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Astrophysics

astro-ph new abstracts, Thu, 6 Apr 06 00:00:13 GMT
0604074 -- 0604112 received

astro-ph/0604074 [abs, ps, pdf, other] :

Title: Molecular Outflows in Galaxy Merger Simulations with Embedded AGN
Authors: Desika Narayanan (1), Thomas J. Cox (2), Brant Robertson (2), Romeel Dave (1), Tiziana Di Matteo (3), Lars Hernquist (2), Philip Hopkins (2), Craig Kulesa (1), Christopher Walker (1) (1) University of Arizona (2) CfA; (3) Carnegie Mellon University)
Comments: 5 pages, 2 figures; Accepted to The Astrophysical Journal Letters

We study the effects of feedback from active galactic nuclei (AGN) on emission from molecular gas in galaxy mergers by combining hydrodynamic simulations which include black holes with a three-dimensional, non-local thermodynamic equilibrium (LTE) radiative transfer code. We find that molecular clouds entrained in AGN winds produce an extended CO morphology with significant off-nuclear emission, which may be detectable via contour mapping. Furthermore, kinematic signatures of these molecular outflows are visible in emission line profiles when the outflow has a large line of sight velocity. Our results can help interpret current and upcoming observations of luminous infrared galaxies, as well as provide a detailed test of subresolution prescriptions for supermassive black hole growth in galaxy-scale hydrodynamic simulations.

 

astro-ph/0604075 [abs, ps, pdf, other] :

Title: Perturbation Theory Reloaded: Analytical Calculation of Non-linearity in Baryonic Oscillations in the Real Space Matter Power Spectrum
Authors: Donghui Jeong, Eiichiro Komatsu (Univ. of Texas at Austin)
Comments: 5 pages, 3 figures, submitted to ApJ

We compare the non-linear matter power spectrum in real space calculated analytically from 3rd-order perturbation theory with N-body simulations at 1<z<6. We find that the perturbation theory prediction agrees with the simulations to better than 1% accuracy in the weakly non-linear regime where the dimensionless power spectrum, Delta^2(k)=k^3P(k)/2pi^2, which approximately gives variance of matter density field at a given k, is less than 0.4. While the baryonic acoustic oscillation features are preserved in the weakly non-linear regime at z>1, the shape of oscillations is distorted from the linear theory prediction. Nevertheless, our results suggest that one can correct the distortion caused by non-linearity almost exactly. We also find that perturbation theory, which does not contain any free parameters, provides a significantly better fit to the simulations than the conventional approaches based on empirical fitting functions to simulations. The future work would include perturbation theory calculations of non-linearity in redshift space distortion and halo biasing in the weakly non-linear regime.

 

astro-ph/0604076 [abs, ps, pdf, other] :

Title: A Debris Disk Around An Isolated Young Neutron Star
Authors: Zhongxiang Wang, Deepto Chakrabarty, David L. Kaplan (MIT)
Comments: 5 pages, 3 figures. To appear in Nature (6 Apr 2006)

Pulsars are rotating, magnetized neutron stars that are born in supernova explosions following the collapse of the cores of massive stars. If some of the explosion ejecta fails to escape, it may fall back onto the neutron star or it may possess sufficient angular momentum to form a disk. Such 'fallback' is both a general prediction of current supernova models and, if the material pushes the neutron star over its stability limit, a possible mode of black hole formation. Fallback disks could dramatically affect the early evolution of pulsars, yet there are few observational constraints on whether significant fallback occurs or even the actual existence of such disks. Here we report the discovery of mid-infrared emission from a cool disk around an isolated young X-ray pulsar. The disk does not power the pulsar's X-ray emission but is passively illuminated by these X-rays. The estimated mass of the disk is of order 10 Earth masses, and its lifetime (at least a million years) significantly exceeds the spin-down age of the pulsar, supporting a supernova fallback origin. The disk resembles protoplanetary disks seen around ordinary young stars, suggesting the possibility of planet formation around young neutron stars.

 

astro-ph/0604077 [abs, ps, pdf, other] :

Title: Predictions for the correlation between giant and terrestrial extrasolar planets in dynamically evolved systems
Authors: Dimitri Veras, Philip J. Armitage
Comments: 8 pages, 8 figures, 1 table, accepted for publication in ApJ

The large eccentricities of many giant extrasolar planets may represent the endpoint of gravitational scattering in initially more crowded systems. If so, the early evolution of the giant planets is likely to be more restrictive of terrestrial planet formation than would be inferred from the current, dynamically quiescent, configurations. Here, we study statistically the extent of the anti-correlation between giant planets and terrestrial planets expected in a scattering model. We use marginally stable systems of three giant planets, with a realistic range of planetary masses, as a simple model for the initial conditions prior to scattering, and show that after scattering the surviving planets reproduce well the known extrasolar planet eccentricities beyond a > 0.5 AU. By tracking the minimum periastron values of all planets during the evolution, we derive the distribution of orbital radii across which strong perturbations (from crossing orbits) are likely to affect low mass planet formation. We find that scattering affects inner planet formation at orbital separations less than 50% of the final periastron distance of the innermost massive planet in approximately 30% of the realizations, and can occasionally influence planet formation at orbital separations less than 20% of the final periastron distance of the innermost massive planet. The domain of influence of the scattering massive planets increases as the mass differential between the massive planets decreases. Observational study of the correlation between massive and terrestrial extrasolar planets in the same system has the potential to constrain the origin of planetary eccentricity.

 

astro-ph/0604078 [abs, ps, pdf, other] :

Title: Protoplanet Dynamics in a Shear-Dominated Disk
Authors: Benjamin F. Collins, Re'em Sari
Comments: 8 pages, 2 figures

The velocity dispersion, or eccentricity distribution, of protoplanets interacting with planetesimals is set by a balance between dynamical friction and viscous stirring. We calculate analytically the eccentricity distribution function of protoplanets embedded in a cold, shear-dominated planetesimal swarm. We find a distinctly non-Rayleigh distribution with a simple analytical form. The peak of the distribution lies much lower than the root-mean-squared value, indicating that while most of the bodies have similarly small eccentricities, a small subset of the population contains most of the thermal energy. We also measure the shear-dominated eccentricity distribution using numerical simulations. The numerical code treats each protoplanet explicitly and adds an additional force term to each body to represent the dynamical friction of the planetesimals. Without fitting any parameters, the eccentricity distribution of protoplanets in the N-body simulation agrees with the analytical results. This distribution function provides a useful tool for testing hybrid numerical simulations of late-stage planet formation.

 

astro-ph/0604079 [abs, ps, pdf, other] :

Title: Shoemaker-Levy 9 Impact Modeling: I. High-Resolution 3D Bolides
Authors: D. G. Korycansky, Joseph Harrington, Drake Deming, Mathew E. Kulick

We have run high-resolution, three-dimensional, hydrodynamic simulations of the impact of comet Shoemaker-Levy 9 into the atmosphere of Jupiter. We find that the energy deposition profile is largely similar to the previous two-dimensional calculations of Mac Low and Zahnle (1994), though perhaps somewhat broader in the range of height over which the energy is deposited. As with similar calculations for impacts into the Venusian atmosphere, there is considerable sensitivity in the results to small changes in the initial conditions, indicating dynamical chaos. We calculated the median depth of energy deposition (the height z at which 50% of the bolide's energy has been released) per run. The mean value among runs is approx 70 km below the 1-bar level, for a 1-km diameter impactor of porous ice of density rho=0.6 g cm^{-3}. The standard deviation among these runs is 14 km. We find little evidence of a trend in these results with the resolution of the calculations (up to 57 cells across the impactor radius, or 8.8-m resolution), suggesting that resolutions as low as 16 grid cells across the radius of the bolide may yield good results for this particular quantity.

 

astro-ph/0604080 [abs, ps, pdf, other] :

Title: Redshifted 21 Centimeter Emission from Minihalos Before Reionization
Authors: Steven Furlanetto (Yale), S. Peng Oh (UCSB)
Comments: 8 pages, 6 figures, submitted to ApJ

Before reionization, the intergalactic medium (IGM) may have been sufficiently cold for low-mass "minihalos" to condense out of the gas and subsequently affect reionization. Previous work has shown that minihalos generate reasonably large 21 cm fluctuations. Here we consider this signal in its proper cosmological context and show that isolating minihalos from the rest of the IGM is extremely difficult. Using the well-known halo model, we compute the power spectrum of 21 cm fluctuations from minihalos and show that the signal decreases rapidly as feedback increases the Jeans mass. We then show that even a small Lyman-alpha background increases the 21 cm fluctuations of the diffuse IGM well beyond those of the minihalos; because the mass fraction in the IGM is much larger, minihalos will lie buried within the IGM signal. The distinctive signatures of non-linear bias and minihalo structure emerge only at much smaller scales, well beyond the resolution of any upcoming instruments. Using simple, but representative, reionization histories, we then show that the required Lyman-alpha background level is most likely achieved at z>15, while minihalos are still rare, so that they are almost always degenerate with the diffuse IGM.

 

astro-ph/0604081 [abs, ps, pdf, other] :

Title: Stellar Properties of Embedded Protostars
Authors: R. J. White (1), T. P. Greene (2), G. W. Doppmann (3), K. R. Covey (4), L. A. Hillenbrand (5) ((1) University of Alabama in Huntsville, (2) National Aeronautics and Space Administration at Ames Research Center, (3) Gemini Observatory, (4) University of Washington, (5) California Institute of Technology)
Comments: Review Chapter for Protostars and Planets V. 16 pages, 8 figures

(Abridged) High dispersion spectrographs on large aperture telescopes have recently allowed observers to study the stellar and accretion properties of deeply embedded young stars, commonly referred to as Class I stars. We summarize these newly determined properties and compare them with observations of more optically revealed Class II (T Tauri) stars. Class I stars have spectral types and stellar luminosities similar to those of Class II stars, suggesting similar masses and ages. Estimates of stellar luminosity and age, however, are especially uncertain given the large extinctions, scattered light emission and continuum excesses typical of Class I stars. Several candidate Class I brown dwarfs are identified. Class I stars appear to rotate more rapidly than T Tauri stars, by roughly a factor of 2. Likewise, Class I disk accretion rates are only a factor of two larger than those of T Tauri stars, less than the mass infall rates predicted by envelope models by 1-2 orders of magnitude. In at least a few cases the discrepancy appears to be caused by T Tauri stars being misclassified as Class I stars because of their edge-on disk orientation. Stars where the envelope density and infall velocity have been determined directly and unambiguously imply that stellar mass is not acquired in a steady-state fashion, but instead through brief outbursts of enhanced accretion. If some Class I stars are in fact as old as T Tauri stars, replenishment may be necessary to sustain the long-lived envelopes, possibly via continued dynamical interactions with cloud material.

 

astro-ph/0604082 [abs, ps, pdf, other] :

Title: Evolution of the Color-Magnitude relation in High-Redshift Clusters: Early-type Galaxies in the Lynx Supercluster at z~1.26
Authors: Simona Mei, Brad P. Holden, John P. Blakeslee, Piero Rosati, Marc Postman, Myungkook J. Jee, Alessandro Rettura, Marco Sirianni, Ricardo Demarco, Holland C. Ford, Marijn Franx, Nicole L. Homeier, Garth D. Illingworth
Comments: 22 pages, 8 figures; ApJ, in press

Color-magnitude relations have been derived in the clusters RX J0849+4452 and RX J0848+4453 at z~1.26. The color-magnitude relation was determined from Advanced Camera for Surveys imaging in the WFC F775W (i_775) and F850LP (z_850) filters combined with ground-based spectroscopy. Early-type cluster candidates have been identified according to the Postman et al. morphological classification. In both clusters the bright red early-type population defines a tight color-magnitude relation very similar in color, although the two clusters present different X-ray luminosities and shapes, with RX J0849+4452 being three times more X-ray luminous and more compact, and having a temperature two times higher. The elliptical galaxy color-magnitude relations (CMR) in RX J0849+4452 and RX J0848+4453 show an intrinsic (i_775-z_850) color scatter of 0.026 +/- 0.012 mag and 0.024 +/- 0.015 mag, respectively, within 2 arcminutes (~1Mpc at z=1.26) from the cluster X-ray emission centers. Simple modeling of the scatters using stellar population models from Bruzual and Charlot, gives a mean luminosity-weighted age t > 2.5 Gyr (z_f > 2.75). S0 galaxies follow the elliptical CMR; they show larger scatters about the CMR.
The intrinsic scatter decreases and the CMR slopes are steeper at smaller radii, within both clusters. We conclude that old stellar populations in cluster elliptical galaxies are already in place at z=1.26, both in the more evolved cluster RX J0849+4452, and in its less evolved companion RX J0848+4453. Even at a lookback time of 9 Gyr, in the early merging and buildup of massive clusters, the bulk of the stellar content of the bright elliptical galaxy population was in place - apparently formed some 2.5~Gyr earlier at z~3

 

astro-ph/0604083 [abs, ps, pdf, other] :

Title: On the Early Time X-ray Spectra of Swift Afterglows I: Evidence for Anomalous Soft X-ray Emission
Authors: Nathaniel R. Butler
Comments: 36 pages, 10 tables, 17 figures, Submitted to ApJ

We have conducted a thorough and blind search for emission lines in >70 Swift X-ray afterglows of total exposure ~10^7s. We find that most afterglows are consistent with pure power-laws plus extinction. Significant outliers to the population exist at the 5-10% level and have anomalously soft, possibly thermal spectra. Four bursts are singled out via detections of 2-5 lines: GRBs 060218, 060202, 050822, and 050714B. The most significant soft component detections in the full data set of ~2000 spectra correspond to GRB060218/SN2006aj, with significances ranging up to ~20sigma. A thermal plasma model fit to the data indicates that the flux is primarily due to L-shell transitions of Fe at ~solar abundance. We associate (>4sigma significant) line emission from the 3 other events with K-shell transitions in light metals. We favor a model where the line emission in these afterglows arises from the mildly relativistic cocoon of matter surrounding the GRB jet as it penetrates and exits the surface of the progenitor star. The detections point to emitting material at similar distances \~10^12-10^13 cm, at similar densities ~10^17 cm^-3, and subject to similar fluxes of ionizing radiation. The lines may correlate with the X-ray flaring. Alternatively, a blackbody model with kT ~0.1-0.5 keV can describe the soft emission in each afterglow. The blackbody spectrum may also be due to break out of the GRB shock or plasma cocoon from the progenitor star, as recently suggested for GRB060218 (Campana et al. 2006). Due to the low z of GRB060218, bursts faint in Gamma-rays with fluxes dominated by this soft X-ray component could outnumber classical GRBs 100-1.

 

astro-ph/0604084 [abs, ps, pdf, other] :

Title: Flat-Spacetime Covariant Gravity, its Quantization and Solar Application
Authors: Jin He
Comments: 12 pages

General relativity (GR) is a covariant theory of gravity on curved spacetime (general covariance) which, however, denies a covariant Hamiltonian. Most alternative works on flat spacetime present non-covariant Lagrangians which treat time $t$ as a parameter entirely distinct from the spatial coordinates. A covariant formulation requires that the parameter $p$ in the Lagrangian functional $L(x^\alpha, dx^\alpha /dp, p)$ be an invariant parameter. He (2005c) suggests a theory of gravity which is the same as GR except that spacetime is flat. The theory is general covariant with respect to all curvilinear coordinate transformation (including non-curvilinear Lorentz transformation). Following the same principle, the present paper considers the metric form to be a Lagrangian defined on flat spacetime. In the case of the gravitational field of a single point mass, the common procedure, $\vec{P} -> i\tilde h\nabla $ leads to the quantization of the corresponding Hamiltonian where $\tilde h$ is the quantization constant of macrophysics. For the Schwarzschild radius $r_g =GM/c^2$ of solar mass, the first order approximation of the solution is the well-known wave function $\Psi_{nl}$ of Hydrogen atom. Nottale, Schumacher, and Gay (1997) gave an excellent fit of the wave function to the distributions of planetary distances and planetary masses. The wave function here is independent of the quantization constant $\tilde h $. The observable quantities depend only on quantum numbers.

 

astro-ph/0604085 [abs, ps, pdf, other] :

Title: Formation and evolution of compact binaries in globular clusters: I.Binaries with white dwarfs
Authors: N. Ivanova, C.O. Heinke, F.A. Rasio, R.E. Taam, K. Belczynski, J. Fregeau
Comments: 17 pages (MNRAS style), 18 figures, submitted to MNRAS (the version with full resolution figures is available at this http URL)

In this paper, the first of a series, we study the stellar dynamical and evolutionary processes leading to the formation of compact binaries containing white dwarfs in dense globular clusters. We examine the processes leading to the creation of X-ray binaries such as cataclysmic variables and AM CVn systems. Using numerical simulations, we identify the dominant formation channels and we predict the expected numbers and characteristics of detectable systems, emphasizing how the cluster sources differ from the field population. We explore the dependence of formation rates on cluster properties and we explain in particular why the distribution of cataclysmic variables has only a weak dependence on cluster density. We also discuss the frequency of dwarf nova outbursts in globular clusters and their connection with moderately strong white dwarf magnetic fields. We examine the rate of Type Ia supernovae via both single and double degenerate channels in clusters and we argue that those rates may contribute to the total SN Ia rate in elliptical galaxies. Considering coalescing white dwarf binaries we discuss possible constraints on the common envelope evolution of their progenitors and we derive theoretical expectations for gravitational wave detection by LISA.

 

astro-ph/0604086 [abs, pdf] :

Title: Black Hole Censorhip of Varying Fundamental Constants
Authors: Jane H MacGibbon
Comments: 8 pages

Here it is shown that the variation in the fine structure constant measured by Webb et al. matches the theoretically derived value for the maximum variation in the electronic charge permitted by the Generalized Second Law of Thermodynamics for black holes accreting and emitting in the present cosmic microwave background. It is postulated that the constants of nature, independently or dependently, vary at the maximal rate allowed by the Generalized Second Law of Thermodynamics.

 

astro-ph/0604087 [abs, ps, pdf, other] :

Title: A note on cosmological parameters and the topology of the universe
Authors: M.J. Reboucas, J.S. Alcaniz
Comments: 5 pages, 2 figures, revtex4. Prepared for 100 Years of Relativity: International Conference on Classical and Quantum Aspects of Gravity and Cosmology, Sao Paulo, Brazil, 22-24 Aug 2005
Journal-ref: Braz.J.Phys. 35 (2005) 1062-1066

Given the wealth of increasingly accurate cosmological observations, especially the recent results from the WMAP, and the development of methods and strategies in the search for cosmic topology, it is reasonable to expect that we should be able to detect the spatial topology of the Universe in the near future. Motivated by this, we examine to what extent a possible detection of a nontrivial topology of positively curved universe may be used to place constraints on the matter content of the Universe. We show through concrete examples that the knowledge of the spatial topology allows to place constraints on the density parameters associated to dark matter ($\Omega_m$) and dark energy ($\Omega_{\Lambda}$)

 

astro-ph/0604088 [abs, ps, pdf, other] :

Title: Resolving Stellar Atmospheres I: The H alpha line and comparisons to microlensing observations
Authors: Christine Thurl, Penny D. Sackett, Peter H. Hauschildt
Comments: 11 pages, 9 figures, accepted to Astronomy & Astrophysics

We present work on H alpha spectral line characteristics in PHOENIX stellar model atmospheres and their comparison to microlensing observations. We examine in detail the H alpha equivalent width (EW) and the line shape characteristics for effective temperatures of 4500K< Teff < 5600K where H alpha is a strong spectral feature. We find that H alpha EW in models calculated under the assumption of local thermodynamic equilibrium (LTE) is up to 15% smaller than in models without this assumption, non-LTE models (NLTE) and that line shapes vary significantly for the two model types. A comparison with available high quality microlensing data, capable of tracing H alpha absorption across the face of one G5III giant, shows that the LTE model that fits the EW best is about 100K hotter than and the best-fitting NLTE model has a similar Teff as predicted by the spectral type analysis of the observed star but agree within the uncertainties of the observationally derived temperature. Neither LTE nor NLTE models fit the line shape well. We suspect unmodelled chromospheric emission. Line shape diagnostics suggest lower gravities than derived for the star and are unacceptable low in the case of the LTE models. We show that EW alone is insufficient for comparison to stellar model atmospheres, but combined with a new shape parameter we define is promising. In stellar parameter ranges where the H alpha line is strong, a NLTE approach of modeling stellar atmospheres is not only beneficial but mandatory.

 

astro-ph/0604089 [abs, ps, pdf, other] :

Title: Searching for flickering variability in several symbiotic stars and related objects: BX Mon, V471 Per, RS Oph, V627 Cas, CI Cam V886 Her, Z And, T CrB, MWC 560, V407 Cyg
Authors: M. Gromadzki, M. Mikolajewski, T. Tomov, I. Bellas-Velidis, A. Dapergolas, C. Galan
Comments: 30 pages, 13 figures, accepted for publication in Acta Astronomica

UBVRI photometry observations of 10 symbiotic stars and related objects obtained in the period 2002-2003 are presented. Analysing differential light curves we found rapid light variations with timescales of tens of minutes and significant amplitudes in the well-known flickerers MWC 560, RS Oph, V407 Cyg and T CrB. MWC 560 and V407 Cyg demonstrate quasi periodic oscillations (QPO) with similar amplitudes and timescales. Flickering and unusual flare in V627 Cas as well as some indications of flickering presence in BX Mon are detected. The existence of 29 minutes oscillations in Z And with an amplitude about 0.02 mag in U band is confirmed. Only one symbiotic star, V471 Per, and both non symbiotic, CI Cam and V886 Her seem to be constant on flickering timescales. Nevertheless, small night to night changes in the brightness of V886 Her were observed as well.

 

astro-ph/0604090 [abs, ps, pdf, other] :

Title: Crossing the phantom barrier with purely kinetic multiple k-essence
Authors: Luis P. Chimento, Ruth Lazkoz
Comments: 5 pages, revtex

We consider multiple k-essence sources and obtain the conditions their kinetic functions must satisfy so that purely kinetic k-essences lead to models with phantom barrier crossing. After that, we show that polynomial kinetic functions allow the integration of the dynamical equations determining the geometry and the k-fields. The models thus obtained have accelerated expansion at late cosmic time, representing universes ending at a finite time with a big rip singularity. In addition, these models begin to evolve from an initial singularity, so they have a finite time span.

 

astro-ph/0604091 [abs, ps, pdf, other] :

Title: A Survey of O VI, C III, and H I in Highly Ionized High-Velocity Clouds
Authors: Andrew J. Fox, Blair D. Savage, Bart P. Wakker
Comments: 36 pages, 14 figures (3 in color), accepted to ApJS. Some figures downgraded to limit file size

(ABRIDGED) We present a Far-Ultraviolet Spectroscopic Explorer survey of highly ionized high-velocity clouds (HVCs) in 66 extragalactic sight lines. We find a total of 63 high-velocity O VI absorbers, 16 with 21 cm-emitting H I counterparts and 47 ``highly ionized'' absorbers without 21 cm emission. 11 of these high-velocity O VI absorbers are positive-velocity wings (broad O VI features extending asymmetrically to velocities of up to 300 km/s). The highly ionized HVC population is characterized by <b(O VI)>=38+/-10 km/s and <log N_a(O VI)>=13.83+/-0.36. We find that 81% (30/37) of high-velocity O VI absorbers have clear accompanying C III absorption, and 76% (29/38) have accompanying H I absorption in the Lyman series. The lower average width of the high-velocity H I absorbers implies the H I lines arise in a separate, lower temperature phase than the O VI. We find that the shape of the wing profiles is well reproduced by a radiatively cooling, vertical outflow. However, the outflow has to be patchy and out of ionization equilibrium. An alternative model, consistent with the observations, is one where the highly ionized HVCs represent the low N(H I) tail of the HVC population, with the O VI formed at the interfaces around the embedded H I cores. Though we cannot rule out a Local Group explanation, we favor a Galactic origin. This is based on the recent evidence that both H I HVCs and the million-degree gas detected in X-ray absorption are Galactic phenomena. Since the highly ionized HVCs appear to trace the interface between these two Galactic phases, it follows that highly ionized HVCs are Galactic themselves. However, the non-detection of high-velocity O VI in halo star spectra implies that any Galactic high-velocity O VI exists at z-distances beyond a few kpc.

 

astro-ph/0604092 [abs, ps, pdf, other] :

Title: Curvelet analysis of asteroseismic data I: Method description and application to simulated sun-like stars
Authors: P. Lambert, S. Pires, J. Ballot, R. A. Garcia, J.-L. Starck, S. Turck-Chieze
Comments: 8 pages, 6 figures, accepted for publication in A&A

Context: The detection and identification of oscillation modes (in terms of their $\ell$, $m$ and successive $n$) is a great challenge for present and future asteroseismic space missions. The "peak tagging" is an important step in the analysis of these data to provide estimations of stellar oscillation mode parameters, i.e., frequencies, rotation rates, and further studies on the stellar structure.
Aims: To increase the signal-to-noise ratio of the asteroseismic spectra computed from time series representative of MOST and CoRoT observations (30- and 150-day observations).
Methods: We apply the curvelet transform -- a recent image processing technique which looks for curved patterns -- to echelle diagrams built using asteroseismic power spectra. In this diagram the eigenfrequencies appear as smooth continuous ridges. To test the method we use Monte Carlo simulations of several sun-like stars with different combinations of rotation rates, rotation-axis inclination and signal-to-noise ratios.
Results: The filtered diagrams enhance the contrast between the ridges of the modes and the background allowing a better tagging of the modes and a better extraction of some stellar parameters. Monte Carlo simulations have also shown that the region where modes can be detected is enlarged at lower and higher frequencies compared to the raw spectra. Even more, the extraction of the mean rotational splitting from modes at low frequency can be done more easily than using the raw spectrum.

 

astro-ph/0604093 [abs, ps, pdf, other] :

Title: Physics of the GRB 030328 afterglow and its environment
Authors: E. Maiorano, N. Masetti, E. Palazzi, S. Savaglio, E. Rol, P.M. Vreeswijk, E. Pian, P.A. Price, B.A. Peterson, M. Jelinek, , L. Amati, M.I. Andersen, A.J. Castro-Tirado, J.M. Castro Cerón, A. de Ugarte Postigo, F. Frontera, A.S. Fruchter, J.P.U. Fynbo, J. Gorosabel, A.A. Henden, J. Hjorth, B.L. Jensen, S. Klose, C. Kouveliotou, G. Masi, P. Møller, L. Nicastro, E.O. Ofek, S.B. Pandey, J. Rhoads, N.R. Tanvir, R.A.M.J. Wijers, E.P.J. van den Heuvel
Comments: 14 pages, 6 figures, 3 tables, accepted for publication on A&A, main journal. Abstract changed to comply with the new format of A&A abstracts

We report on the photometric, spectroscopic and polarimetric, monitoring of the optical afterglow of Gamma-Ray Burst (GRB) 030328 detected by HETE-2. Photometry, collected at 7 different telescopes, shows that a smoothly broken powerlaw decay, with indices alpha_1 = 0.76 +/- 0.03, alpha_2 = 1.50 +/- 0.07 and a break at t_b = 0.48 +/- 0.03 days after the GRB, provides the best fit of the optical afterglow decline. This shape is interpreted as due to collimated emission, for which we determine a jet opening angle theta_{jet} of about 3.2 degrees. An achromatic bump starting around 0.2 d after the GRB is possibly marginally detected in the optical light curves. Optical spectroscopy shows the presence of two rest-frame ultraviolet metal absorption systems at z = 1.5216 +/- 0.0006 and at z = 1.295 +/- 0.001, the former likely associated with the GRB host galaxy. Analysis of the absorption lines at z = 1.5216 suggests that the host of this GRB may be a Damped Lyman-alpha Absorber. The optical V-band afterglow appears polarized, with P= (2.4 +/- 0.6) % and theta = (170 +/- 7) degrees, suggesting an asymmetric blastwave expansion. An X-ray-to-optical spectral flux distribution of the GRB 030328 afterglow was obtained at 0.78 days after the GRB and fitted using a broken powerlaw, with an optical spectral slope beta_{opt} = 0.47 +/- 0.15, and an X-ray slope beta_{X} = 1.0 +/- 0.2. The discussion of these results in the context of the "fireball model" shows that the preferred scenario for this afterglow is collimated structured jet with fixed opening angle in a homogeneous medium.

 

astro-ph/0604094 [abs, ps, pdf, other] :

Title: New Technique for the Measurement of the Scintillation Efficiency of Nuclear Recoils
Authors: Jelena Ninkovic, Peter Christ, Godehard Angloher, Dieter Hauff, Partick Huff, Emilija Pantic, Federica Petricca, Franz Proebst, Wolfgang Seidel (Max-Planck-Institut fuer Physik (Werner-Heisenberg-Institut) Foehringer Ring 6, Munich, Germany)
Comments: Preprint accepted for publication in NIM A

We present a new technique developed for the measurement of the scintillation efficiency of nuclear recoils in solid scintillators. Using this technique we measured the quenching of the scintillation efficiency for the various recoiling nuclei in CaWO4 crystals which are used in direct Dark Matter searches.

 

astro-ph/0604095 [abs, ps, pdf, other] :

Title: Radio-Loudness of Active Galactic Nuclei: Observational Facts and Theoretical Implications
Authors: M. Sikora, L. Stawarz, J.-P. Lasota
Comments: 29 pages, 4 figures and 5 tables included. MNRAS submitted

We investigate how the radio-loudness of Active Galactic Nuclei (AGN) depends on their bolometric luminosity expressed in Eddington units (Eddington ratio) and on the central black-hole mass. In our sample we include the, often missed in other similar studies, broad-line-radio-galaxies (BLRG) and find that for the same range of Eddington ratios, the radio-loudness of BLRG is on average 3 orders larger than the radio-loudness of Seyfert galaxies. Both sub-samples form sequences along which the radio-loudness decreases with increasing Eddington ratio with slopes corresponding roughly to constant radio luminosity. At very low luminosities these two AGN sequences match respectively the sequences of FRI radio-galaxies and LINERS, but with an indication of a radio-loudness saturation. We show that these features can be naturally reproduced by assuming that the normalization of the dependence of radio-loudness on the Eddington ratio is determined by the black-hole spin and that central black holes in giant elliptical galaxies have on average spins much larger than black holes in the centers of spiral/disc galaxies. Finally, we address the issue of radio-loudness dichotomy among the near-Eddington AGN, and suggest that at high accretion rates jet production, and therefore radio-loudness, might be suppressed as is directly observed in several X-ray binaries.

 

astro-ph/0604096 [abs, ps, pdf, other] :

Title: Connection between accretion disk and superluminal radio jets and the role of radio plateau state in GRS 1915+105
Authors: J. S. Yadav
Comments: 9 pages, 6 Figures, Accepted for publication in ApJ

We investigate the association between the accretion disk during radio plateau state and the following superluminal relativistic radio jets with peak intensity varies from 200 mJy to 1000 mJy observed over a period of five years and present the evidences of direct accretion disc-jet connection in microquasar GRS 1915+105. We have analysed RXTE PCA/HEXTE X-ray data and have found that the accretion rate, $\dot{m}_{accr}$, as inferred from the X-ray flux, is very high during the radio plateaux. We suggest that the accretion disk during the radio plateaux always associated with radiation-driven wind which is manifested in the form of enhanced absorption column density for X-ray and the depleted IR emission. We find that the wind density increases with the accretion disk luminosity during the radio plateaux. The wind density is similar to the density of the warm absorber proposed in extragalactic AGNs and Quasars. We suggest a simple model for the origin of superluminal relativistic jets. Finally, We discuss the implications of this work for galactic microquasars and the extragalactic AGNs and Quasars.

 

astro-ph/0604097 [abs, ps, pdf, other] :

Title: The XMM-Newton view of the Crab
Authors: M.G.F. Kirsch, G. Schoenherr, E. Kendziorra, M.J. Freyberg, M. Martin, J. Wilms, K. Mukerjee, M.G.Breitfellner, M.J.S. Smith, R. Staubert
Comments: 9 pages

We discuss the current X-ray view of the Crab Nebula and Pulsar, summarising our analysis of observations of the source with the EPIC-pn camera on board the XMM-Newton observatory. Different modes of EPIC-pn were combined in order to yield a complete scenario of the spectral properties of the Crab resolved in space and time (pulse phase). In addition we give a description of the special EPIC-pn Burst mode and guidance for data reduction in that mode.
We analysed spectra for the nebula and pulsar separately in the 0.6-12.0 keV energy band. All data were processed with the SAS 6.0.0 XMM-Newton Scientific Analysis System package; models were fitted to the data with XSPEC 11. The high time resolution of EPIC-pn in its Burst mode (7 micros) was used for a phase resolved analysis of the pulsar spectrum, after determination of the period with epoch folding techniques. Data from the Small Window mode were processed and corrected for pile-up allowing for spectroscopy simultaneously resolved in space and time.
The spatial variation of the spectrum over the entire region of the Crab shows a gradual spectral softening from the inner pulsar region to the outer nebula region with a variation in photon index, Gamma, from 2.0 to 2.4. Pulse phase resolved spectroscopy of the Crab Pulsar reveals a phase dependent modulation of the photon index in form of a significant hardening of the spectrum in the inter-peak phase from Gamma =1.7 during the pulse peak to Gamma =1.5.

 

astro-ph/0604098 [abs, ps, pdf, other] :

Title: Mid-Infrared Images of Stars and Dust in Irregular Galaxies
Authors: Deidre A. Hunter (1), Bruce G. Elmegreen (2), Emily Martin (1) ((1) Lowell Observatory, (2) IBM T. J. Watson Research Center)
Comments: To be published in AJ Available from ftp.lowell.edu/pub/dah/papers/irac

We present mid-infrared to optical properties of 22 representative irregular galaxies: 18 Im, 3 BCDs, and one Sm. The mid-IR is based on images from the Spitzer Space Telescope archives. The 3.6 and 4.5 micron bands and the UBVJHK images are used to examine disk morphology and the integrated and azimuthally averaged magnitudes and colors of stars. The non-stellar contribution to the 4.5 micron images is used to trace hot dust. The 5.8 and 8.0 micron images reveal emission from hot dust and PAHs, and both may contribute to these passbands, although we refer to the non-stellar emission as PAH emission. We compare the 8.0 micron images to Halpha. Im galaxies have no hidden bars, and those with double-exponential optical light profiles have the same at mid-IR. Most galaxies have similar optical mid-IR scale lengths. Four galaxies have super star clusters that are not visible at optical bands. Galaxies with higher area-normalized star formation rates have more dust and PAH emission relative to starlight. Hot dust and PAH emission is found mostly in high surface brightness HII regions, implying that massive stars are the primary source of heating. Galaxies with intense, wide-spread star formation have more extended PAH emssion. The ratio of PAH to Halpha emission is not constant on small scales. PAHs are associated with shell and giant filaments, so they are not destroyed during shell formation.

 

astro-ph/0604099 [abs, ps, pdf, other] :

Title: Unexpected Dynamical Instabilities In Differentially Rotating Neutron Stars
Authors: Shangli Ou, Joel Tohline
Comments: 37 pages, to be submitted to ApJ, comments welcomed

A one-armed spiral instability has been found to develop in differentially rotating stellar models that have a relatively stiff, $n=1$ polytropic equation of state and a wide range of rotational energies. This suggests that such instabilities can arise in neutron stars that are differentially, although not necessarily rapidly, rotating. The instability seems to be directly triggered by the presence of a corotation resonance inside the star. Our analysis also suggests that a resonant cavity resulting from a local minimum in the radial vortensity profile of the star plays an important role in amplifying the unstable mode. Hence, it appears as through this instability is closely related to the so-called ``Rossby wave instability'' \citep{LLCN99} that has been found to arise in accretion disks. In addition to the one-armed ($m=1$) spiral mode, we have found that higher-order ($m = 2$ and $m=3$) nonaxisymmetric modes also can become unstable if corotation points that resonate with the eigenfrequencies of these higher-order modes also appear inside the star. The growth rate of each mode seems to depend on the location of its corotation radius with respect to the vortensity profile (or on the depth of its corotation radius inside the vortensity well). The existence of such instabilities makes the stability criterion for differentially rotating neutron stars non-unique. Also, the gravitational-waves emitted from such unstable systems generally will not have a monochromatic frequency spectrum.

 

astro-ph/0604100 [abs, ps, pdf, other] :

Title: Abundance analysis of 5 early-type stars in the young open cluster IC2391
Authors: Ch. Stuetz (1 and 2), S. Bagnulo (2), E. Jehin (2), C. Ledoux (2), R. Cabanac (3), C. Melo, J.V. Smoker (2 and 4) ((1) Inst. of Astron, Vienna, (2) ESO, (3) CFHT, (4) Queens Univ. of Belfast)
Comments: 7 pages + online material

It is unclear whether chemically peculiar stars of the upper main sequence represent a class completely distinct from normal A-type stars, or whether there exists a continuous transition from the normal to the most peculiar late F- to early B-type stars. A systematic abundance analysis of open cluster early-type stars would help to relate the observed differences of the chemical abundances of the photospheres to other stellar characteristics, without being concerned by possible different original chemical composition. Furthermore, if a continuous transition region from the very peculiar to the so called normal A-F stars exists, it should be possible to detect objects with mild peculiarities. As a first step of a larger project, an abundance analysis of 5 F-A type stars in the young cluster IC2391 was performed using high resolution spectra obtained with the UVES instrument of the ESO VLT. Our targets seem to follow a general abundance pattern: close to solar abundance of the light elements and iron peak elements, heavy elements are slightly overabundant with respect to the sun, similar to what was found in previous studies of normal field A-type stars of the galactic plane. We detected a weakly chemically peculiar star, HD74044. Its element pattern contains characteristics of CP1 as well as CP2 stars, enhanced abundances of iron peak elements and also higher abundances of Sc, Y, Ba and Ce. We did not detect a magnetic field in this star (detection limit was 2kG). We also studied the star SHJM2, proposed as a pre-main sequence object in previous works. Using spectroscopy we found a high surface gravity, which suggests that the star is very close to the ZAMS.

 

astro-ph/0604101 [abs, pdf] :

Title: Task Force on Cosmic Microwave Background Research
Authors: James Bock (Caltech/JPL), Sarah Church (Stanford), Mark Devlin (Penn), Gary Hinshaw (GSFC), Andrew Lange (Caltech), Adrian Lee (Berkeley/LBNL), Lyman Page (Princeton), Bruce Partridge (Haverford), John Ruhl (Case Western), Max Tegmark (MIT), Peter Timbie (Wisconsin), Rainer Weiss (MIT, chair), Bruce Winstein (Chicago), Matias Zaldarriaga (Harvard)
Comments: This is the final report of the DoE/NASA/NSF interagency task force on CMB research chaired by Rai Weiss. 87 pages

One of the most spectacular scientific breakthroughs in past decades was using measurements of the fluctuations in the cosmic microwave background (CMB) to test precisely our understanding of the history and composition of the Universe. This report presents a roadmap for leading CMB research to its logical next step, using precision polarization measurements to learn about ultra-high-energy physics and the Big Bang itself.

 

astro-ph/0604102 [abs, ps, pdf, other] :

Title: The impact of meridional circulation on stellar butterfly diagrams and polar caps
Authors: V. Holzwarth (1), D.H. Mackay (2), M. Jardine (1) ((1) School of Physics and Astronomy, University of St Andrews, Scotland, (2) School of Mathematics and Statistics, University of St Andrews, Scotland)
Comments: 15 pages, 12 figures, accepted for publication by MNRAS

Observations of rapidly rotating solar-like stars show a significant mixture of opposite-polarity magnetic fields within their polar regions. To explain these observations, models describing the surface transport of magnetic flux demand the presence of fast meridional flows. Here, we link sub-surface and surface magnetic flux transport simulations to investigate (i) the impact of meridional circulations with peak velocities of <125m/s on the latitudinal eruption pattern of magnetic flux tubes and (ii) the influence of the resulting butterfly diagrams on polar magnetic field properties. Prior to their eruption, magnetic flux tubes with low field strengths and initial cross sections below about 300km experience an enhanced poleward deflection through meridional flows. In particular flux tubes which originate between low and intermediate latitudes within the convective overshoot region are strongly affected. This latitude-dependent poleward deflection of erupting magnetic flux renders the wings of stellar butterfly diagrams distinctively convex. The subsequent evolution of the surface magnetic field shows that the increased number of newly emerging bipoles at higher latitudes promotes the intermingling of opposite polarities of polar magnetic fields. The associated magnetic flux densities are about 20% higher than in the case disregarding the pre-eruptive deflection, which eases the necessity for fast meridional flows predicted by previous investigations. In order to reproduce the observed polar field properties, the rate of the meridional circulation has to be on the order of 100m/s, and the latitudinal range from which magnetic flux tubes originate at the base of the convective zone (<50degrees) must be larger than in the solar case (<35degrees).

 

astro-ph/0604103 [abs, ps, pdf, other] :

Title: Statistics of Giant Radio Halos from Electron Reacceleration Models
Authors: R. Cassano, G. Brunetti, G. Setti
Comments: 21 pages, 17 figures, accepted for publication in MNRAS

The most important evidence of non-thermal phenomena in galaxy clusters comes from Giant Radio Halos (GRHs), synchrotron radio sources extended over Mpc scales, detected in a growing number of massive galaxy clusters. A promising possibility to explain these sources is given by "in situ" stochastic reacceleration of relativistic electrons by turbulence generated in the cluster volume during merger events. Cassano & Brunetti (2005) have recently shown that the expected fraction of clusters with GRHs and the increase of such a fraction with cluster mass can be reconciled with present observations provided that a fraction of 20-30 % of the turbulence in clusters is in the form of compressible modes. In this work we extend these calculations by including a scaling of the magnetic field strength with cluster mass. We show that the observed correlations between the synchrotron radio power of a sample of 17 GRHs and the X-ray properties of the hosting clusters are consistent with, and actually predicted by a magnetic field dependence on the virial mass of the form B \propto M^b, with b>0.5 and typical micro Gauss strengths of the average B intensity. The occurrence of GRHs as a function of both cluster mass and redshift is obtained. The most relevant findings are that the predicted luminosity functions of GRHs are peaked around a power P_{1.4 GHz} 10^{24} W/Hz, and severely cut-off at low radio powers due to the decrease of the electron reacceleration in smaller galaxy clusters. We expect a total number of GRHs to be discovered at ~mJy radio fluxes of ~100 at 1.4 GHz. Finally, the occurrence of GRHs and their number counts at 150 MHz are estimated in view of the fortcoming operation of low frequency observatories (LOFAR, LWA) and compared with those at higher radio frequencies.

 

astro-ph/0604104 [abs, ps, pdf, other] :

Title: A Correlation Between Pre-Main Sequence Stellar Rotation Rates and IRAC Excesses in Orion
Authors: L. M. Rebull, J. R. Stauffer, S. T. Megeath, J. L. Hora, L. Hartmann
Comments: 19 pages, 7 figures, accepted by ApJ

Early observations of T Tauri stars suggested that stars with evidence of circumstellar accretion disks rotated slower than stars without such evidence, but more recent results are not as clear. Near-IR circumstellar disk indicators, though the most widely available, are subject to uncertainties that can result from inner disk holes and/or the system inclination. Mid-infrared observations are less sensitive to such effects, but until now, these observations have been difficult to obtain. The Spitzer Space Telescope now easily enables mid-infrared measurements of large samples of PMS stars covering a broad mass range in nearby star-forming regions. Megeath and collaborators surveyed the Orion Molecular Clouds (~1 Myr) with the IRAC instrument (3.6, 4.5, 5.8, 8 microns) as part of a joint IRAC and MIPS GTO program. We examine the relationship between rotation and Spitzer mid-IR fluxes for ~900 stars in Orion for stars between 3 and 0.1 Msun. We find in these Spitzer data the clearest indication to date that stars with longer periods are more likely than those with short periods to have IR excesses suggestive of disks.

 

astro-ph/0604105 [abs, ps, pdf, other] :

Title: Evidence for chromatic X-ray light-curve breaks in Swift GRB afterglows and their theoretical implications
Authors: A. Panaitescu, P. Meszaros, D. Burrows, J. Nousek, N. Gehrels, P. O'Brien, R. Willingale
Comments: 6 pages, sumbitted to MNRAS

The power-law decay of the X-ray emission of GRB afterglows 050319, 050401, 050607, 050713A, 050802 and 050922C exhibits a steepening at about 1--4 hours after the burst which, surprisingly, is not accompanied by a break in the optical emission. If it is assumed that both the optical and X-ray afterglows arise from the same outflow then, in the framework of the standard forward shock model, the chromaticity of the X-ray light-curve breaks indicates that they do not arise solely from a mechanism related to the outflow dynamics (e.g. energy injection) or the angular distribution of the blast-wave kinetic energy (structured outflows or jets). The lack of a spectral evolution accompanying the X-ray light-curve breaks shows that these breaks do not arise from the passage of a spectral break (e.g. the cooling frequency) either. Under these circumstances, the decoupling of the X-ray and optical decays requires that the microphysical parameters for the electron and magnetic energies in the forward shock evolve in time, whether the X-ray afterglow is synchrotron or inverse-Compton emission. For a steady evolution of these parameters with the Lorentz factor of the forward shock and an X-ray light-curve break arising from cessation of energy injection into the blast-wave, the optical and X-ray properties of the above six Swift afterglows require a circumburst medium with a r^{-2} radial stratification, as expected for a massive star origin for long GRBs. Alternatively, the chromatic X-ray light-curve breaks may indicate that the optical and X-ray emissions arise from different outflows. Neither feature (evolution of microphysical parameters or the different origin of the optical and X-ray emissions) were clearly required by pre-Swift afterglows.

 

astro-ph/0604106 [abs, ps, pdf, other] :

Title: A Chandra ACIS Observation of the X-ray Luminous SN1988Z
Authors: E. M. Schlegel, R. Petre
Comments: accepted ApJ; 11 manuscript pages, 3 figures

SN1988Z is the most luminous X-ray-emitting supernova, initially detected in 1995 using the ROSAT HRI with a luminosity of ~8x10^40 erg s^-1 (Fabian & Terlevich 1996). Its high luminosity was ascribed to expansion of the blast wave into an especially dense circumstellar medium. In this paper, we describe a recent observation of SN1988Z using the ACIS detector on Chandra. We readily detect SN1988Z, obtaining ~30 net counts which corresponds to a 0.2-2 keV luminosity of ~3.2x10^39 erg s^-1. The calculated quantiles for the extracted counts allow a broad range of temperatures, but require a temperature hotter than 5 keV if there is no intrinsic absorption. The long term light curve (1995-2005) declines as t^-2.6+/-0.6. This is one of the steepest X-ray light curves. The X-ray luminosity indicates that the emitting region has a high density (>10^5 cm^-3) and that the density profile is not consistent with a constant mass loss stellar wind during the ~5000 years before the explosion. If the circumstellar medium is due to progenitor mass loss, then the mass loss rate is extremely high (~10^-3 M_sol yr^-1(v_w / 10 km s^-1)). The X-ray results are compared with the predictions of models of SN1988Z.

 

astro-ph/0604107 [abs, ps, pdf, other] :

Title: Simulations of Cosmic Chemical Enrichment
Authors: Chiaki Kobayashi, Volker Springel, Simon D. M. White
Comments: 15 pages, 19 figures, submitted to MNRAS

Using a new numerical model for cosmic chemical evolution, we study the influence of hypernova feedback on the star formation and metal enrichment history of the universe. For assumptions which produce plausible results in idealized collapse models of individual galaxies, our cosmological simulations of the standard Lambda-CDM cosmology show a peak of the cosmic star formation rate at z~4, with ~10% of the baryons turning into stars. We find that the majority of stars in present-day massive galaxies formed in much smaller galaxies at high redshifts, giving them a mean stellar age as old as 10 Gyr, despite their late assembly times. The hypernova feedback drives galactic outflows efficiently in low mass galaxies, and these winds eject heavy elements into the intergalactic medium. The ejected baryon fraction is larger for less massive galaxies, correlates well with stellar metallicity, and amounts to ~20% of all baryons in total. The resulting enrichment history is broadly consistent with the observed abundances of Lyman break galaxies, of damped Lyman alpha systems, and of the intergalactic medium. The metallicity of the cold gas in galaxies increases with galaxy mass, which is comparable to observations with a significant scatter. The stellar mass-metallicity relation of the observed galaxy population is well reproduced by the simulation model as a result of mass-dependent galactic winds, occurring after z~5. However, star formation does not terminate in massive galaxies at late times in our model, and too few dwarf galaxies are still forming stars. These problems may be due to a lack of resolution, to inappropriate modelling of supernova feedback, or to a neglect of other feedback processes such as active galactic nuclei.

 

astro-ph/0604108 [abs, ps, pdf, other] :

Title: A Massive Spiral Galaxy in the Zone of Avoidance
Authors: J. L. Donley, B. S. Koribalski, L. Staveley-Smith, R. C. Kraan-Korteweg, A. Schroeder, P. A. Henning
Comments: Accepted for publication in MNRAS, 17 pages, 11 figures, 7 tables, version with high-resolution figures available at: this http URL

We report the discovery of a very HI-massive disk galaxy, HIZOA J0836-43, at a velocity of v_hel = 10689 km/s, corresponding to a distance of 148 Mpc (assuming H_0=75 km/s/Mpc). It was found during the course of a systematic HI survey of the southern Zone of Avoidance (|b| < 5 deg) with the multibeam system at the 64m Parkes radio telescope. Follow-up observations with the Australia Telescope Compact Array (ATCA) reveal an extended HI disk. We derive an HI mass of 7.5 x 10^10 Msun. Using the HI radius, we estimate a total dynamical mass of 1.4 x 10^12 Msun, similar to the most massive known disk galaxies such as Malin 1. HIZOA J0836-43 lies deep in the Zone of Avoidance (l, b = 262.48 deg, -1.64 deg) where the optical extinction is very high, A_B = 9.8. However, in the near-infrared wavebands, where the extinction is considerably lower, HIZOA J0836-43 is clearly detected by both DENIS and 2MASS. Deep AAT near-infrared (Ks and H-band) images show that HIZOA J0836-43 is an inclined disk galaxy with a prominent bulge (scale length 2.5 arcsec or 1.7 kpc), and an extended disk (scale length 7 arcsec or 4.7 kpc) which can be traced along the major axis out to a radius of 20 arcsec or 13.4 kpc (at 20 mag/arcsec^2 in Ks). The HI disk is much more extended, having a radius of 66 kpc at 1 Msun/pc^2. Detections in the radio continuum at 1.4 GHz and at 60 micron (IRAS) are consistent with HIZOA J0836-43 forming stars at a rate of ~35 Msun/yr. We compare the properties of HIZOA J0836-43 with those of the most HI-massive galaxies currently known, UGC 4288, UGC 1752 and Malin 1, all of which are classified as giant low surface brightness galaxies.

 

astro-ph/0604109 [abs, ps, pdf, other] :

Title: Hypernova Signatures in the Late Rebrightening of GRB 050525A
Authors: M. Della Valle, D. Malesani, J.S. Bloom, S. Benetti, G. Chincarini, P. D'Avanzo, R.J. Foley, S. Covino, A. Melandri, S. Piranomonte, G. Tagliaferri, L. Stella, R. Gilmozzi, L.A. Antonelli, S. Campana, H.-W. Chen, P. Filliatre, F. Fiore, D. Fugazza, N. Gehrels, K. Hurley, I.F. Mirabel, L.J. Pellizza, L. Piro, J.X. Prochaska
Comments: 5 pages, 4 ps figures. Accepted for publication in ApJ Letters

We report observations of GRB 050525A, for which a Gemini North spectrum shows its redshift to be z = 0.606. This is the third closest long GRB discovered by Swift. We observed its afterglow using the VLT, Gemini and TNG telescopes to search for an associated SN. We find that the early-time light curve is described by a broken power law with a break at t ~ 0.3 d after the burst. About 5 d after the burst, a flattening is apparent, followed by a further dimming. Both the magnitude and the shape of the light curve suggest that a supernova was emerging during the late decay of the afterglow. This supernova, dubbed SN 2005nc, had a rise time faster than SN 1998bw and a long-lasting maximum. A spectrum obtained about 20 d (rest-frame) after the GRB resembles the spectrum of SN 1998bw obtained close to maximum light.

 

astro-ph/0604110 [abs, ps, pdf, other] :

Title: Supernova and GRB connection: Observations and Questions
Authors: Massimo Della Valle (INAF/Arcetri)
Comments: Invited review talk to appear in proceedings of the 16th Annual October Astrophysics Conference in Maryland, "Gamma Ray Bursts in the Swift Era", eds. S. Holt, N. Gehrels and J. Nousek; 13 pages, 5 figures, 2 tables

We review the observational status of the supernova/gamma-ray burst connection. Present data suggest that SNe associated with GRBs form a heterogeneous class of objects including both bright and faint hypernovae and perhaps also `standard' Ib/c events. Evidence for association with other types of core-collapse SNe (e.g. IIn) is much weaker. After combining the local GRB rate with the local SN-Ibc rate and beaming estimates, we find the ratio GRB/SNe-Ibc in the range ~0.5-4%. In most SN/GRB associations so far discovered, the SN and GRB events appear to go off simultaneously. In some cases data do not exclude that the SN explosion may have preceded the GRB by a few days. Finally we discuss a number of novel questions started by recent cases of GRB-SN associations.

 

astro-ph/0604111 [abs, ps, pdf, other] :

Title: Hypervelocity Stars I: The Spectroscopic Survey
Authors: Warren R. Brown, Margaret J. Geller, Scott J. Kenyon, Michael J. Kurtz (SAO)
Comments: 10 pages, uses emulateapj, submitted to ApJ

We discuss our targeted search for hypervelocity stars (HVSs), stars traveling with velocities so extreme that dynamical ejection from a massive black hole is their only suggested origin. Our survey, now half complete, has successfully identified a total of four probable HVSs plus a number of other unusual objects. Here we report the most recently discovered two HVSs: SDSS J110557.45+093439.5 and possibly SDSS J113312.12+010824, traveling with Galactic rest-frame velocities at least +508+-12 and +418+-10 km/s, respectively. The other late B-type objects in our survey are consistent with a population of post main-sequence stars or blue stragglers in the Galactic halo, with mean metallicity [Fe/H]=-1.3 and velocity dispersion 108+-5 km/s. Interestingly, the velocity distribution shows a tail of objects with large positive velocities that may be a mix of low-velocity HVSs and high-velocity runaway stars. Our survey also includes a number of DA white dwarfs with unusually red colors, possibly extremely low mass objects. Two of our objects are B supergiants in the Leo A dwarf, providing the first spectroscopic evidence for star formation in this dwarf galaxy within the last ~30 Myr.

 

astro-ph/0604112 [abs, ps, pdf, other] :

Title: uvbyCa H beta CCD Photometry of Clusters. VII. The Intermediate-Age Anticenter Cluster Melotte 71
Authors: B. A. Twarog (1), S. Corder (1 and 2), B. J. Anthony-Twarog (1) ((1) Univ. of Kansas, (2) Cal Tech)
Comments: Accepted for Astronomical Journal. 38 page latex file includes 11 figures and short version of data table. Full table will appear in online AJ or may be requested from authors

CCD photometry on the intermediate-band uvbyCa H beta system is presented for the anticenter, intermediate-age open cluster, Melotte 71. Restricting the data to probable single members of the cluster using the color-magnitude diagram and the photometric indices alone generates a sample of 48 F dwarfs on the unevolved main sequence. The average E(b-y) = 0.148 +/- 0.003 (s.e.m.) or E(B-V) = 0.202 +/- 0.004 (s.e.m.), where the errors refer to internal errors alone. With this reddening, [Fe/H] is derived from both m1 and hk, using H beta and b-y as the temperature index, with excellent agreement among the four approaches and a final weighted average of [Fe/H] = -0.17 +/- 0.02 (s.e.m.) for the cluster, on a scale where the Hyades has [Fe/H] = +0.12. When adjusted for the higher reddening estimate, the previous metallicity estimates from Washington photometry and from spectroscopy are now in agreement with the intermediate-band result. From comparisons to isochrones of appropriate metallicity, the cluster age and distance are determined as 0.9 +/- 0.1 Gyr and (m-M) = 12.2 +/- 0.1 or (m-M)_0 = 11.6 +/- 0.1. At this distance from the sun, Mel 71 has a galactocentric distance of 10.0 kpc on a scale where the sun is 8.5 kpc from the galactic center. Based upon its age, distance, and elemental abundances, Mel 71 appears to be a less populous analog to NGC 3960.