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astro-ph new abstracts, Mon, 24 Apr 06 00:00:10 GMT
0604441 -- 0604476 received

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astro-ph/0604441 [abs, ps, pdf, other] :

Title: Cosmic Neutrinos and the Energy Budget of Galactic and Extragalactic Cosmic Rays
Authors: Francis Halzen
Comments: 13 pages, Latex2e, 3 postscript figures included. Talk presented at the International Workshop on Energy Budget in the High Energy Universe, Kashiwa, Japan, February 2006

Although kilometer-scale neutrino detectors such as IceCube are discovery instruments, their conceptual design is very much anchored to the observational fact that Nature produces protons and photons with energies in excess of 10^{20} eV and 10^{13} eV, respectively. The puzzle of where and how Nature accelerates the highest energy cosmic particles is unresolved almost a century after their discovery. We will discuss how the cosmic ray connection sets the scale of the anticipated cosmic neutrino fluxes. In this context, we discuss the first results of the completed AMANDA detector and the science reach of its extension, IceCube.

 

astro-ph/0604442 [abs, ps, pdf, other] :

Title: The Survey for Ionization in Neutral Gas Galaxies- II. The Star Formation Rate Density of the Local Universe
Authors: D.J. Hanish, G.R. Meurer, H.C. Ferguson, M.A. Zwaan, T.M. Heckman, L. Staveley-Smith, J. Bland-Hawthorn, V.A. Kilborn, B.S. Koribalski, M.E. Putman, E.V. Ryan-Weber, M.S. Oey, R.C. Kennicutt, Jr., P.M. Knezek, M.J. Meyer, R.C. Smith, R.L. Webster, M.A. Dopita, M.T. Doyle, M.J. Drinkwater, K.C. Freeman, J.K. Werk
Comments: 15 pages, 5 figures

We derive observed Halpha and R band luminosity densities of an HI-selected sample of nearby galaxies using the SINGG sample to be l_Halpha' = (9.4 +/- 1.8) x 10^38 h_70 erg s^-1 Mpc^-3 for Halpha and l_R' = (4.4 +/- 0.7) x 10^37 h_70 erg s^-1 A^-1 Mpc^-3 in the R band. This R band luminosity density is approximately 70% of that found by the Sloan Digital Sky Survey. This leads to a local star formation rate density of log(SFRD) = -1.80 +0.13/-0.07(random) +/- 0.03(systematic) + log(h_70) after applying a mean internal extinction correction of 0.82 magnitudes. The gas cycling time of this sample is found to be t_gas = 7.5 +1.3/-2.1 Gyr, and the volume-averaged equivalent width of the SINGG galaxies is EW(Halpha) = 28.8 +7.2/-4.7 A (21.2 +4.2/-3.5 A without internal dust correction). As with similar surveys, these results imply that SFRD(z) decreases drastically from z ~ 1.5 to the present. A comparison of the dynamical masses of the SINGG galaxies evaluated a their optical limits with their stellar and HI masses shows significant evidence of downsizing: the most massive galaxies have a larger fraction of their mass locked up in stars compared with HI, while the opposite is true for less massive galaxies. We show that the application of the Kennicutt star formation law to a galaxy having the median orbital time at the optical limit of this sample results in a star formation rate decay with cosmic time similar to that given by the SFRD(z) evolution. This implies that the SFRD(z) evolution is primarily due to the secular evolution of galaxies, rather than interactions or mergers. This is consistent with the morphologies predominantly seen in the SINGG sample.

 

astro-ph/0604443 [abs, ps, pdf, other] :

Title: How Rare is the Bullet Cluster?
Authors: Eric Hayashi, Simon D.M. White (MPA Garching)
Comments: 5 pages, 3 figures, accepted for publication in MNRAS

The galaxy cluster 1E 0657-56 has a bullet-like subcluster that is moving away from the centre of the main cluster at high speed. Markevitch et al. (2004) recently estimated a relative velocity of V_bullet = 4500 +1100/-800 km/s, based on observations of the bow shock in front of the subcluster. The weak lensing analysis of Clowe et al. (2004) indicates that a substantial secondary mass peak is associated with this subcluster. We estimate the likelihood of such a configuration by examining the distribution of subhalo velocities for clusters in the Millennium Run, a large LCDM cosmological simulation. We find that the most massive subhalo has a velocity as high as that of the bullet subcluster in only about 1 out of every 100 cluster-sized halos. This estimate is strongly dependent on the precise velocity adopted for the bullet. One of the ten most massive subhalos has such a high velocity about 40% of the time. We conclude that the velocity of the bullet subcluster is not exceptionally high for a cluster substructure, and can be accommodated within the currently favoured LCDM comogony.

 

astro-ph/0604444 [abs, ps, pdf, other] :

Title: The Survey for Ionization in Neutral Gas Galaxies: I. Description and Initial Results
Authors: Gerhardt R. Meurer, D.J. Hanish, H.C. Ferguson, P.M. Knezek, M.E. Putman, R.C. Smith, B. Koribalski, M. Meyer, M.S. Oey, E.V. Ryan-Weber, M.A. Zwaan, T.M. Heckman, R.C. Kennicutt, Jr., J.C. Lee, R.L. Webster, J. Bland-Hawthorn M.A. Dopita, K.C. Freeman, M.T. Doyle, M.J. Drinkwater, L. Staveley-Smith, J. Werk
Comments: 28 pages, ApJS, in press. Full resolution version with all panels of Fig. 8 available at this http URL . On line data available at this http URL

We introduce the Survey for Ionization in Neutral Gas Galaxies (SINGG), a census of star formation in HI-selected galaxies. The survey consists of H-alpha and R-band imaging of a sample of 468 galaxies selected from the HI Parkes All Sky Survey (HIPASS). The sample spans three decades in HI mass and is free of many of the biases that affect other star forming galaxy samples. We present the criteria for sample selection, list the entire sample, discuss our observational techniques, and describe the data reduction and calibration methods. This paper focuses on 93 SINGG targets whose observations have been fully reduced and analyzed to date. The majority of these show a single Emission Line Galaxy (ELG). We see multiple ELGs in 13 fields, with up to four ELGs in a single field. All of the targets in this sample are detected in H-alpha indicating that dormant (non-star forming) galaxies with M(HI) > ~3e7 M_sun are very rare. A database of the measured global properties of the ELGs is presented. The ELG sample spans four orders of magnitude in luminosity (H-alpha and R-band), and H-alpha surface brightness, nearly three orders of magnitude in R surface brightness and nearly two orders of magnitude in H-alpha equivalent width (EW). The surface brightness distribution of our sample is broader than that of the Sloan Digital Sky Survey spectroscopic sample, the (EW) distribution is broader than prism-selected samples, and the morphologies found include all common types of star forming galaxies (e.g. irregular, spiral, blue compact dwarf, starbursts, merging and colliding systems, and even residual star formation in S0 and Sa spirals). (abridged)

 

astro-ph/0604445 [abs, ps, pdf, other] :

Title: Low-Mass Binary Induced Outflows from Asymptotic Giant Branch Stars
Authors: J. Nordhaus (Univ. Rochester), E. G. Blackman (Univ. Rochester)
Comments: 24 pages, 6 figures, submitted to MNRAS

A significant fraction of planetary nebulae (PNe) and proto-planetary nebulae (PPNe) exhibit aspherical, axisymmetric structures, many of which are highly collimated. The origin of these structures is not entirely understood, however recent evidence suggests that many observed PNe harbor binary systems, which may play a role in their shaping. In an effort to understand how binaries may produce such asymmetries, we study the effect of low-mass (< 0.3 M_sun) companions (planets, brown dwarfs and low-mass main sequence stars) embedded into the envelope of a 3.0 M_sun star during three epochs of its evolution (Red Giant Branch, Asymptotic Giant Branch (AGB), interpulse AGB). We find that common envelope evolution can lead to three qualitatively different consequences: (i) direct ejection of envelope material resulting in a predominately equatorial outflow, (ii) spin-up of the envelope resulting in the possibility of powering an explosive dynamo driven jet and (iii) tidal shredding of the companion into a disc which facilitates a disc driven jet. We study how these features depend on the secondary's mass and discuss observational consequences.

 

astro-ph/0604446 [abs, ps, pdf, other] :

Title: Flux-transport dynamos with Lorentz force feedback on differential rotation and meridional flow: Saturation mechanism and torsional oscillations
Authors: Matthias Rempel (HAO/NCAR, Boulder CO, USA)
Comments: 15 pages, 12 figures, Accepted for publication in ApJ

In this paper we discuss a dynamic flux-transport dynamo model that includes the feedback of the induced magnetic field on differential rotation and meridional flow. We consider two different approaches for the feedback: meanfield Lorentz force and quenching of transport coefficients such as turbulent viscosity and heat conductivity. We find that even strong feedback on the meridional flow does not change the character of the flux-transport dynamo significantly; however it leads to a significant reduction of differential rotation. To a large degree independent from the dynamo parameters, the saturation takes place when the toroidal field at the base of the convection zone reaches between 1.2 an 1.5 T, the energy converted intomagnetic energy corresponds to about 0.1 to 0.2% of the solar luminosity. The torsional oscillations produced through Lorentz force feedback on differential rotation show a dominant poleward propagating branch with the correct phase relation to the magnetic cycle. We show that incorporating enhanced surface cooling of the active region belt (as proposed by Spruit 2003, Sol. Phys. 213, 1) leads to an equatorward propagating branch in good agreement with observations.

 

astro-ph/0604447 [abs, ps, pdf, other] :

Title: Implications of WMAP Three Year Data for Reionization
Authors: Marcelo A. Alvarez, Paul R. Shapiro, Kyungjin Ahn, Ilian T. Iliev
Comments: 4 pages, 3 figures, submitted to ApJ Letters

New results on the anisotropy of the cosmic microwave background (CMB) and its polarization based upon the first three years of data from the Wilkinson Microwave Anisotropy Probe (WMAP) have revised the electron scattering optical depth downward from tau_es= 0.17+/-0.07 to tau_es=0.09+/-0.03. This implies a shift of the effective reionization redshift from z_r~17 to z_r~11. Previous attempts to explain the high redshift of reionization inferred from the WMAP 1-year data have led to widespread speculation that the sources of reionization must have been much more efficient than those associated with the star formation observed at low redshift. This is consistent, for example, with the suggestion that early star formation involved massive, Pop III stars which early-on produced most of the ionizing radiation escaping from halos. It is, therefore, tempting to interpret the new WMAP results as implying that we can now relax those previous high demands on the efficiency of the sources of reionization and perhaps even turn the argument around as evidence against such high efficiency. We show that this is not the case, however. The new WMAP results also find that the primordial density fluctuation power spectrum has a lower amplitude, sigma_8, and departs substantially from the scale-invariant spectrum. We show that these effects combine to cancel the impact of the later reionization implied by the new value of tau_es on the required ionizing efficiency per collapsed baryon. The delay of reionization is surprisingly well-matched by a comparable delay (by a factor of ~1.4 in scale factor) in the formation of the halos responsible for reionization.

 

astro-ph/0604448 [abs, ps, pdf, other] :

Title: Astronomy: Trouble at first light
Authors: Piero Madau
Comments: 6 pages, 1 figure, Nature News and Views on Aharonian et al. 2006, Nature, 440, 1018-1021
Journal-ref: Nature 440 (2006), 1002-1003

The question of how much light the first stars produced is fundamental to models of the Universe's development. But observations have so far failed to agree: is the answer a lot, or not very much at all?

 

astro-ph/0604449 [abs, ps, pdf, other] :

Title: Color and Variability Characteristics of Point Sources in the Faint Sky Variability Survey
Authors: M. E. Huber, M. E. Everett, S. B. Howell
Comments: 34 pages, 16 figures, accepted in AJ

We present an analysis of the color and variability characteristics for point sources in the Faint Sky Variability Survey (FSVS). The FSVS cataloged ~23 square degrees in BVI filters from ~16--24 mag to investigate variability in faint sources at moderate to high Galactic latitudes. Point source completeness is found to be >83% for a selected representative sample (V=17.5--22.0 mag, B-V=0.0--1.5) containing both photometric B, V detections and 80% of the time-sampled V data available compared to a basic internal source completeness of 99%. Multi-epoch (10--30) observations in V spanning minutes to years modeled by light curve simulations reveal amplitude sensitivities to 0.015--0.075 mag over a representative V=18--22 mag range. Periodicity determinations appear viable to time-scales of an order 1 day or less using the most sampled fields (~30 epochs). The fraction of point sources is found to be generally variable at 5--8% over V=17.5--22.0 mag. For V brighter than 19 mag, the variable population is dominated by low amplitude (<0.05 mag) and blue (B-V<0.35) sources, possibly representing a population of gamma Doradus stars. Overall, the dominant population of variable sources are bluer than B-V=0.65 and have Main Sequence colors, likely reflecting larger populations of RR Lyrae, SX Phe, gamma Doradus, and W UMa variables.

 

astro-ph/0604450 [abs, ps, pdf, other] :

Title: First Year Performance of The IceCube Neutrino Telescope
Authors: The IceCube Collaboration

The first sensors of the IceCube neutrino observatory were deployed at the South Pole during the austral summer of 2004-05 and have been producing data since February 2005. One string of 60 sensors buried in the ice and a surface array of 8 ice Cherenkov tanks took data until December 2005 when deployment of the next set of strings and tanks began. We have analyzed these data, demonstrating that the performance of the system meets or exceeds design requirements. Times are determined across the whole array to a relative precision of better than 3 nanoseconds, allowing reconstruction of muon tracks and light bursts in the ice, of air-showers in the surface array and of events seen in coincidence by surface and deep-ice detectors separated by up to 2.5 km.

 

astro-ph/0604451 [abs, ps, pdf, other] :

Title: Solar differential rotation and meridional flow: The role of a subadiabatic tachocline for the Taylor-Proudman balance
Authors: Matthias Rempel (HAO/NCAR, Boulder CO, USA)
Comments: 15 pages, 7 figures
Journal-ref: Astrophys. J. 622 (2005) 1320-1332

We present a simple model for the solar differential rotation and meridional circulation based on a mean field parameterization of the Reynolds stresses that drive the differential rotation. We include the subadiabatic part of the tachocline and show that this, in conjunction with turbulent heat conductivity within the convection zone and overshoot region, provides the key physics to break the Taylor-Proudman constraint, which dictates differential rotation with contour lines parallel to the axis of rotation in case of an isentropic stratification. We show that differential rotation with contour lines inclined by 10 - 30 degrees with respect to the axis of rotation is a robust result of the model, which does not depend on the details of the Reynolds stress and the assumed viscosity, as long as the Reynolds stress transports angular momentum toward the equator. The meridional flow is more sensitive with respect to the details of the assumed Reynolds stress, but a flow cell, equatorward at the base of the convection zone and poleward in the upper half of the convection zone, is the preferred flow pattern.

 

astro-ph/0604452 [abs, ps, pdf, other] :

Title: Radial Kinematics of Isolated Elliptical Galaxies
Authors: George Hau, Duncan Forbes
Comments: 11 pages, 6 figures, MNRAS accepted

Ellipticals in very low density environments are extremely rare but hold important clues about galaxy formation and evolution. In this paper we continue our study of isolated elliptical galaxies, presenting results on the radial stellar kinematics for 13 isolated early-type galaxies. We derive radial rotation velocity, velocity dispersion and hermite terms to ~1 effective radius. We observe a dichotomy in kinematic properties similar to that in the elliptical population as a whole, where low luminosity ellipticals tend to be rotationally supported. For all galaxies the V/sigma ratio increases with radius. We find kinematically distinct cores (KDCs), or velocity substructure, in ~40% of the galaxies for which we have major axis spectra. Such a fraction is similar to that observed for ellipticals in higher density environments. Most galaxies in the sample reveal kinematic evidence for a nuclear disk. The non-relaxed kinematics in several galaxies suggests that they have undergone a merger or accretion event. Isolated ellipticals generally follow the fundamental plane defined by cluster ellipticals -- exceptions being those galaxies with evidence for young stellar populations. Overall, we find isolated ellipticals have similar kinematic properties to their counterparts in higher density environments.

 

astro-ph/0604453 [abs, ps, pdf, other] :

Title: Solar-like oscillations in the metal-poor subgiant nu Indi: constraining the mass and age using asteroseismology
Authors: Timothy R. Bedding, R. Paul Butler, Fabien Carrier, Francois Bouchy, Brendon J. Brewer, Patrick Eggenberger, Frank Grundahl, Hans Kjeldsen, Chris McCarthy, Tine Bjorn Nielsen, Alon Retter, Christopher G. Tinney
Comments: accpted for publication in ApJ

Asteroseismology is a powerful method for determining fundamental properties of stars. We report the first application to a metal-poor object, namely the subgiant star nu Ind. We measured precise velocities from two sites, allowing us to detect oscillations and infer a large frequency separation of Delta_nu = 24.25 +/- 0.25 microHz. Combining this value with the location of the star in the H-R diagram and comparing with standard evolutionary models, we were able to place constraints on the stellar parameters. In particular, our results indicate that nu Ind has a low mass (0.85 +/- 0.04 M_sun) and is at least 9 Gyr old.

 

astro-ph/0604454 [abs, ps, pdf, other] :

Title: Diffuse Far-ultraviolet Observation of the Lupus Loop Region
Authors: Jong-Ho Shinn, Kyoung Wook Min, Chi-Na Lee, Jerry Edelstein, Eric J. Korpela, Barry Y. Welsh, Wonyong Han, Uk-Won Nam, Ho Jin, Dae-Hee Lee
Comments: Accepted in ApJL, 4 pages, 3 figures

Diffuse far-ultraviolet (FUV) emissions from the Lupus Loop region (SNR 330.0+15.0) have been observed with Spectroscopy of Plasma Evolution from Astrophysical Radiation (SPEAR), also known as Far-ultraviolet IMaging Spectrograph (FIMS). We have detected several important ionic lines, including Si II*, C IV, and N IV], which characterize the warm, hot ionized gas in this region. The spatial variations in the line intensities of Si II* and C IV have also been studied in comparison with X-ray and dust observations. The result shows that they originate from the interface between the hot gas seen in the X-ray and the cooler H I shell with which dust is associated. The interface is rather diffuse, and the gases with different temperatures seem to co-exist in this region. A shock may exist upfront in the interface, but its velocity should be very small as no shock-related distinguished feature is seen in Ha.

 

astro-ph/0604455 [abs, ps, pdf, other] :

Title: Observations of the intense and ultra-long burst GRB041219a with the Germanium Spectrometer on INTEGRAL
Authors: S. McBreen, L. Hanlon, S. McGlynn, B. McBreen, S. Foley, R. Preece, A. von Kienlin, O.R. Williams
Comments: Accepted for publication in Astronomy and Astrophysics, 10 pages, 6 figures

GRB041219a is the brightest burst localised by INTEGRAL. The intense burst occurred about ~250s after the precursor and the long delay enabled optical and near infrared telescopes to observe the prompt emission. We present comprehensive results of the temporal and spectral analyses, including line and afterglow searches using the spectrometer, SPI, aboard INTEGRAL, BAT on Swift and ASM on RXTE. We avail of multi-wavelength data to generate broadband spectra of GRB041219a and afterglow. Spectra for the burst and sub-intervals were fit by the Band model and also by the quasithermal model. The high resolution Germanium spectrometer data were searched for emission and absorption features and for gamma-ray afterglow. The overall burst and sub-intervals are well fit by the Band model. The photon index below the break energy shows a marked change after the quiescent time interval. In addition the spectra are well described by a black body component with a power law. The burst was detected by BAT and ASM during the long quiescent interval in SPI indicating the central engine might not be dormant but that the emission occurs in different bands. No significant emission or absorption features were found and limits of 900 eV and 120 eV are set on the most significant features. No gamma-ray afterglow was detected from the end of the prompt phase to ~12 hours post-burst. Broadband spectra of the prompt emission were generated in 7 time intervals using gamma-ray, x-ray, optical and near-infrared data and these were compared to the high-redshift burst GRB050904. The optical and gamma-ray emission are correlated in GRB041219a. The spectral lag was determined using data from the BAT and it changes throughout the burst. A number of pseudo-redshifts were evaluated and large dispersion in values was found.

 

astro-ph/0604456 [abs, ps, pdf, other] :

Title: Exact Analytic Spectrum of Relic Gravitational Waves in Accelerating Universe
Authors: Y. Zhang, X. Z. Er, T. Y. Xia, W. Zhao, H. X. Miao
Comments: 17 pages, 14figures, Class.Quant.Grav. accepted

An exact analytic calculation is presented for the spectrum of relic gravitational waves in the scenario of accelerating Universe $\Omega_{\Lambda}+\Omega_m = 1$. The spectrum formula contains explicitly the parameters of acceleration, inflation, reheating, and the (tensor/scalar) ratio, so that it can be employed for a variety of cosmological models. We find that the spectrum depends on the behavior of the present accelerating expansion. The amplitude of gravitational waves for the model $\Omega_{\Lambda}=0.65$ is about $\sim 50 %$ greater than that of the model $\Omega_{\Lambda}=0.7$, an effect accessible to the designed sensitivities of LIGO and LISA. The spectrum sensitively depends on the inflationary models with $a(\tau) \propto |\tau|^{1+\beta}$, and a larger $\beta$ yields a flatter spectrum, producing more power. The current LIGO results rule out the inflationary models of $\beta \geq -1.8$. The LIGO with its design sensitivity and the LISA will also be able to test the model of $\beta=-1.9$. We also examine the constraints on the spectral energy density of relic gravitational waves. Both the LIGO bound and the nucleosynthesis bound point out that the model $\beta=-1.8$ is ruled out, but the model $\beta=-2.0$ is still alive. The exact analytic results also confirm the approximate spectrum and the numerical one in our previous work.

 

astro-ph/0604457 [abs, ps, pdf, other] :

Title: The Coincidence Problem in YM Field Dark Energy Models
Authors: Wen Zhao, Yang Zhang
Comments: 8 pages, 4 figures

The coincidence problem is one of the most troubles for the dark energy models, which has been gracefully answered in the quintessence and k-essence models with tracking solutions. In this letter, we study this problem in the Yang-Mills (YM) field dark energy models, and find that it is also naturally avoided in this kind of models: no matter what kind of initial condition the YM fields have, at present, they get the similar state. We also constrain the present equation-of-state (EoS) of the YM field by the observation result of the Big Bang nucleosynthesis (BBN), and find $\omega_{y_0}<-0.94$.

 

astro-ph/0604458 [abs, ps, pdf, other] :

Title: The Constraints on the Relic Gravitational Waves By the Current and Future Observations
Authors: Wen Zhao, Yang Zhang
Comments: 13 pages, 4 figures

In this paper, by solving the inflationary flow equations, we study the constraints on the relic gravitational waves (RGW) from the observations at large scale, i.e. cosmic microwave background radiation (CMB) and large scale structure (LSS), and those at small scale, i.e. the pulsar timing, LIGO, and big bang nucleosynthesis (BBN). We find that the current constraints on the RGW from small scale are too loose to be ignored, but which from large scale is relatively tight. Under the current constraints, and considering the the damping effect of neutrino and accelerating expansion of the Universe, we also study the detection ability on RGW of the future observation projects at large scale. i.e. Planck, CMBPol, and those at small scale, i.e. advanced LIGO, LISA, ASTROD, BBO and DECIGO. We find the observations from CMB observations have larger probability to detect RGW ($>50%$). But the laser interferometers have relatively smaller probability for detection: the BBO and Ultimate DECIGO project are the possible ways to detect RGW at small scale ($\sim3.48%$ and $\sim26.49%$), but the Advanced LIGO, LISA and ASTROD have little change to detect them.

 

astro-ph/0604459 [abs, ps, pdf, other] :

Title: The Quintessence With Oscillating State Equation and Its Potential
Authors: Wen Zhao
Comments: 9 pages, 7 figures

In this paper, I discuss the quintessence with oscillating state equation and the function form of its potential. From the constructed potential, which has the state equation $\omega_{\phi}=w_0+w_1\sin z$, I find it is also an oscillating function, but the oscillating amplitude is decreasing (increasing) with the field. From the evolution equation of the field $\phi$, I find which is for the effect of the cosmic expansion is the resistant to the kinetic energy of the field $\phi$. Also for this reason, it is very difficult to build a potential which can follow the state equation oscillating forever. But it is easy to build a model with state equation being oscillating for a period. Then we discuss three models of the quintessence with potentials, which are the combinations of the invert power law function and the oscillating function of the field $\phi$. I find that they all indeed give the oscillating state equation.

 

astro-ph/0604460 [abs, ps, pdf, other] :

Title: The Quintom Models With State Equation Crossing -1
Authors: Wen Zhao
Comments: 18 pages, 8 figures

In this paper, we study a kind of special quintom models, which are made of a quintessence field $\phi_1$ and a phantom field $\phi_2$, and their potential functions have the form of $V(\phi_1^2-\phi_2^2)$. These models have simple kinetic functions, so the analysis of them is simple. These models are separated into two kind: the hessence models, which have $\phi_1^2>\phi_2^2$, and the hantom, which have $\phi_1^2<\phi_2^2$. We discuss the evolution of these models in the plane defined by $\omega$ (the state equation parameter of the dark energy) and $\omega'$ (the derivative of $\omega$ with respect to the logarithm of the scale factor), and find that the $\omega$-$\omega'$ plane can be divided into four parts according to two conditions: one is that the field being quintessence-like or phantom-like; the other is that the potential being climbed up or rolled down. For the late time attractor solutions, if existing, which are always quintessence-like or $\Lambda$-like for hessence fields, so the Big Rip doesn't exist in this kind of models. But for hantom fields, their late time attractor solutions can be phantom-like or $\Lambda$-like, and the Big Rip is unavoidable for some hantom models. As two example hessence models with the exponential potential and power law potential, we study their evolution in the $\omega$-$\omega'$ plane. At last, we show the way to construct the potential function from the parametrized state equations $\omega(z)$. For five kind of parametrized methods, where $\omega$ crossing -1 can exist, we build their potential functions, and find they all can be realized in hessence models. Especially, we discuss a kind of oscillating $\omega(z)$, and find its potential is also an oscillating function.

 

astro-ph/0604461 [abs, ps, pdf, other] :

Title: Detection of Wolf-Rayet stars in host galaxies of Gamma-Ray Bursts (GRBs): are GRBs produced by runaway massive stars ejected from high stellar density regions ?
Authors: F. Hammer (1), H. Flores (1), D. Schaerer (2,3), M. Dessauges-Zavadsky (2), E. Le Floc'h (4,1), M. Puech (1)
Comments: (1) GEPI, Observatoire de Paris-Meudon, France, (2) Observatoire de Geneve, Switzerland, (3) Laboratoire d'Astrophysique Toulouse-Tarbes, France,(4) Steward Observatory, University of Arizona, USA

We have obtained deep spectroscopic observations of several nearby gamma-ray burst (GRB) host galaxies revealing for the first time the presence of Wolf-Rayet (WR) stars and numerous O stars located in rich and compact clusters or star forming regions. Surprisingly, high spatial resolution imaging shows that the GRBs and the associated supernovae did not occur in these regions, but several hundreds of parsec away. Considering various scenarios for GRB progenitors, we do not find any simple explanation of why they should be preferentially born in regions with low stellar densities. All the examined GRBs and associated SNe have occurred 400 to 800 pc from very high density stellar environments including large numbers of WR stars. Such distances can be travelled through at velocities of 100 km/s or larger, assuming the travel time to be the typical life time of WR stars. It leads us to suggest that GRB progenitors may be runaway massive stars ejected from compact massive star clusters. The ejection from such super star clusters may lead to a spin-up of these stars, producing the loss of the hydrogen and/or helium envelopes leading to the origin of the type Ibc supernovae associated with GRBs. If this scenario applies tocd text/Sc all GRBs, it provides a natural explanation of the very small fraction of massive stars that emit a GRB at the end of their life. An alternative to this scenario could be a binary origin for GRBs, but this still requires an explanation of why it would preferentially occur in low stellar density regions.

 

astro-ph/0604462 [abs, ps, pdf, other] :

Title: Magnetic Field Seeding by Galactic Winds
Authors: Serena Bertone (University of Sussex), Corina Vogt (ASTRON), Torsten Ensslin (MPA Garching)
Comments: 14 pages, 5 figures. Accepted for publications by MNRAS. A high resolution version of the paper is available at this http URL

The origin of intergalactic magnetic fields is still a mystery and several scenarios have been proposed so far: among them, primordial phase transitions, structure formation shocks and galactic outflows. In this work we investigate how efficiently galactic winds can provide an intense and widespread "seed" magnetisation. This may be used to explain the magnetic fields observed today in clusters of galaxies and in the intergalactic medium (IGM). We use semi-analytic simulations of magnetised galactic winds coupled to high resolution N-body simulations of structure formation to estimate lower and upper limits for the fraction of the IGM which can be magnetised up to a specified level. We find that galactic winds are able to seed a substantial fraction of the cosmic volume with magnetic fields. Most regions affected by winds have magnetic fields in the range -12 < Log B < -8 G, while higher seed fields can be obtained only rarely and in close proximity to wind-blowing galaxies. These seed fields are sufficiently intense for a moderately efficient turbulent dynamo to amplify them to the observed values. The volume filling factor of the magnetised regions strongly depends on the efficiency of winds to load mass from the ambient medium. However, winds never completely fill the whole Universe and pristine gas can be found in cosmic voids and regions unaffected by feedback even at z=0. This means that, in principle, there might be the possibility to probe the existence of primordial magnetic fields in such regions.

 

astro-ph/0604463 [abs, ps, pdf, other] :

Title: Millimetric Properties of Gamma Ray Burst Host Galaxies
Authors: R.S. Priddey (1), N.R. Tanvir (1), A.J. Levan (1), A.S. Fruchter (2), C. Kouveliotou (3), I.A. Smith (4), R.A.M.J. Wijers (5) ((1) University of Hertfordshire (2) Space Telescope Science Institute (3) NASA Marshall Space Flight Center (4) Rice University (5) University of Amsterdam)
Comments: MNRAS, in press. 8 pages, 4 figures

We present millimetre (mm) and submillimetre (submm) photometry of a sample of host galaxies of Gamma Ray Bursts (GRBs), obtained using the MAMBO2 and SCUBA bolometer arrays respectively. These observations were obtained as part of an ongoing project to investigate the status of GRBs as indicators of star formation. Our targets include two of the most unusual GRB host galaxies, selected as likely candidate submm galaxies: the extremely red (R-K approx 5) host of GRB 030115, and the extremely faint (R>29.5) host of GRB 020124. Neither of these galaxies is detected, but the deep upper limits for GRB 030115 impose constraints on its spectral energy distribution.
As a framework for interpreting these data, and for predicting the results of forthcoming submm surveys of Swift-derived host samples, we model the expected flux and redshift distributions based on luminosity functions of both submm galaxies and GRBs, assuming a direct proportionality between the GRB rate density and the global star formation rate density. We derive the effects of possible sources of uncertainty in these assumptions, including an anticorrelation between GRB rate and the global average metallicity.

 

astro-ph/0604464 [abs, ps, pdf, other] :

Title: Dynamical Masses of Young Star Clusters: Constraints on the Stellar IMF and Star-Formation Efficiency
Authors: N. Bastian (1), S.P. Goodwin (2) ((1) University College London, UK; (2) University of Sheffield, UK)
Comments: 4 pages, contribution to "Globular Clusters: Guides to Galaxies", March 6th-10th, 2006

Many recent works have attempted to constrain the stellar initial mass function (IMF) inside massive clusters by comparing their dynamical mass estimates to the measured light. These studies have come to different conclusions, with some claiming standard Kroupa-type IMFs, while others have claimed extreme non-standard IMFs. However, the results appear to be correlated with the age of the clusters, as older clusters (>80 Myr) all appear to be well fit by a Kroupa-type IMF whereas younger clusters display significant scatter in their best fitting IMF. Here we show that this is likely due to the fact that young clusters are out of virial equilibrium and therefore cannot be used for such studies. Hence only the older clusters are suitable for IMF studies. Using only these clusters we find that the IMF does not vary significantly. The youngest clusters can be used instead to constrain the star-formation efficiency (SFE) within clusters. We find that the SFE varies between 20 and 60% and we conclude that approximately 60% of young clusters are unbound and will not survive for more than a few 10's of Myr (i.e. "infant mortality").

 

astro-ph/0604465 [abs, ps, pdf, other] :

Title: Warm-hot intergalactic medium contribution to baryonic matter
Authors: Andrzej M. Soltan (Nicolaus Copernicus Astronomical Center, Warsaw)
Comments: 9 pages, 6 figures

Hydrodynamical simulations indicate that substantial fraction of baryons in the Universe remains in a diffuse component - Warm-Hot Intergalactic Medium (WHIM). To determine physical properties (spatial distribution, temperature and density) of the WHIM, spatial structure of the soft extended X-ray emission surrounding field galaxies is carefully investigated using the XMM-Newton EPIC/MOS observations. Angular correlations between the galaxy distribution and the soft X-ray background extending over several arcmin are determined. The correlations at large scales result from the clustering of galaxies. At small scales (below ~2 arcmin) the excess of the soft flux is interpreted as the genuine emission from halos of the WHIM surrounding individual galaxies. Bulk parameters of the average WHIM halos associated with galaxies in the sample are estimated. Typical halo has a characteristic radius of ~0.3 Mpc and a mass of 4 - 7 x 10^11 M_sun. The average density of the WHIM in the local universe amounts to 7 - 11 x 10^{-32} g cm^{-3} (Omega_WHIM = 0.7 - 1.2 %). Observations of the X-ray WHIM emission are in good agreement with the numerical simulations, but accuracy of the observational material is insufficient to constrain the theory of WHIM. A series of deep observations of a moderately numerous sample of low redshift galaxies with high resolution instruments of Chandra would significantly improve our estimates of the WHIM parameters.

 

astro-ph/0604466 [abs, ps, pdf, other] :

Title: Discovery of a 3.6-hr Eclipsing Luminous X-Ray Binary in the Galaxy NGC 4214
Authors: Kajal K. Ghosh, Saul Rappaport, Allyn F. Tennant, Douglas A. Swartz, David Pooley, N. Madhusudhan
Comments: 7 pages, submitted to ApJ

We report the discovery of an eclipsing X-ray binary with a 3.62-hr period within 24'' of the center of the dwarf starburst galaxy NGC 4214. The orbital period places interesting constraints on the nature of the binary, and allows for a few very different interpretations. The most likely possibility is that the source lies within NGC 4214 and has an X-ray luminosity of up to 0.7e39 erg/s or just below the ultraluminous X-ray source range. In this case the binary may well be comprised of a naked He-burning donor star with a neutron-star accretor, though a stellar-mass black-hole accretor cannot be completely excluded. There is no obvious evidence for a strong stellar wind in the X-ray orbital light curve that would be expected from a massive He star; thus, the mass of the He star should be < 3-4 solar masses. If correct, this would represent a new class of very luminous X-ray binary -- perhaps related to Cyg X-3. Other less likely possibilities include a conventional low-mass X-ray binary that somehow manages to produce such a high X-ray luminosity and is apparently persistent over an interval of years; or a foreground AM Her binary of much lower luminosity that fortuitously lies in the direction of NGC 4214. Any model for this system must accommodate the lack of an optical counterpart down to a limiting magnitude of 22.6 in the visible.

 

astro-ph/0604467 [abs, pdf] :

Title: Global Dynamics in Galactic Triaxial Systems I
Authors: Pablo M.Cincotta (1), Claudia M. Giordano (1), Josefa Perez (1,2). ((1) FCAGLP - UNLP, Argentine; (2) IAFE - Buenos Aires, Argentine)
Comments: Accepted A&A

In this paper we present a theoretical analysis of the global dynamics in a triaxial galactic system using a 3D integrable Hamiltonian as a simple representation. We include a thorough discussion on the effect of adding a generic non--integrable perturbation to the global dynamics of the system. We adopt the triaxial Stackel Hamiltonian as the integrable model and compute its resonance structure in order to understand its global dynamics when a perturbation is introduced. Also do we take profit of this example in order to provide a theoretical discussion about diffussive processes taking place in phase space.

 

astro-ph/0604468 [abs, ps, pdf, other] :

Title: Spectral line variability amplitudes in AGNs
Authors: W. Kollatschny, M. Zetzl, M. Dietrich
Comments: 17 pages, 21 figures. Astronomy & Astrophysics, in press

We present the results of a long-term variability campaign of very broad-line AGNs with line widths broader than FWHM $>$ 5000 kms$^{-1}$. The main goal of our investigation was to study whether the widths of the optical broad emission lines are correlated with the optical intensity variations on timescales of years. Our AGN sample consisted of 10 objects. We detected a significant correlation between optical continuum variability amplitudes and H$\beta$ emission line widths (FWHM) and, to a lesser degree, between H$\beta$ line intensity variations and H$\beta$ equivalent widths. We add the spectroscopic data of variable AGNs from the literature to supplement our sample. The AGNs from other optical variability campaigns with different line-widths helped to improve the statistical significance of our very broad-line AGN sample. After including the data on 35 additional galaxies, the correlation between optical continuum variability amplitudes and H$\beta$ emission line widths becomes even more significant and the probability that this is a random correlation drops to 0.7 percent.

 

astro-ph/0604469 [abs, ps, pdf, other] :

Title: Modeling the Large Scale Structures of Astrophysical Jets in the Magnetically Dominated Limit
Authors: Hui Li, et al
Comments: Accepted to ApJ, May 10, 2006 issue, 12 figures total (3 color figures)

We suggest a new approach that could be used for modeling both the large scale behavior of astrophysical jets and the magnetically dominated explosions in astrophysics. We describe a method for modeling the injection of magnetic fields and their subsequent evolution in a regime where the free energy is magnetically dominated. The injected magnetic fields, along with their associated currents, have both poloidal and toroidal components, and they are not force free. The dynamic expansion driven by the Lorentz force of the injected fields is studied using 3-dimensional ideal magnetohydrodynamic simulations. The generic behavior of magnetic field expansion, the interactions with the background medium, and the dependence on various parameters are investigated.

 

astro-ph/0604470 [abs, ps, pdf, other] :

Title: LP 400-22, A very low-mass and high-velocity white dwarf
Authors: Adela Kawka, Stephane Vennes, Terry D. Oswalt, J. Allyn Smith, Nicole M. Silvestri
Comments: accepted for publication in Apj Letters

We report the identification of LP 400-22 (WD 2234+222) as a very low-mass and high-velocity white dwarf. The ultraviolet GALEX and optical photometric colors and a spectral line analysis of LP 400-22 show this star to have an effective temperature of 11080+/-140 K and a surface gravity of log g = 6.32+/-0.08. Therefore, this is a helium core white dwarf with a mass of 0.17 M_solar. The tangential velocity of this white dwarf is 414+/-43 km/s, making it one of the fastest moving white dwarfs known. We discuss probable evolutionary scenarios for this remarkable object.

 

astro-ph/0604471 [abs, ps, pdf, other] :

Title: Explosive Nucleosynthesis from GRB and Hypernova Progenitors: Direct Collapse versus Fallback
Authors: Christopher L. Fryer, Patrick A. Young, Aimee L. Hungerford
Comments: 47 pages, submitted to ApJ

The collapsar engine behind long-duration gamma-ray bursts extracts the energy released from the rapid accretion of a collapsing star onto a stellar-massed black hole. In a collapsing star, this black hole can form in two ways: the direct collapse of the stellar core into a black hole and the delayed collapse of a black hole caused by fallback in a weak supernova explosion. In the case of a delayed-collapse black hole, the strong collapsar-driven explosion overtakes the weak supernova explosion before shock breakout, and it is very difficult to distinguish this black hole formation scenario from the direct collapse scenario. However, the delayed-collapse mechanism, with its double explosion, produces explosive nucleosynthetic yields that are very different from the direct collapse scenario. We present 1-dimensional studies of the nucleosynthetic yields from both black hole formation scenarios, deriving differences and trends in their nucleosynthetic yields.

 

astro-ph/0604472 [abs, ps, pdf, other] :

Title: Exploring the spectroscopic diversity of Type Ia Supernovae
Authors: S. Hachinger, P. A. Mazzali, S. Benetti
Comments: 25 pages, 22 figures, accepted for publication in MNRAS

The velocities and equivalent widths of a set of absorption features are measured for a sample of 28 well-observed Type Ia Supernovae covering a wide range of properties. The values of these quantities at maximum are obtained through inter-/extrapolation and plotted against the decline rate, and so are various line ratios. The SNe are divided according to their velocity evolution into the three classes defined by Benetti et al. (2005): LVG, HVG, and FAINT. It is found that all LVG have approximately uniform velocities at B maximum, while the FAINT SNe have values that decrease with increasing Delta m_15(B), and the HVG have a large spread. The EWs of the Fe-dominated features are approximately constant in all SNe, while those of Intermediate Mass Elements lines have larger values for intermediate decliners and smaller values for brighter and FAINT SNe. The HVG SNe have stronger Si II 6355 A lines, with no correlation with Delta m_15(B). It is also shown that the Si II 5972 A EW and three EW ratios, including one analogous to the R(Si II) ratio of Nugent et al. (1995), are good spectroscopic indicators of luminosity. The data suggest that all LVG have approximately constant kinetic energy, since burning to IME extends to similar velocities. The FAINT SNe may have somewhat lower energies. The large velocities and EWs of the IME lines in the HVG appear correlated with each other, but are not correlated with the presence of high velocity features in the Ca II IR triplet in the earliest spectra for the SNe for which such data exist.

 

astro-ph/0604473 [abs, ps, pdf, other] :

Title: Active Galaxies in the UV
Authors: Wolfram Kollatschny, Wang Ting-Gui
Comments: 29 pages, 13 figures, Ap&SS in press

In this article we present different aspects of AGN studies demonstrating the importance of the UV spectral range. Most important diagnostic lines for studying the general physical conditions as well as the metalicities in the central broad line region in AGN are emitted in the UV. The UV/FUV continuum in AGN excites not only the emission lines in the immediate surrounding but it is responsible for the ionization of the intergalactic medium in the early stages of the universe. Variability studies of the emission line profiles of AGN in the UV give us information on the structure and kinematics of the immediate surrounding of the central supermassive black hole as well as on its mass itself.

 

astro-ph/0604474 [abs, ps, pdf, other] :

Title: Simulations of Mixed Morphology Supernova Remnants With Anisotropic Thermal Conduction
Authors: David A. Tilley, Dinshaw S. Balsara, J. Christopher Howk
Comments: 8 pages, 5 figures, submitted to Monthly Notices

We explore the role of anisotropic thermal conduction on the evolution of supernova remnants through interstellar media with a range of densities via numerical simulations. We find that a remnant expanding in a dense environment can produce centre-bright hard x-ray emission within 20 kyr, and centre-bright soft x-ray emission within 60 kyr of the supernova event. In a more tenuous environment, the appearance of a centre-bright structure in hard x-rays is delayed until about 60 kyr. The soft x-ray emission from such a remnant may not become centre bright during its observable lifetime. This can explain the observations that show that mixed-morphology supernova remnants preferentially occur close to denser, molecular environments. Remnants expanding into denser environments tend to be smaller, making it easier for thermal conduction to make larger changes in the temperatures of their hot gas bubbles. We show that the lower temperatures make it very favorable to use high-stage ions as diagnostics of the hot gas bubbles in SNRs. In particular, the distribution of O VIII transitions from shell-bright at early epochs to centre-bright at later epochs in the evolution of an SNR expanding in a dense ISM when the physics of thermal conduction is included.

 

astro-ph/0604475 [abs, ps, pdf, other] :

Title: A toy model of a fake inflation
Authors: M. Novello, E. Huguet, J. Queva

Discontinuities in non linear field theories propagate through null geodesics in an effective metric that depends on its dynamics and on the background geometry. Once information of the geometry of the universe comes mostly from photons, one should carefully analyze the effects of possible nonlinearities on Electrodynamics in the cosmic geometry. Such phenomenon of induced metric is rather general and may occurs for any nonlinear theory independently of its spin properties. We limit our analysis here to the simplest case of non linear scalar field. We show that a class of theories that have been analyzed in the literature, having regular configuration in the Minkowski space-time background is such that the field propagates like free waves in an effective deSitter geometry. The observation of these waves would led us to infer, erroneously, that we live in a deSitter universe.

 

astro-ph/0604476 [abs, ps, pdf, other] :

Title: Gravitational lensing of distant gamma-ray bursts mimics evolving dark energy
Authors: Masamune Oguri, Keitaro Takahashi (Princeton)
Comments: 4 pages, 3 figures

Gamma-Ray Bursts (GRBs) offer a potentially powerful way to extend the Hubble diagram to very high redshifts and to constrain the nature of dark energy in a way complementary to distant type Ia supernovae. We show that its usage as a cosmological probe is limited by gravitational lensing. In addition to increasing the dispersions of distance measurements, lensing systematically brightens distant GRBs through the magnification bias, which mimics evolving dark energy in the Hubble diagram. We perform Monte-Carlo simulations of GRBs assuming a cosmological constant dominated universe and then constrain the dark energy equation of state neglecting gravitational lens effects. We find that the originally assumed model is rejected by 68% confidence limit when the dispersion of inferred luminosities is comparable to that of type Ia supernovae. The precise degree of the bias in cosmological parameter determinations depends strongly on the shape of the luminosity function of GRBs, implying that the deconvolution of gravitational lensing effects is quite challenging.

 

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