m$ number counts at flux levels of $S_{\nu}< 1 mJy$; 2.) the strong evolution of the number density of the ULIGs from redshift $z\sim 0$ to $\sim 1$ predicted by our models is consistent with the current deep $\rm 1.4 GHz$ radio surveys and accounts for the upturn in the $\rm 1.4 GHz$ differential counts at the sub-mJy flux level; and 3.) comparing with number counts at near infrared bands, as well as the background measurements using DIRBE and 2MASS, shows that only a fraction of the stellar mass in the Universe is included in our models of dusty starburst mergers at $z \sim 1 - 2$.
The GSC-II white dwarf survey was designed to identify faint and high proper motion objects, which we used to define a new and independent sample of cool white dwarfs. With this survey we aim to derive new constraints on the halo white dwarf space density. Also, these data can provide information on the age of thick disk and halo through the analysis of the luminosity function. On the basis of astrometric and photometric parameters, we selected candidates with mu > 0.28 as/yr and R_F > 16 in an area of 1150 square degrees. Then, we separated white dwarfs from late type dwarfs and subdwarfs by means of the reduced proper motion diagram. Finally, spectroscopic follow-up observations were carried out to confirm the white dwarf nature of the selected candidates. We found 41 white dwarfs of which 24 are new discoveries. Here we present the full sample and for each object provide positions, absolute proper motions, photometry, and spectroscopy.
We report the discovery of a new M9.0 dwarf at only 7 pc, which we identified in by our search for nearby ultracool dwarf (I-J >= 3.0, later than M8.0) in the DENIS database. We measure a very high proper motion of 2.5 arc-sec/yr. The PC3 index measured from its low-resolution spectrum gives a spectrophotometric distance of 7 pc. This makes it the third closest M9.0 dwarf.
Single-dish spectra and interferometric maps of (sub)mm lines of H2O-18 and HDO are used to study the chemistry of water in eight regions of high-mass star formation. The spectra indicate HDO excitation temperatures of ~110 K and column densities in an 11'' beam of ~2x10^14 cm^-2 for HDO and ~2x10^17 cm^-2 for H2O, with the N(HDO)/N(H2O) ratio increasing with decreasing temperature. Simultaneous observations of CH3OH and SO2 indicate that 20-50% of the single-dish line flux arises in the molecular outflows of these objects. The outflow contribution to the H2O-18 and HDO emission is estimated to be 10-20%. Radiative transfer models indicate that the water abundance is low (~10^-6) outside a critical radius corresponding to a temperature in the protostellar envelope of ~100 K, and `jumps' to H2O/H2 ~ 10^-4 inside this radius. This value corresponds to the observed abundance of solid water and together with the derived HDO/H2O abundance ratios of ~1/1000 suggests that the origin of the observed water is evaporation of grain mantles. This idea is confirmed in the case of AFGL 2591 by interferometer observations of HDO, H2O-18 and SO2 lines, which reveal compact (~800 AU) emission with a systematic velocity gradient. This size is similar to that of the 1.3 mm continuum towards AFGL 2591, from which we estimate a mass of ~0.8 M0, or ~5% of the mass of the central star. We speculate that we may be observing a circumstellar disk in an almost face-on orientation.
We present our VLT/FORS1 deep spectroscopic observations of the gravitationally lensed quasar SDSS J0924+0219, as well as archival HST/NICMOS and ACS images of the same object. The two-epoch spectra, obtained in the Multi Object Spectroscopy (MOS) mode, allow for very accurate flux calibration, spatial deconvolution of the data, and provide the redshift of the lensing galaxy z=0.394 +/- 0.001. These spectra, taken 15 days apart, show only slight continuum variations, while the broad emission lines display obvious changes in the red wing of the Mg II line, in the Fe II bands, and in the central part of the C III] line. Even though variations in the line profiles are present, we do not see any significant differences between the continuum and emission line flux ratios of images A and B of the quasar, from 4000 A to 8000 A. Spatial deconvolution of the HST images reveals a double Einstein ring. One ring corresponds to the lensed quasar host galaxy at z=1.524 and a second bluer one, is the image either of a star-forming region in the host galaxy, or of another unrelated lower redshift object. We find that a broad range of lens models gives a satisfactory fit to the data. However, they predict very different time delays, making SDSS J0924+0219 an object of particular interest for photometric monitoring. In addition, the lens models reconstructed using exclusively the constraints from the Einstein rings, or using exclusively the astrometry of the quasar images, are not compatible. This suggests that substructures play an important role in SDSS J0924+0219.
We study the conditions for operation of the 22 GHz ortho-water maser in a dusty medium near late-type stars. The main physical processes, such as exchange of energy between dust and gas in the radiation field of a star, radiative cooling by water molecules and pumping of water masers are described self-consistently. We show that the presence of dust grains of various types (or of one type with size distribution) strongly affects the maser action. The pumping mechanism based on the presence of the dust of different optical properties is able to explain water masers in the silicate carbon star V778 Cyg. However, the masers in the winds from asymptotic giant branch stars require an additional source of heating, for instance due to the dust drift through the gas.
The abundance of galaxy clusters as a function of mass is determined using the 2dFGRS Percolation-Inferred Galaxy Group (2PIGG) catalogue. This is used to estimate the amplitude of the matter fluctuation spectrum, parametrised by the linear theory rms density fluctuations in spheres of 8Mpc/h, sigma_8. The best-fitting value for this parameter is highly correlated with the mean matter density in the Universe, Omega_m, and is found to satisfy sigma_8=0.25Omega_m^{-0.92-4.5(Omega_m-0.22)^2}+/-10%(statistical)+/-20% (systematic) for 0.18<Omega_m<0.50, assuming that Omega_m+Omega_Lambda=1. This gives sigma_8=0.89 when Omega_m=0.25. A ~20% correction has been applied to undo the systematic bias inherent in the measurement procedure. Mock catalogues, constructed from large cosmological N-body simulations, are used to help understand and model these systematic errors. The abundance of galaxy groups as a function of group bj band luminosity is also determined. This is used in conjunction with the halo mass function, determined from simulations, to infer the variation of halo mass-to-light ratio over four orders of magnitude in halo mass. The mass-to-light ratio shows a minimum value of 100hM_sol/L_sol in the bj band at a total group luminosity of L_bJ ~ 5.10^9Lsol/h^2. Together with the observed Tully-Fisher relation, this implies that the observed rotation speed of Tully-Fisher galaxies is within ~10% of the typical circular speed of haloes hosting brightest galaxies of the same luminosity.
We propose a new kind of seismic diagram, based on the determination of the locations of sharp acoustic features inside a star. We show that by combining the information about the position of the base of the convective envelope or the second helium ionisation zone with a measure of the average large separation, it is possible to constrain the unknown parameters characterising the physical processes in the stellar interior. We demonstrate the application of this technique to the analysis of mock data for a CoRoT target star.
We present the results of a 45 ks Chandra observation of the high-mass X-ray binary Cen X--3 at orbital phases between 0.13 and 0.40 (in the eclipse post-egress phases). Here we concentrate on the study of discrete features in the energy spectrum at energies between 6 and 7 keV, i.e. on the iron K$_\alpha$ line region, using the High Energy Transmission Grating Spectrometer on board the Chandra satellite. We clearly see a K$_\alpha$ neutral iron line at $\sim 6.40$ keV and were able to distinguish the three lines of the \ion{Fe}{25} triplet at 6.61 keV, 6.67 keV, and 6.72 keV, with an equivalent width of 6 eV, 9 eV, and 5 eV, respectively. The equivalent width of the K$_\alpha$ neutral iron line is 13 eV, an order of magnitude lower than previous measures. We discuss the possibility that the small equivalent width is due to a decrease of the solid angle subtended by the reflector.
In main-sequence stars, the chemical composition gradient that develops at the edge of the convective core is responsible for a non-uniform period spacing of high-order gravity modes. In this work we investigate, in the case of a 1.6 Msun star, the effects on the period-spacing of extra mixing processes in the core (such as diffusion and overshooting).
Previous authors have calculated the mass function of primordial black holes only on scales which are well outside the horizon at the end of inflation. Here we extend the calculation to sub-horizon scales, on which the density perturbation never becomes classical. Regarding the formation of black holes as a `measurement' of the (high peaks) of the density perturbation, we estimate a mass function by assuming that black holes form as soon as inflation ends, in those rare regions where the Bardeen potential exceeds a threshold value of $Psi_c\simeq 0.5$.
In this paper we consider the viability of these non-gravitational torques to take the gas off the plane. We show that magnetically generated warps are clearly flawed because they would wrap up into a spiral in less than two or three galactic rotations. The inclusion of any magnetic diffusivity to dilute the wrapping effect, causes the amplitude of the warp to damp. We also consider observational consequences of the accretion of an intergalactic plane-parallel flow at infinity. We have computed the amplitude and warp asymmetry in the accretion model, for a disc embedded in a flattened dark matter halo, including self-consistently the contribution of the modes with azimuthal wavenumbers m=0 and m=1. This model predicts quite asymmetric warps, maximum gas displacements on the two sides in the ratio 3:2 for the preferred Galactic parameters,and the presence of a fraction ~3.5% of U-shaped warps, at least. The azimuthal dependence of the moment transfer by the ram pressure would produce a strong asymmetry in the thickness of the HI layer and asymmetric density distributions in z, in conflict with observational data for the warp in our Galaxy and in external galaxies. The amount of accretion that is required to explain the Galactic warp would give gas scaleheights in the far outer disc that are too small. We conclude that accretion of a flow with no net angular momentum, cannot be the main and only cause of warps.
We present an analysis of light variations in UBVRI of the anomalous object in the center of planetary nebula Sh 2-71. We refined the linear ephemeris of the light maxima to JD{max} = 2449862.0 + 68.101 (E-96), but also identified long-term, obviously non-periodic variations. The latter manifest themselves in large O-C shifts, a variable profile of light curves (hereafter LC) and changes in the mean brightness of the object. Our spectroscopic observations suggested the presence of a superdense nebula in the center of Sh 2-71.
Among radio galaxies containing nuclear dust disks, the bipolar jet axis is generally expected to be perpendicular to the disk major axis. However, the FR I radio source 3C 449, possessing a nearly parallel jet/disk orientation on the sky, is an extreme example of a system that does not conform to this expectation. We examine the 600 pc dusty disk in this galaxy with images from the {\it Hubble Space Telescope}. We find that a colormap of the disk exhibits a twist in its isocolor contours (isochromes). We model the colormap by integrating galactic starlight through an absorptive disk, and find that the anomalous twist in the isochromes can be reproduced in the model with a vertically thin, warped disk. The model predicts that the disk is nearly perpendicular to the jet axis within 100 pc of the nucleus. We discuss physical mechanisms capable of causing such a warp. We show that a torque on the disk arising from a possible binary black hole in the AGN or radiation pressure from the AGN causes precession on a timescale that is too long to generate such a warp. However, we estimate that the pressure in the X-ray emitting interstellar medium is large enough to perturb the disk. In this way, the warped disk in 3C 449 may be a new manifestation of feedback from an active galactic nucleus.
(abridged) Two XMM-Newton observations of the black-hole binary GRS1915+105 were triggered in 2004, during a long "plateau" state of the source. (...) While the light curves show just small amplitude variations (a few percent) at timescales longer than a few seconds, a QPO is seen at about 0.6 Hz (...). The pn spectrum is well fitted without invoking thermal disk emission, on the base of four main components: a primary one (...), absorbed by cold matter with abundances different than those of standard ISM; reprocessing from an ionized disk; emission and absorption lines; and a soft X-ray excess around 1 keV. However, the latter is not confirmed by the RGS spectra, whose difference from the EPIC-pn ones actually lacks of a fully satisfactory explanation. If real, the soft X-ray excess may be due to reflection from an optically thin, photoionized disk wind; in this case it may yield a way to disentangle intrinsic from interstellar absorption.
We calculate the heating and cooling of the accreting white dwarf (WD) in the ultracompact AM Canum Venaticorum (AM CVn) binaries and show that the WD can contribute significantly to their optical and ultraviolet emission. We estimate the WD's effective temperature, Teff, using the optical continuum for a number of observed binaries, and show that it agrees well with our theoretical calculations. Driven by gravitational radiation losses, the time averaged accretion rate, <Mdot>, decreases monotonically with increasing Porb, covering six orders of magnitude. If the short period (Porb<10 min) systems accrete at a rate consistent with gravitational radiation via direct impact, we predict their unpulsed optical/UV light to be that of the Teff>50,000$ K accreting WD. At longer Porb we calculate the Teff and absolute visual magnitude, M_V, that the accreting WD will have during low accretion states, and find that the WD naturally crosses the pulsational instability strip. Discovery and study of pulsations could allow for the measurement of the accumulated helium mass on the accreting WD, as well as its rotation rate. Accretion heats the WD core, but for Porb>40 minutes, the WD's Teff is set by its cooling as <Mdot> plummets. For the two long period AM CVn binaries with measured parallaxes, GP Com and CE 315, we show that the optical broadband colors and intensity are that expected from a pure helium atmosphere WD. This confirms that the WD brightness sets the minimum light in wide AM CVn binaries, allowing for meaningful constraints on their population density from deep optical searches, both in the field and in Globular Clusters.
Hypermassive neutron stars (HMNSs) -- equilibrium configurations supported against collapse by rapid differential rotation -- are possible transient remnants of binary neutron star mergers. Using newly developed codes for magnetohydrodynamic simulations in dynamical spacetimes, we are able to track the evolution of a magnetized HMNS in full general relativity for the first time. We find that secular angular momentum transport due to magnetic braking and the magnetorotational instability results in the collapse of an HMNS to a rotating black hole, accompanied by a gravitational wave burst. The nascent black hole is surrounded by a hot, massive torus undergoing quasistationary accretion and a collimated magnetic field. This scenario suggests that HMNS collapse is a possible candidate for the central engine of short gamma-ray bursts.
(Abridged) The spatial distribution of the Galactic satellite system plays an important role in Galactic dynamics and cosmology, where its successful reproduction is a key test of simulations of galaxy halo formation. Here, we examine its representative nature by conducting an analysis of the 3-dimensional spatial distribution of the M31 subgroup of galaxies. We begin by a discussion of distance estimates and incompleteness concerns, before revisiting the question of membership of the M31 subgroup. Comparison of the distribution of M31 and Galactic satellites relative to the galactic disks suggests that the Galactic system is probably modestly incomplete at low latitudes by ~20%. We find that the radial distribution of satellites around M31 is more extended than the Galactic subgroup; 50% of the Galactic satellites are found within ~100kpc of the Galaxy, compared to ~200kpc for M31. We search for ``ghostly streams'' of satellites around M31, in the same way others have done for the Galaxy, and find several. The lack of M31-centric kinematic data, however, means we are unable to probe whether these streams represent real physical associations. Finally, we find that the M31 satellites are asymmetrically distributed with respect to our line-of-sight to this object, so that the majority of its satellites are on its near side with respect to our line-of-sight. We quantify this result and find it to be significant at the ~3 sigma level. Until such time as a satisfactory explanation for this finding is presented, our results warn against treating the M31 subgroup as complete, unbiased and relaxed.
We report the discovery of a galaxy cluster at z = 1.41. ISCS J143809+341419 was found in the Spitzer/IRAC Shallow Survey of the Bootes field in the NOAO Deep Wide-Field Survey carried out by IRAC. The cluster candidate was initially identified as a high density region of objects with photometric redshifts in the range 1.3 < z < 1.5. Optical spectroscopy of a limited number of objects in the region shows that 5 galaxies within a ~120 arcsec diameter region lie at z = 1.41 +/- 0.01. Most of these member galaxies have broad--band colors consistent with the expected spectral energy distribution of a passively--evolving elliptical galaxy formed at high redshift. The redshift of ISCS J143809+341419 is the highest currently known for a spectroscopically-confirmed cluster of galaxies.
One can solve the Jeans equation analytically for equilibrated dark matter structures, once given two pieces of input from numerical simulations. These inputs are 1) a connection between phase-space density and radius, and 2) a connection between velocity anisotropy and density slope, the \alpha-\beta relation. The first (phase-space density v.s. radius) has already been analysed through several different simulations, however the second (\alpha-\beta relation) has not been quantified yet. We perform a large set of numerical experiments in order to quantify the slope and zero-point of the \alpha-\beta relation. We find strong indication that the relation is indeed an attractor. When combined with the assumption of phase-space being a power-law in radius, this allows us to conclude that equilibrated dark matter structures indeed have zero central velocity anisotropy \beta_0 = 0, central density slope of \alpha_0 = -0.8, and outer anisotropy of \beta_\infty = 0.5.
Using HST images, we separate the bulge-like (pbulge) and disk-like (pdisk) components of 71 galaxies in the rich cluster MS1054-03 and of 21 in the field. Our key finding is that luminous pbulges are very red with restframe U-B ~ 0.45$, while predicted colors are bluer by 0.20 mag. Moreover, these very red colors appear to be independent of environment, pbulge luminosity, pdisk color, and pbulge fraction. These results challenge any models of hierarchical galaxy formation that predict the colors of distant (z ~ 0.8) luminous field and cluster bulges would differ. Our findings also disagree with other claims that 30% to 50% of bright bulges and ellipticals at z ~ 1 are very blue (U-B < 0).
XTE J1739-302 is a transient X-ray source with unusually short outbursts, lasting on the order of hours. Here we give a summary of X-ray observations we have made of this object in outburst with the Rossi X-ray Timing Explorer (RXTE) and at a low level of activity with the Chandra X-ray Observatory, as well as observations made by other groups. Visible and infrared spectroscopy of the mass donor of XTE J1739-302 are presented in a companion paper. The X-ray spectrum is hard both at low levels and in outburst, but somewhat variable, and there is strong variability in the absorption column from one outburst to another. Although no pulsation has been observed, the outburst data from multiple observatories show a characteristic timescale for variability on the order of 1500-2000 s. The Chandra localization (right ascension 17h 39m 11.58s, declination -30o 20' 37.6'', J2000) shows that despite being located less than 2 degrees from the Galactic Center and highly absorbed, XTE J1739-302 is actually a foreground object with a bright optical counterpart. The combination of a very short outburst timescale and a supergiant companion is shared with several other recently-discovered systems, forming a class we designate as Supergiant Fast X-ray Transients (SFXTs). Three persistently bright X-ray binaries with similar supergiant companions have also produced extremely short, bright outbursts: Cyg X-1, Vela X-1, and 1E 1145.1-6141.
We investigate the rotational equilibrium state of a disk accreting magnetized stars using axisymmetric magnetohydrodynamic (MHD) simulations. In this ``locked'' state, the spin-up torque balances the spin-down torque so that the net average torque on the star is zero. We investigated two types of initial conditions, one with a relatively weak stellar magnetic field and a high coronal density, and the other with a stronger stellar field and a lower coronal density. We observed that for both initial conditions the rotation of the star is locked to the rotation of the disk. In the second case, the radial field lines carry significant angular momentum out of the star. However, this did not appreciably change the condition for locking of the rotation of the star. We find that in the equilibrium state the corotation radius $r_{co}$ is related to the magnetospheric radius $r_A$ as $r_{co}/r_A\approx 1.2-1.3$ for case (1) and $r_{co}/r_A\approx 1.4-1.5$ for case (2). We estimated periods of rotation in the equilibrium state for classical T Tauri stars, dwarf novae and X-ray millisecond pulsars.
We present a new radiative transfer code for axi-symmetric stellar atmospheres and compare test results against 1D and 2D models with and without velocity fields. The code uses the short characteristic method with modifications to handle axi-symmetric and non-monotonic 3D wind velocities, and allows for distributed calculations. The formal solution along a characteristic is evaluated with a resolution that is proportional to the velocity gradient along the characteristic. This allows us to accurately map the variation of the opacities and emissivities as a function of frequency and spatial coordinates, but avoids unnecessary work in low velocity regions. We represent a characteristic with an impact-parameter vector p (a vector that is normal to the plane containing the characteristic and the origin) rather than the traditional unit vector in the direction of the ray. The code calculates the incoming intensities for the characteristics by a single latitudinal interpolation without any further interpolation in the radiation angles. Using this representation also provides a venue for distributed calculations since the radiative transfer can be done independently for each p.
We report detailed studies of the X-ray emission from the kpc scale jet in the nearest active galaxy, Cen A. 41 compact sources were found within the jet, 13 of which were newly identified. We construct the luminosity function for the detected jet-knots and argue that the remaining emission is most likely to be truly diffuse, rather than resulting from the pile-up of unresolved faint knots. The transverse jet profile reveals that the extended emission has the intensity peak at the jet boundaries. We note that limb-brightened jet morphologies have been observed previously at radio frequencies in some jet sources, but never so clearly at higher photon energies. Our result therefore supports a stratified jet model, consisting of a relativistic outflow including a boundary layer with a velocity shear. In addition, we found that the X-ray spectrum of the diffuse component is almost uniform across and along the jet. We discuss this spectral behavior within a framework of shock and stochastic particle acceleration processes. We note some evidence for a possible spectral hardening at the outer sheath of the jet. Due to the limited photon statistics of the present data, further deep observations of Cen A are required to determine the reality of this finding, however we note that the existence of the hard X-ray features at outer jet boundaries would provide an important challenge to theories for the evolution of ultra-relativistic particles within the jets.
Using the near-infrared spectrometer SpeX and its slit-viewing camera at the IRTF, I have resolved a low-mass member of the Upper Scorpius OB association into a double star. From K-band images of the pair, DENIS-P J161833.2-251750.4 A and B, I measure a separation of 0.96" and a magnitude difference of dK=0.42 mag. I present resolved 0.8-2.5 micron spectroscopy of the two objects, both of which exhibit signatures of youth in the shape of their H- and K-band continua, demonstrating that both are members of Upper Scorpius rather than field stars. In addition, through a comparison to optically-classified pre-main-sequence objects, I derive a spectral type near M5 for each component, corresponding to a mass of ~0.15 Msun with the evolutionary models of Chabrier and Baraffe. The probability that this pair is composed of unrelated M-type members of Upper Scorpius is ~10^-5. When added to the recent discoveries of other wide, easily disrupted low-mass binaries, this new system further establishes that the formation of low-mass stars and brown dwarfs does not require ejection from multiple systems. These observations also indicate that wide low-mass binaries can form in OB associations as well as in smaller clusters where the previously known wide pairs have been found. Thus, the available data show no perceptible effect of star-forming environment on the prevalence of loosely bound low-mass systems.
We calculate the photon energy dependence of the pulsed amplitude of neutron star (NS) surface modes. Simple approximations demonstrate that it depends most strongly on the bursting NS surface temperature. This result compares well with full integrations that include Doppler shifts from rotation and general relativistic corrections to photon propagation. We show that the energy dependence of type I X-ray burst oscillations agrees with that of a surface mode, lending further support to the hypothesis that they originate from surface waves. The energy dependence of the pulsed emission is rather insensitive to the NS inclination, mass and radius, or type of mode, thus hindering constraints on these parameters. We also show that, for this energy-amplitude relation, the majority of the signal (relative to the noise) comes in the 2-25 keV band, so that the current burst oscillation searches with the Rossi X-Ray Timing Explorer are close to optimal. The critical test of the mode hypothesis for X-ray burst oscillations would be a measurement of the energy dependence of burst oscillations from an accreting millisecond pulsar.
Using the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope, we have obtained mid-infrared photometry for 25 and 18 low-mass members of the IC 348 and Chamaeleon I star-forming clusters, respectively (>M6, M<=0.08 Msun). We find that 42+/-13% and 50+/-17% of the two samples exhibit excess emission indicative of circumstellar disks. In comparison, the disk fractions for stellar members of these clusters are 33+/-4% and 45+/-7% (M0-M6, 0.7 Msun>=M>=0.1 Msun). The similarity of the disk fractions of stars and brown dwarfs is consistent with a common formation mechanism and indicates that the raw materials for planet formation are available around brown dwarfs as often as around stars.
We have derived a new age estimate for the nearby young star AB Dor and have investigated the resulting implications for testing theoretical evolutionary models with the data reported by Close and coworkers for the low-mass companion AB Dor C. Using color-magnitude diagrams, we find that the AB Dor moving group is roughly coeval with the Pleiades (100-125 Myr) and is clearly older than IC 2391 (35-50 Myr). In fact, based on a comparison of the kinematics of AB Dor and the Pleiades, we suggest that the stars identified by Zuckerman and coworkers as members of a moving group with AB Dor are remnants of the large scale star-formation event that formed the Pleiades. Using the age of 50+50/-20 Myr adopted by Close, the luminosity predicted by the models of Chabrier and Baraffe for AB Dor C is larger than the value reported by Close, but is still within the quoted uncertainties. Meanwhile, the agreement is good when our age estimate for AB Dor C is adopted. Thus, we find no evidence in the data presented by Close for AB Dor C to suggest that previous studies using the models of Chabrier and Baraffe and bolometric luminosity as the mass indicator have significantly underestimated the masses of young low-mass stars and brown dwarfs.
We assess the constraints on the evolutionary models of young low-mass objects that are provided by the measurements of the companion AB Dor C by Close and coworkers and by a new comparison of model-derived IMFs of star-forming regions to the well-calibrated IMF of the solar neighborhood. After performing an independent analysis of Close's imaging and spectroscopic data for AB Dor C, we find that AB Dor C is not detected at a significant level (SN 1.2) in the SDI images when one narrow-band image is subtracted from another, but that it does appear in the individual SDI frames as well as the images at JHK. Using the age of 75-150 Myr for AB Dor from Luhman, Stauffer, & Mamajek, the luminosity predicted by the models of Chabrier & Baraffe is consistent with the value that we estimate. We measure a spectral type of M6+/-1 from the K-band spectrum of AB Dor C, which is earlier than the value of M8+/-1 from Close and is consistent with the model predictions when a dwarf temperature scale is adopted. In a test of these models at much younger ages, we show that the low-mass IMFs that they produce for star-forming regions are similar to the IMF of the solar neighborhood. If the masses of the low-mass stars and brown dwarfs in these IMFs of star-forming regions were underestimated by a factor of two as suggested by Close, then the IMF characterizing the current generation of Galactic star formation would have to be radically different from the IMF of the solar neighborhood.
We present the results of our study of the emission from the transient burster MX 0836-42 using its observations by the INTEGRAL and RXTE X-ray and gamma-ray observatories in the period 2003-2004. The source's broadband X-ray spectrum in the energy range 3-120 keV has been obtained and investigated for the first time. We have detected 39 X-ray bursts from this source. Their analysis shows that the maximum 3-20 keV flux varies significantly from burst to burst, F ~ (0.5-1.5)x10^{-8} erg cm^{-2} s^{-1}. Using the flux at the maximum of the brightest detected burst, we determined an upper limit for the distance to the source, D ~ 8 kpc.
(abridged) Numerical simulations of binary neutron stars by Wilson, Mathews, and Marronetti indicated that neutron stars that are stable in isolation can be made to collapse to black holes when placed in a binary. This claim was surprising as it ran counter to the Newtonian expectation that a neutron star in a binary should be more stable, not less. After correcting an error found by Flanagan, Wilson and Mathews found that the compression of the neutron stars was significantly reduced but not eliminated. This has motivated us to ask the following general question: Under what circumstances can general relativistic tidal interactions cause an otherwise stable neutron star to be compressed? We have found that if a non-rotating neutron star possess a current quadrupole moment, interactions with a gravitomagnetic tidal field can lead to a compressive force on the star. If this current quadrupole is induced by the gravitomagnetic tidal field, it is related to the tidal field by an equation-of-state-dependent constant called the gravitomagnetic Love number. This is analogous to the Newtonian Love number that relates the strength of a Newtonian tidal field to the induced mass quadrupole moment of a star. The compressive force is almost never larger than the Newtonian tidal interaction that stabilizes the neutron star against collapse. In the case in which a current quadrupole is already present in the star (perhaps as an artifact of a numerical simulation), the compressive force can exceed the stabilizing one, leading to a net increase in the central density of the star. This increase is small (<~1%) but could, in principle, cause gravitational collapse in a star that is close to its maximum mass.
In all four microquasars which show double peak kHz QPOs, the ratio of the two frequencies is 3:2. This strongly supports the suggestion that twin peak kHz QPOs are due to a resonance between some modes of accretion disk oscillations. Here, we stress that fits to observations of the hypothetical resonances between vertical and radial epicyclic frequencies (particularly of the parametric resonance) give an accurate estimate of the spin for the three microquasars with known mass. Measurement of double peak QPOs frequencies in the Galaxy centre seems also to be consistent with the 3:2 ratio established by previous observations in microquasars, however the Sgr A* data are rather difficult for the same exact analysis. If confirmed, the 3:2 ratio of double peak QPOs in Sgr A* would be of a fundamental importance for the black hole accretion theory and the precise measurement could help to solve the question of QPOs nature.
We present first results from our Very Large Telescope large program to study the dynamical evolution of Ultraluminous Infrared Galaxies (ULIRGs), which are the products of mergers of gas-rich galaxies. The full data set consists of high resolution, long-slit, H- and K-band spectra of 38 ULIRGs and 12 QSOs (between 0.042<z<0.268). In this paper, we present the sources that have not fully coalesced, and therefore have two distinct nuclei. This sub-sample consists of 21 ULIRGs, the nuclear separation of which varies between 1.6 and 23.3 kpc. From the CO bandheads that appear in our spectra, we extract the stellar velocity dispersion, sigma, and the rotational velocity, V_rot. The stellar dispersion equals 142 km/s on average, while V_rot is often of the same order. We combine our spectroscopic results with high-resolution infrared (IR) imaging data to study the conditions for ULIRG activity in interacting pairs. We find that the majority of ULIRGs are triggered by almost equal-mass major mergers of 1.5:1 average ratio. Less frequently, 3:1 encounters are also observed in our sample. However, less violent mergers of mass ratio >3:1 typically do not force enough gas into the center to generate ULIRG luminosities.
We have developed a complete model of the hydrogen molecule as part of the spectral simulation code Cloudy. Our goal is to apply this to spectra of high-redshift star-forming regions where H2 absorption is seen, but where few other details are known, to understand its implication for star formation. The microphysics of H2 is intricate, and it is important to validate these numerical simulations in better-understood environments. This paper studies a well-defined line-of-sight through the Galactic interstellar medium (ISM) as a test of the microphysics and methods we use. We present a self-consistent calculation of the observed absorption-line spectrum to derive the physical conditions in the ISM towards HD185418, a line-of-sight with many observables. We deduce density, temperature, local radiation field, cosmic ray ionization rate, chemical composition and compare these conclusions with conditions deduced from analytical calculations. We find a higher density, similar abundances, and require a cosmic ray flux enhanced over the Galactic background value, consistent with enhancements predicted by MHD simulations.
We present results from optical narrow-band lambda_c = 8150A ~ and Delta lambda = 120A) observations of the Great Observatories Origins Deep Survey (GOODS) fields, using Suprime-Cam on the Subaru Telescope. Using these narrow-band data, we then perform a survey of Lyman alpha Emitters (LAEs) at z~5.7. The LAE survey covers an area of approx 320 arcmin^2 and a co-moving volume of ~8.0 x 10^4 Mpc^3. We found a total of 10 (GOODS-N) and 4 (GOODS-S) LAE candidates at z~5.7. We perform a study of the spatial distribution, space density, and star formation properties of the LAEs at z~5.7.
We consider a class of five-dimensional cosmological solutions which contains two arbitrary function $\mu(t)$ and $\nu(t)$. We found that the arbitrary function $\mu(t)$ contained in the solutions can be rewritten in terms of the redshift $z$ as a new arbitrary function $f(z)$. We further showed that this new arbitrary function $f(z)$ could be solved out for four known parameterized equations of state of dark energy. Then the $5D$ models can be reconstructed and the evolution of the density and deceleration parameters of the universe can be determined.
The radio quiet neutron star 1E1207.4-5209 has been the target of a 260 ks XMM-Newton observation, which yielded, as a by product, an harvest of about 200 serendipitous X-ray sources above a limiting flux of 2E-15 erg/cm2/s, in the 0.3-8 keV energy range. In view of the intermediate latitude of our field (b~10 deg), it comes as no surprise that the logN-logS distribution of our serendipitous sources is different from those measured either in the Galactic Plane or at high galactic latitudes. Here we shall concentrate on the analysis of the brightest sources in our sample, which unveiled a previously unknown Seyfert-2 galaxy.
The weak X-ray transient XTE J1739-302, characterized by extremely short outbursts, has recently been identified with a reddened star. Here we present spectroscopy and photometry of the counterpart, identifying it as a O8Iab(f) supergiant at a distance of ~2.3 kpc. XTE J1739-302 becomes thus characterized as the prototype of the new class of Supergiant Fast X-ray Transients. The optical and infrared spectra of the counterpart to XTE J1739-302 do not reveal any obvious characteristics setting it apart from other X-ray binaries with supergiant companions, which display a very different type of X-ray lightcurve.
We present observations of the dusty emission from the young planetary nebulae Hen2-113 and CPD-56 obtained with VLT/NACO, VLTI/MIDI. The central stars of these two objects are Wolf-Rayet stars of the same spectral type [WC10]. They share an impressive number of characteristics and are located at similar distance, making the detection of any differences in their close environment of great interest. Hen2-113 exhibits a clear ring-like structure of about 0.4 arcsec in diameter, superimposed to a more diffuse environment visible in L', M' and 8.7 micron bands. No clear core could be detected for this object with MIDI through the N band. The dusty environment of CPD-568032 is much more compact, dominated by a bright, barely resolved, core whereas the visible nebula exhibits an amazing complexity. From MIDI 8.7 micron acquisition images (dominated by PAHs emission), the extension and geometry of the core have been estimated and compared to the STIS/HST observations (De Marco et al., 1997 and 2002). Moreover, high SNR fringes at low level have been detected with projected baselines between 40 and 45 meters. This clear signal is interpreted in terms of the bright inner rim of a dusty disk exposed to the flux from the Wolf-Rayet star. The geometrical parameters of the N band flux distribution are well constrained by means of simple geometrical models and a simple radiative transfer model has been developed to extract the physical parameters of the disk.
This paper discusses Swift observations of the gamma-ray burst GRB 050315 (z=1.949) from 80 s to 10 days after the onset of the burst. The X-ray light curve displayed a steep early decay (t^-5) for ~200 s and several breaks. However, both the prompt hard X-ray/gamma-ray emission (observed by the BAT) and the first ~ 300 s of X-ray emission (observed by the XRT) can be explained by exponential decays, with similar decay constants. Extrapolating the BAT light curve into the XRT band suggests the rapidly decaying, early X-ray emission was simply a continuation of the fading prompt emission; this strong similarity between the prompt gamma-ray and early X-ray emission may be related to the simple temporal and spectral character of this X-ray rich GRB. The prompt (BAT) spectrum was a steep down to 15 keV, and appeared to continue through the XRT bandpass, implying a low peak energy, inconsistent with the Amati relation. Following the initial steep decline the X-ray afterglow did not fade for ~1.2*10^4 s, after which time it decayed with a temporal index of alpha ~ 0.7, followed by a second break at ~2.5*10^5 s to a slope of alpha ~ 2. The apparent `plateau' in the X-ray light curve, after the early rapid decay, makes this one of the most extreme examples of the steep-flat-steep X-ray light curves revealed by Swift. If the second afterglow break is identified with a jet break then the jet opening angle was theta_0 ~ 5 deg, and implying E_gamma > 10^50 erg.
Recent theoretical calculations of stellar evolutionary tracks for rotating high-mass stars suggests that the chemical composition of the surface layers changes even whilst the star is evolving on the Main Sequence. The abundance analysis of binary components with precisely known fundamental stellar quantities allows a powerful comparison with theory. The observed spectra of close binary stars can be separated into the individual spectra of the component stars using the method of spectral disentangling on a time-series of spectra taken over the orbital cycle. Recently, Pavlovski & Hensberge (2005, A&A, 439, 309) have shown that, even with moderately high line-broadening, metal abundances can be derived from disentangled spectra with a precision of 0.1 dex. In a continuation of this project we have undertaken a detailed abundance analysis of the components of another two high-mass binaries, V453 Cyg, and V380 Cyg. Both binaries are well-studied systems with modern solutions. The components are close to the TAMS and therefore very suitable for an observational test of early mixing in high-mass stars.
Some of the Galactic outer halo globular clusters are excellent tools to probe gravitational theories in the regime of weak accelerations (Baumgardt et al. 2005). The measurement of the line-of-sight velocity dispersion among stars in these clusters will differentiate between the validity of Newtonian dynamics (low velocity dispersion) and the possiblity of modified Newtonian dynamics (MOND) or dark matter dominated globular clusters (high velocity dispersion). In this paper, the properties of probable member stars of the three best-case gravitational theory-testing clusters AM 1, Pal 3 and Pal 14 are presented. The member selection is based on VLT photometry in Johnson BV. The positions of the stars were determined with an accuracy of the order <=0.2 arcsec, allowing their direct use for follow-up spectroscopy. The distance, reddening, age, and metallicities of the clusters were estimated from isochrone fitting. Furthermore, improved structural parameters, like central coordinates, ellipticit
In view of recent developments attention is directed again at two aspects of the well known 'knee' in the cosmic ray energy spectrum at 3 PeV: the mass of the predominant particles at this energy and their source. It is inevitable in a subject such as this that ideas - and conclusions - evolve. Earlier, we had used a particular acceleration model and the nature of the local ISM to infer that the particles are mainly oxygen nuclei. Initially no specific source was identified. More recently, however, we have specified the Monogem Ring supernova remnant (SNR) as the likely source; this is at just the right distance and age and the energies are reasonable. Concerning the mass composition at the knee, a quantity more difficult to determine, recent direct measurements, which extend to higher energies than hitherto, show a likely flattening in the spectrum above 10^4 GeV/nucleon for He-nuclei, a flattening which, if extrapolated to higher energies, would meet the measured spectrum in the knee region. The other nuclei do not show this feature. He-nuclei in the knee region would also be marginally consistent with KASCADE EAS data, although there are serious problems with EAS mass estimates in that experiment. Concerning the acceleration, recent models applied to the Monogem Ring SNR allow a satisfactory explanation in terms of either oxygen or helium, but with the latter being a distinct possibility and perhaps more likely.
The nonlinear dynamics of a warped accretion disc is investigated in the important case of a thin Keplerian disc with negligible viscosity and self-gravity. A one-dimensional evolutionary equation is formally derived that describes the primary nonlinear and dispersive effects on propagating bending waves other than parametric instabilities. It has the form of a derivative nonlinear Schroedinger equation with coefficients that are obtained explicitly for a particular model of a disc. The properties of this equation are analysed in some detail and illustrative numerical solutions are presented. The nonlinear and dispersive effects both depend on the compressibility of the gas through its adiabatic index Gamma. In the physically realistic case Gamma<3, nonlinearity does not lead to the steepening of bending waves but instead enhances their linear dispersion. In the opposite case Gamma>3, nonlinearity leads to wave steepening and solitary waves are supported. The effects of a small effective viscosity, which may suppress parametric instabilities, are also considered. This analysis may provide a useful point of comparison between theory and numerical simulations of warped accretion discs.
We investigate the long-term evolution and observability of remnants originating from the merger of compact binary systems and discuss the differences to supernova remnants. Compact binary mergers expel much smaller amounts of mass at much higher velocities, as compared to supernovae and therefore the free expansion phase of the remnant will be short (~ 1 - 10 yr). In general the remnants will be observable for a considerable time (~ 10^6 - 10^7 yr). Events releasing large amounts of kinetic energy may be responsible for a subsample of observed giant HI holes of unknown origin as compact binaries merge far away from star forming regions. If the ejecta consist primarily of actinides, on long timescales the expelled material will contain mainly the few quasi-stable nuclei in the actinides range. Consequently the abundance of each isotope in the ejecta might be of the order of a few percent. During their decay some actinides will produce observational signatures in form of gamma ray lines. We particularly investigate the gamma ray emission of Am 243, Cm 247, Cm 248 and Bi 208 and estimate their observability in nearby remnants. Detections of the gamma ray lines with INTEGRAL will be possible only in very advantageous cases but these remnants are promising targets for future instruments using focusing optics for soft gamma rays. Due to the low mass expelled in mergers and due to the lack of free electrons in the ejecta, the merger remnants might be significantly fainter in bremsstrahlung and synchrotron radiation than comparable supernova remnants. Hence merger remnants might represent a candidate for very recently discovered 'dark accelerators' which are hard gamma ray sources with no apparent emission in other bands.
We give an introduction to interferometrical concepts and their applicability to Be stars. The first part of the paper concentrates on a short historic overview and basic principles of two-beam interferometric observations. In the second part, the VLTI/MIDI instrument is introduced and its first results on Be stars, obtained on alpha Ara and delta Cen, are outlined.
The thermal emission from isolated neutron stars is not well understood. The X-ray spectrum is very close to a blackbody but there is a systematic optical excess flux with respect to the extrapolation to low energy of the best blackbody fit. This fact, in combination with the observed pulsations in the X-ray flux, can be explained by anisotropies in the surface temperature distribution.We study the thermal emission from neutron stars with strong magnetic fields in order to explain the origin of the anisotropy. We find (numerically) stationary solutions in axial symmetry of the heat transportequations in the neutron star crust and the condensed envelope. The anisotropy in the conductivity tensor is included consistently. The presence of magnetic fields of the expected strength leads to anisotropy in the surface temperature. Models with toroidal components similar to or larger than the poloidal field reproduce qualitatively the observed spectral properties and variability of isolated neutron stars. Our models also predict spectral features at energies between 0.2 and 0.6 keV.
Motivated by theoretical predictions that first stars were predominantly very massive, we investigate the physics of the transition from an early epoch dominated by massive Pop III stars to a later epoch dominated by familiar low-mass Pop II/I stars by means of a numerically-generated catalogue of dark matter halos coupled with a self-consistent treatment of chemical and radiative feedback. Depending on the strength of the chemical feedback, Pop III stars can contribute a substantial fraction (several percent) of the cosmic star formation activity even at moderate redshifts, z = 5. We find that the three z = 10 sources tentatively detected in NICMOS UDFs should be powered by Pop III stars, if these are massive; however, this scenario fails to reproduce the derived WMAP electron scattering optical depth. Instead, both the UDFs and WMAP constraints can be fulfilled if stars at any time form with a more standard, slightly top-heavy, Larson IMF in the range 1 Msun < M < 100 Msun.
Far-ultraviolet (900-1200A) spectral synthesis of nine giant extragalactic HII regions in M 33 and M 101 is performed to study their massive stellar content. Several parameters are quantified, predicted, and compared to the literature: age, stellar mass, IMF slope, number of O-type and Wolf-Rayet stars, Halpha and 5500A continuum fluxes. The results of this particular technique are consistent with other methods and observations. This work shows that a total stellar mass of a few 10^3 Msun is needed to populate the IMF bins well enough at high masses to obtain accurate results from the spectral synthesis technique in the far-ultraviolet. A flat IMF slope seems to characterize better the stellar line profiles of these objects, which is likely the first sign of a small number statistic effect on the IMF. Finally, the HII region NGC 5461 is identified as a good candidate for hosting a second generation of stars, not yet seen at far-ultraviolet wavelengths.
We present a new mechanism for core-collapse supernova explosions that relies upon acoustic power generated in the inner core as the driver. In our simulation using an 11-solar-mass progenitor, a strong advective-acoustic oscillation a la Foglizzo with a period of ~25-30 milliseconds (ms) arises ~200 ms after bounce. Its growth saturates due to the generation of secondary shocks, and kinks in the resulting shock structure funnel and regulate subsequent accretion onto the inner core. However, this instability is not the primary agent of explosion. Rather, it is the acoustic power generated in the inner turbulent region and most importantly by the excitation and sonic damping of core g-mode oscillations. An l=1 mode with a period of ~3 ms grows to be prominent around ~500 ms after bounce. The accreting protoneutron star is a self-excited oscillator. The associated acoustic power seen in our 11-solar-mass simulation is sufficient to drive the explosion. The angular distribution of the emitted sound is fundamentally aspherical. The sound pulses radiated from the core steepen into shock waves that merge as they propagate into the outer mantle and deposit their energy and momentum with high efficiency. The core oscillation acts like a transducer to convert accretion energy into sound. An advantage of the acoustic mechanism is that acoustic power does not abate until accretion subsides, so that it is available as long as it may be needed to explode the star. [abridged]
A review of the literature reveals that while parallel computing is sometimes employed by astronomers for custom, large-scale calculations, no package fosters the routine application of parallel methods to standard problems in astronomical data analysis. This paper describes our attempt to close that gap by wrapping the Parallel Virtual Machine (PVM) as a scriptable S-Lang module. Using PVM within ISIS, the Interactive Spectral Interpretation System, we've distributed a number of representive calculations over a network of 25+ CPUs to achieve dramatic reductions in execution times. We discuss how the approach applies to a wide class of modeling problems, outline our efforts to make it more transparent for common use, and note its growing importance in the context of the large, multi-wavelength datasets used in modern analysis.
Thanks to observations at the ESO VLT with the GIRAFFE multifibers spectrograph, we have obtained spectra of 177 and 346 B-type stars in the Large and Small Magellanic Clouds respectively. We have discovered 25 and 90 new Be stars among the 47 and 131 Be stars observed in the LMC and SMC respectively. We have determined the fundamental parameters of these stars and examined the effect of the metallicity, star formation conditions and evolution on the behaviour of the rotational velocities. We discovered that only the future B-type stars with a strong rotational velocity in ZAMS will become a Be star. We have also discovered short term variability in several SMC Be stars thanks to a cross-correlation with the MACHO database.
We present optical spectroscopy of the emission line star BAL 224 (V=17.3, B-V=0.46). This star also named KWBBE 485, [MA93]906 is located at the periphery of the young SMC cluster NGC 330; it is known as a photometric variable with a possible period around 1 day (Balona 1992). Furthermore it was reported as the optical counterpart of the prominent mid-infrared source (MIR1) by Kucinskas et al. (2000), indicating the presence of a dust shell. The star was included in a sample of B-type stars observed using the ESO VLT-FLAMES facilities. The presence of emission lines such as Fe II,[Fe II], [S II] make this object like a B[e] star. The H$ alpha$, H$ gamma$ and H$ delta$ lines show an asymmetrical double-peaked emission profile suggesting the presence of an accretion disk. Moreover the MACHO and OGLE light curves were analyzed; in addition to a long-term variability ($ simeq$ 2300d), a short period very close to 1 day has been detected using different methods, confirming the variability previously reported by Balona (1992). Finally the nature of this object is reconsidered.
In a previous paper (Ouyed et al. 2004) we presented a new model for soft gamma-ray repeaters (SGR), based on the onset of colour superconductivity in quark stars. In this model, the bursts result from the reorganization of the exterior magnetic field following the formation of vortices that confine the internal magnetic field (the \mz effect). Here we extend the model by presenting full 3-dimensional simulations of the evolution of the inclined exterior magnetic field immediately following vortex formation. The simulations capture the violent reconnection events in the entangled surface magnetic field as it evolves into a smooth, more stable, configuration which consists of a dipole field aligned with the star's rotation axis. The total magnetic energy dissipated in this process is found to be of the order of $10^{44} \mathrm{erg}$ and, if it is emitted as synchrotron radiation, peaks typically at $280 \mathrm{keV}$. The intensity decays temporally in a way resembling SGRs and AXPs (anomalous X-ray pulsars), with a tail lasting from a few to a few hundred times the rotation period of the star, depending on the initial inclination between the rotation and dipole axis. One of the obvious consequences of our model's final state (aligned rotator) is the suppression of radio-emission in SGRs and AXPs following their bursting era. We suggest that magnetar-like magnetic field strength alone cannot be responsible for the properties of SGRs and AXPs, while a quark star entering the ``Meissner phase'' is compatible with the observational facts. We compare our model to observations and highlight our predictions.
In this comment, we show explicitly that the phenomenological model in which the quintessence field depends linearly on the energy density of the spatial curvature can provide acceleration for the universe, contrary to recent claims it could not.
Recent Chandra and XMM-Newton observations of a number of X-ray ``dim'' pulsating neutron stars revealed quite unexpected features in the emission from these sources. Their soft thermal spectrum, believed to originate directly from the star surface, shows evidence for a phase-varying absorption line at some hundred eVs. The pulse modulation is relatively large (pulsed fractions in the range ~12%-35%), the pulse shape is often non-sinusoidal, and the hard X-ray color appears to be anti-correlated in phase with the total emission. Moreover, the prototype of this class, RX J0720.4-3125, has been found to undergo rather sensible changes both in its spectral and timing properties over a timescale of a few years. All these new findings seem difficult to reconcile with the standard picture of a cooling neutron star endowed with a purely dipolar magnetic field, at least if surface emission is produced in an atmosphere on top of the crust. In this paper we explore how a dipolar+quadrupolar star-centered field influence the properties of the observed lightcurves. The phase-resolved spectrum has been evaluated accounting for both radiative transfer in a magnetized atmosphere and general relativistic ray-bending. We computed over 78000 lightcurves varying the quadrupolar components and the viewing geometry. A comparison of the data with our model indicate that higher order multipoles are required to reproduce the observations.
Central black hole masses for 118 spiral galaxies representing morphological
stages S0/a through Sc and taken from the large spectroscopic survey of Ho,
Filippenko & Sargent (1997) are derived using 2MASS Ks data. Black hole (BH)
masses are found using a calibrated black-hole - Ks bulge luminosity relation,
while bulge luminosities are measured using GALFIT, a two-dimensional
bulge/disk decomposition routine.
The BH masses are correlated against a variety of nuclear and host-galaxy
properties. Nuclear properties such as line width and line ratios show a very
high degree of correlation with BH mass. The excellent correlation with
line-width supports the view that the emission-line gas is in virial
equilibrium with either the BH or bulge potential. The very good emission-line
ratio correlations may indicate a change in ionizing continuum shape with BH
mass in the sense that more massive BHs generate harder spectra.
Apart from the inclination-corrected rotational velocity, no excellent
correlations are found between BH mass and host-galaxy properties.
Significant differences are found between the distributions of BH masses in
early-, mid- and later-type spiral galaxies in the sense that early-type
galaxies have preferentially larger central BHs. The line-width distributions
show a marked difference among the subsamples in the sense that earlier-type
galaxies have larger line widths. There are also clear differences in line
ratios between subsamples likely related to the level of ionization in the gas.
Finally, a Ks-band Simien & de Vaucouleurs diagram shows excellent agreement
with the original B-band relation.
The effects of rapid rotation and bi--stability upon the density contrast between the equatorial and polar directions of a B[e] supergiant are investigated. Based on a new slow solution for different high rotational radiation--driven winds and the fact that bi--stability allows a change in the line--force parameters ($\alpha$, $k$, and $\delta$), the equatorial densities are about $10^2$--$10^3$ times higher than the polar ones. These values are in qualitative agreement with the observations. This calculation also permits to obtain the aperture angle of the disk.