arXiv.org > astro-ph

Astrophysics

astro-ph new abstracts, Tue, 23 May 06 00:00:12 GMT
0605512 -- 0605570 received

astro-ph/0605512 [abs, ps, pdf, other] :

Title: Black Hole-Neutron Star Binary Merger Calculations: GRB Progenitors and the Stability of Mass Transfer
Authors: Joshua A. Faber, Thomas W. Baumgarte, Stuart L. Shapiro, Keisuke Taniguchi, Frederic A. Rasio
Comments: 8 pages, 5 figures, in Proceedings of the Albert Einstein Century International Conference, Paris, France, 2005, edited by Jean-Michel Alimi and Andre Fuzfa

We have calculated the first dynamical evolutions of merging black hole-neutron star binaries that treat the combined spacetime in a nonperturbative general relativistic framework. Using the conformal flatness approximation, we have studied how the location of the tidal disruption radius with respect to the the black hole horizon and innermost stable circular orbit (ISCO) affects the qualitative evolution of the system. Based on simple arguments, we show that for a binary mass ratio q>~0.24, tidal disruption occurs outside the ISCO, while the opposite is true for q<~0.24. When tidal disruption occurs sufficiently far outside the ISCO, mass is transferred unstably from the neutron star to the black hole, resulting in the complete disruption of the neutron star. When tidal disruption occurs slightly within the ISCO, we find that some of the mass forms an extremely hot disk around the black hole. The resulting configurations in this case are excellent candidates for the progenitors of short-hard gamma ray bursts.

 

astro-ph/0605513 [abs, ps, pdf, other] :

Title: On the internal structure of starless cores. II. A molecular survey of L1498 and L1517B
Authors: M. Tafalla, J. Santiago, P.C. Myers, P. Caselli, C.M. Walmsley, A. Crapsi
Comments: 22 pages, 12 figures, A&A accepted

[Abridged] We present a molecular survey of the starless cores L1498 and L1517B. These cores have been selected for their relative isolation and close-to-round shape, and they have been observed in a number of lines of 13 molecular species (4 already presented in the first part of this series): CO, CS, N2H+, NH3, CH3OH, SO, C3H2, HC3N, C2S, HCN, H2CO, HCO+, and DCO+. Using a physical model of core structure and a Monte Carlo radiative transfer code, we determine for each core a self-consistent set abundances that fits simultaneously the observed radial profile of integrated intensity and the emergent spectrum towards the core center (for abundant species, optically thin isopologues are used). From this work, we find that L1498 and L1517B have similar abundance patterns, with most species suffering a significant drop toward the core center. This occurs for CO, CS, CH3OH, SO, C3H2, HC3N, C2S, HCN, H2CO, HCO+, and DCO+, which we fit with profiles having a sharp central hole. The size of this hole varies with molecule: DCO+, HCN, and HC3N have the smallest holes while SO, C2S and CO have the largest holes. Only N2H+ and NH3 are present in the gas phase at the core centers. From the different behavior of molecules, we select SO, C2S, and CH3OH as the most sensitive tracers of molecular depletion. Comparing our abundance determinations with the predictions from current chemical models we find order of magnitude discrepancies. Finally, we show how the ``contribution function'' can be used to study the formation of line profiles from the different regions of a core.

 

astro-ph/0605514 [abs, ps, pdf, other] :

Title: Solutions of pulsar equation by finding the source term
Authors: Lukasz Bratek, Marcin Kolonko, Marek Kutschera
Comments: 13 pages, 4 figures

An algorithm of finding numerical solutions of pulsar equation is presented which is essentially different from the one used in the pioneering paper by Contopoulos, Kazanas and Fendt (CKF). The problem of finding the solutions was reduced to finding expansion coefficients of the source term in a base of orthogonal functions on the unit interval by minimizing a multi-variable mismatch function defined on the light cylinder. The whole infinite integration region was mapped onto a single rectangular lattice without the need of splitting it along the light cylinder. To compare the two methods on comparable size grids, the algorithm is applied to Scharlemann and Wagoner boundary conditions by which CKF solution is reconstructed that by construction passes successfully the Gruzinov test of the source function exponent.

 

astro-ph/0605515 [abs, ps, pdf, other] :

Title: Multi-colour Imaging of Ultra-compact Objects in the Fornax Cluster
Authors: A. M. Karick, M. D. Gregg, M. J. Drinkwater, M. Hilker, P. Firth
Comments: 2 pages, 1 figure, poster presentation for "Globular Clusters - Guides to Galaxies", Concepcion, Chile, March 6th-10th, 2006, eds. T. Richtler & S. Larsen (Springer)

Ultra-compact dwarf galaxies (UCDs) are a new type of galaxy we have discovered in the central region of the Fornax and Virgo clusters. Unresolved in ground-based imaging, UCDs have spectra typical of old stellar systems. Ninety-two have been found in Fornax, making them the most numerous galaxy type in the cluster. Here we present multicolour (u'g'r'i'z') imaging of the central region of the Fornax Cluster using the CTIO 4m Mosaic Telescope. The colour-magnitude relation for bright UCDs is qualitatively consistent with UCDs being the stripped nuclei of dE,Ns. However at faint magnitudes, GCs and UCDs cannot be distinguished by colour alone. High resolution spectroscopy to measure their internal velocity dispersions and metallicities, is needed to distinguish between GCs and UCDs.

 

astro-ph/0605516 [abs, ps, pdf, other] :

Title: GRB 060121: Implications of a Short/Intermediate Duration Gamma-Ray Burst at High Redshift
Authors: A. de Ugarte Postigo, A.J. Castro-Tirado, S. Guziy, J. Gorosabel, G. Johannesson, M.A. Aloy, S.McBreen, D.Q. Lamb, N.Benitez, M. Jelinek, S.B. Pandey, D. Coe, M. D. Perez-Ramirez, F.J. Aceituno, M. Alises, J.A. Acosta-Pulido, G. Gomez, R. Lopez, T.Q. Donaghy, Y.E. Nakagawa, T. Sakamoto, G.R. Ricker, F.R. Hearty, M. Bayliss, G. Gyuk, D.G. York
Comments: 4 pages, 4 figures. Submitted to ApJL

Since the discovery of the first short-hard gamma-ray burst afterglows in 2005, the handful of observed events have been found to be embedded in nearby (z < 1), bright underlying galaxies. We present multiwavelength observations of the short-duration burst GRB 060121, which is the first observed to clearly outshine its host galaxy (by a factor >10^2). A photometric redshift for this event places the progenitor at a most probable redshift of z = 4.6, with a less probable scenario of z = 1.7. In either case, GRB 060121 could be the farthermost short-duration GRB detected to date and implies an isotropic-equivalent energy release in gamma-rays comparable to that seen in long-duration bursts. We discuss the implications of the released energy on the nature of the progenitor. These results suggest that GRB 060121 may belong to a family of energetic short-duration events, lying at z > 1 and whose optical afterglows would outshine their host galaxies, unlike the first short-duration GRBs observed in 2005. The possibility of GRB 060121 being an intermediate duration burst is also discussed.

 

astro-ph/0605517 [abs, ps, pdf, other] :

Title: The Initial Mass Function of Stars
Authors: Bruce G. Elmegreen (IBM Research Division, T.J. Watson Research Center)
Comments: 6 pages, from the 7th Pacific Rim Conference on Stellar Astrophysics, 1-5 November 2005, Sejong University, Seoul, Korea, ed. H.-W. Lee

Observations and theory of the IMF are briefly reviewed. Slight variations have been observed, although they are difficult to prove unambiguously. Most detailed theoretical models reproduce the IMF, but because they use different assumptions and conditions, there is no real convergence of explanations yet.

 

astro-ph/0605518 [abs, ps, pdf, other] :

Title: Discovery of new Milky Way star cluster candidates in the 2MASS point source catalog. V. Follow-up observations of the young stellar cluster candidates RCW87, [BDSB2003]164 and [DBSB2003]172
Authors: J.Borissova, V.D.Ivanov, D.Minniti, D.Geisler
Comments: 7 pages, 18 figures, accepted for publication in Astronomy and Astrophysics

Massive clusters are more often found in actively star forming galaxies than in quiescent ones, similar to the Milky Way. We have carried out an extensive survey of obscured Milky Way clusters to determine whether our Galaxy is still forming such objects. Near-infrared spectral classification of some probable cluster members was used to derive the distances to the cluster candidates. Isochrone analysis of deep near-infrared color-magnitude diagrams allowed us to obtain age and mass estimates. We report the physical parameters of three cluster candidates:
RCW87 is approx. 25 Myr old, located at a distance of D=7.6 Kpc in the general direction of the Galactic center. Adding the mass of the suspected cluster members we obtain an estimated total cluster mass of approx. 10300 solar masses. The mid-infrared photometry indicates that some next-generation star formation is on-going in the associated HII region, probably triggered by supernovae or stellar wind from the older stars in RCW87.
The brightest member of [BDSB2003]164 is an O5V type star, based on our spectroscopy. This limits the cluster age to less than a few million years. The estimated total mass is approx. 1760 solar masses and the distance is D=3.2 Kpc.
[DBSB2003]172 lacks central concentration and most likely this is not a cluster but an extended star forming region.
The high mass of RCW87 -- if confirmed -- puts this object in line with Arches and Quintuplet, among the most young massive clusters in the Galaxy. Further study is necessary to confirm this important result.

 

astro-ph/0605519 [abs, ps, pdf, other] :

Title: Hierarchical Formation of Galactic Clusters
Authors: Bruce G. Elmegreen (IBM Research Division, T.J. Watson Research Center)
Comments: 7 pages, From the conference ``Globular Clusters, Guide to Galaxies,'' 6-10 March 2006, University of Concepcion, Chile, ed. T. Richtler, et al., ESO/Springer

Young stellar groupings and clusters have hierarchical patterns ranging from flocculent spiral arms and star complexes on the largest scale to OB associations, OB subgroups, small loose groups, clusters and cluster subclumps on the smallest scales. There is no obvious transition in morphology at the cluster boundary, suggesting that clusters are only the inner parts of the hierarchy where stars have had enough time to mix. The power-law cluster mass function follows from this hierarchical structure: n(M_cl) M_cl^-b for b~2. This value of b is independently required by the observation that the summed IMFs from many clusters in a galaxy equals approximately the IMF of each cluster.

 

astro-ph/0605520 [abs, ps, pdf, other] :

Title: On the Similarity between Cluster and Galactic Stellar Initial Mass Functions
Authors: Bruce G. Elmegreen (IBM Research Division, T.J. Watson Research Center)
Comments: 22 pages, 2 figures, ApJ, 648, in press, September 1, 2006

The stellar initial mass functions (IMFs) for the Galactic bulge, the Milky Way, other galaxies, clusters of galaxies, and the integrated stars in the Universe are composites from countless individual IMFs in star clusters and associations where stars form. These galaxy-scale IMFs, reviewed in detail here, are not steeper than the cluster IMFs except in rare cases. This is true even though low mass clusters generally outnumber high mass clusters and the average maximum stellar mass in a cluster scales with the cluster mass. The implication is that the mass distribution function for clusters and associations is a power law with a slope of -2 or shallower. Steeper slopes, even by a few tenths, upset the observed equality between large and small scale IMFs. Such a cluster function is expected from the hierarchical nature of star formation, which also provides independent evidence for the IMF equality when it is applied on sub-cluster scales. We explain these results with analytical expressions and Monte Carlo simulations. Star clusters appear to be the relaxed inner parts of a widespread hierarchy of star formation and cloud structure. They are defined by their own dynamics rather than pre-existing cloud boundaries.

 

astro-ph/0605521 [abs, ps, pdf, other] :

Title: The Theory of Bodily Tides. I. The Models and the Physics
Authors: Michael Efroimsky
Comments: Submitted to the `Journal of Geophysical Research - Planets.'

The MacDonald-Kaula-Gerstenkorn theory of bodily tides assumes constancy of the geometric lag angle, while the Singer-Mignard theory asserts constancy of the time lag. Each of these two models is tacitly based on a particular law of the geometric lag's dependence upon the main frequency of the tidal flexure. In the theory of Gerstenkorn (1955), MacDonald (1964), and Kaula (1964) the angular lag is proportional to the frequency to the power of zero, while in the theory of Singer (1968) and Mignard (1979, 1980) it is proportional to the frequency to the power of one. The actual dependence of the lag on the frequency is expected to be more complicated. Its functional form will be determined by the rheology of the planet. Each particular functional form of this dependence will unambiguously fix the form of the frequency dependence of the tidal quality factor Q. Since at present we know the functional dependence of Q upon the frequency, it enables us to reverse our line of reasoning and to single out the appropriate actual dependence of the angular lag upon the frequency. This dependence entails considerable alterations in the theory of tides.

 

astro-ph/0605522 [abs, ps, pdf, other] :

Title: Recurrence of the blue wing enhancements in the high ionization lines of SDSS 1004+4112 A
Authors: P. Gomez-Alvarez, E. Mediavilla, J. A. Munoz, S. Arribas, S.F. Sanchez, A. Oscoz, F. Prada, M. Serra-Ricart
Comments: 4 pages, 5 figures

We present integral field spectroscopic observations of the quadruple-lensed QSO SDSS 1004+4112 taken with the fiber system INTEGRAL at the William Herschel Telescope on 2004 January 19. In May 2003 a blueward enhancement in the high ionization lines of SDSS 1004+4112A was detected and then faded. Our observations are the first to note a second event of similar characteristics less than one year after. Although initially attributed to microlensing, the resemblance among the spectra of both events and the absence of microlensing-induced changes in the continuum of component A are puzzling. The lack of a convincing explanation under the microlensing or intrinsic variability hypotheses makes the observed enhancements particularly relevant, calling for close monitoring of this object.

 

astro-ph/0605523 [abs, ps, pdf, other] :

Title: Hierarchical Star Formation in the Spiral Galaxy NGC 628
Authors: Bruce G. Elmegreen (1), Debra Meloy Elmegreen (2), Rupali Chandar (3), Brad Whitmore (4), Michael Regan (4) ((1) IBM T.J. Watson Research Center, (2) Vassar College, (3) The Johns Hopkins University, (4) Space Telescope Science Institute)
Comments: 28 pages, 11 figures, ApJ, 644, June 20, 2006

The distributions of size and luminosity for star-forming regions in the nearby spiral galaxy NGC 628 are studied over a wide range of scales using progressively blurred versions of an image from the Hubble Space Telescope Advanced Camera for Surveys. Four optical filters are considered for the central region, including Halpha. Two filters are used for an outer region. The features in each blurred image are counted and measured using SExtractor. The cumulative size distribution is found to be a power law in all passbands with a slope of about -1.5 over 1.8 orders of magnitudes. The luminosity distribution is approximately a power law too, with a slope of about -1 for logarithmic intervals of luminosity. The results suggest a scale-free nature for stellar aggregates in a galaxy disk. Fractal models of thin disks reproduce the projected size distribution and suggest a projected mass distribution slope of about -0.5 for these extended regions. This mass slope converts to the observed luminosity slope if we account for luminosity evolution and longer lifetimes in larger regions.

 

astro-ph/0605524 [abs, ps, pdf, other] :

Title: IRAC and MIPS Observations of the Interacting Galaxies IC 2163 and NGC 2207: Clumpy Emission
Authors: Debra Meloy Elmegreen (1), Bruce G. Elmegreen (2), Michele Kaufman (3), Kartik Sheth (4), Curtis Struck (5), Magnus Thomasson (6), Elias Brinks (7) ((1) Vassar College, (2) IBM T.J. Watson Research Center, (3) Ohio State University, (4) Spitzer Space Center, (5) Iowa State University, (6) Onsala Space Observatory, (7) University of Hertfordshire)
Comments: 49 pages, 18 figures, ApJ, 642, 158

IC 2163 and NGC 2207 are interacting galaxies that have been well studied at optical and radio wavelengths and simulated in numerical models to reproduce the observed kinematics and morphological features. Spitzer IRAC and MIPS observations reported here show over 200 bright clumps from young star complexes. The brightest IR clump is a morphologically peculiar region of star formation in the western arm of NGC 2207. This clump, which dominates the Halpha and radio continuum emission from both galaxies, accounts for ~12% of the total 24mu m flux. Nearly half of the clumps are regularly spaced along some filamentary structure, whether in the starburst oval of IC 2163 or in the thin spiral arms of NGC 2207. This regularity appears to influence the clump luminosity function, making it peaked at a value nearly a factor of 10 above the completeness limit, particularly in the starburst oval. This is unlike the optical clusters inside the clumps, which have a luminosity function consistent with the usual power law form. The giant IR clumps presumably formed by gravitational instabilities in the compressed gas of the oval and the spiral arms, whereas the individual clusters formed by more chaotic processes, such as turbulence compression, inside these larger-scale structures.

 

astro-ph/0605525 [abs, ps, pdf, other] :

Title: Host galaxies and environments of compact extragalactic radio sources
Authors: Alvaro Labiano
Comments: Thesis dissertation. University of Groningen (The Netherlands) Full version available at: this http URL

The main goal of this thesis is to study the interrelation of powerful radio sources with their hosts. The objects of study are GPS and CSS sources. Due to their small size, GPS/CSS sources are excellent probes of this relation. Furthermore, their young age allows us to compare them to the larger, old radio sources and establish a time-line evolution of this relation.
This thesis combines imaging and spectroscopy of GPS/CSS sources at different wavelengths, and all our studies lead to the same conclusion: the presence and expansion of powerful radio sources clearly affect the properties and evolution of their hosts. All chapters of the thesis find evidence of strong interaction between the host and the radio source. Furthermore, the radio source and host can significantly affect each others evolution. However, this influence takes place in different ways. The influence that the host has on the radio source is somehow indirect. However it can completely change its destiny: depending of the contents, distribution and density of the gas, the radio source will die early, expand and grow into the large FR sources, or remain confined inside its host. In contrast, the influence of the radio source on its host seems to be more direct and takes place during its expansion through the host: the radio source will affect the kinematics and ionization of the emission line gas, and may change the star formation history of the host.

 

astro-ph/0605526 [abs, ps, pdf, other] :

Title: Escape fraction of ionizing photons from galaxies at z=0--6
Authors: Akio K. Inoue (1), Ikuru Iwata (2), Jean-Michel Deharveng (3) ((1) Osaka Sangyo Univ., (2) Okayama Astrophysical Observatory, (3) Laboratoire d'Astrophysique de Marseille)
Comments: MNRAS Letters, in press

The escape fraction of ionizing photons from galaxies is a crucial quantity controlling the cosmic ionizing background radiation and the reionization. Various estimates of this parameter can be obtained in the redshift range, z=0--6, either from direct observations or from the observed ionizing background intensities. We compare them homogeneously in terms of the observed flux density ratio of ionizing ($\sim900$ \AA rest-frame) to non-ionizing ultraviolet ($\sim1500$ \AA rest-frame) corrected for the intergalactic absorption. The escape fraction is found to increase by an order of magnitude, from a value less than 0.01 at $z\la1$ to about 0.1 at $z\ga4$.

 

astro-ph/0605527 [abs, ps, pdf, other] :

Title: Mapping the ionizing sources with CMB polarization measurements
Authors: L.A. Popa (INAF/IASF Bologna, ISS Bucharest) C. Burigana (INAF/IASF Bologna)

We show that the delay of structure formation from WMAP3 can not fully account for the reduction of electron optical depth from WMAP1 to WMAP3 when the radiative transfer effects and feedback mechanisms are took into account in computing the reionization history of the Universe. As the ultimate limit in constraining the reionizatin history of the Universe with Planck will be placed by the our understanding of systematic effects and foregrounds removal, we discuss also these aspects.

 

astro-ph/0605528 [abs, ps, pdf, other] :

Title: Nonlinear Development of Thermal Instability without External Forcing
Authors: Hiroshi Koyama, Shu-ichiro Inutsuka
Comments: 27 pages, 11 figures, submitted to the Astrophysical Journal

Supersonic turbulent motions are the remarkable properties of interstellar medium. Previous numerical simulations have demonstrated that the thermal instability in a shock-compressed layer produces the supersonic turbulent motion that does not decay. In this paper we focus on two- and three-dimensional numerical simulations of the non-linear development of simple thermal instability incorporating physical viscosity but without any external forcing, in order to isolate the effects of various processes responsible for the long-lasting turbulent motion. As the initial condition for our simulations, we set up spatially uniform gas with thermally unstable temperature in a box with periodic boundaries. After the linear growth stage of the thermal instability, two-phase medium forms where cold clumps are embedded in warm medium, and turbulent fluid flow clearly visible as translational motions of the cold clumps does not decay in a viscous dissipation timescale. The amplitude of the turbulent velocity increases when we reduce the Prandtl number that is the non-dimensional ratio of kinetic viscosity to thermal conduction: the saturation amplitude does not change when we increase the viscosity and thermal conduction coefficients simultaneously in order to keep the Prandtl number. This shows that the thermal conduction plays an important role in maintaining turbulent motions against viscous dissipation. The amplitude also increases when we increase the ratio of the computational domain length $L$ to the cooling length $\lambda_{\rm c}$ that is defined by the product of the cooling time and the sound speed, as long as $L \la 100 \lambda_{\rm c}$.

 

astro-ph/0605529 [abs, ps, pdf, other] :

Title: New Chandra observations of the jet in 3C273. I. Softer X-ray than radio spectra and the X-ray emission mechanism
Authors: S. Jester (Fermilab and U. Southampton), D. E. Harris (SAO), H. L. Marshall (MIT Kavli Institute), K. Meisenheimer (MPIA Heidelberg)
Comments: 11 pages, 5 figures, emulateapj. Accepted by ApJ

The jet in 3C273 is a high-power quasar jet with radio, optical and X-ray emission whose size and brightness allow a detailed study of the emission processes acting in it. We present deep Chandra observations of this jet and analyse the spectral properties of the jet emission from radio through X-rays. We find that the X-ray spectra are significantly softer than the radio spectra in all regions of the bright part of the jet except for the first bright "knot A", ruling out a model in which the X-ray emission from the entire jet arises from beamed inverse-Compton scattering of cosmic microwave background photons in a single-zone jet flow. Within two-zone jet models, we find that a synchrotron origin for the jet's X-rays requires fewer additional assumptions than an inverse-Compton model, especially if velocity shear leads to efficient particle acceleration in jet flows.

 

astro-ph/0605530 [abs, ps, pdf, other] :

Title: Shedding New Light on the 3C 273 Jet with the Spitzer Space Telescope
Authors: Y. Uchiyama, C. M. Urry, C. C. Cheung, S. Jester, J. Van Duyne, P. Coppi, R. M. Sambruna, T. Takahashi, F. Tavecchio, L. Maraschi
Comments: 12 pages, 8 figures (2 color figures), accepted for publication in ApJ, color images are also available at this http URL

We have performed infrared imaging of the jet of the quasar 3C 273 at wavelengths 3.6 and 5.8 microns with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. When combined with the radio, optical and X-ray measurements, the IRAC photometry clearly shows that the optical emission is dominated by the high-energy component of the jet, not by the radio synchrotron component, as had been assumed to date. The high-energy component may be due to a second synchrotron component or to IC scattering of ambient photons. In the former case, we argue that the acceleration of protons exceeding 10^16 eV or possibly even to 10^19 eV would be taking place in the jet. In contrast, the IC model, into which highly relativistic Doppler beaming has to be incorporated, requires very low-energy electrons (~ 1 MeV). The present polarization data in the radio and optical would favor the former interpretation in the case of the 3C 273 jet. Sensitive and detailed measurements of optical polarization are important to establish the radiation mechanism responsible for the high-energy emission. The present study offers new clues as to the controversial origin of the X-ray emission seen in many quasar jets.

 

astro-ph/0605531 [abs, ps, pdf, other] :

Title: The History of Galaxy Formation in Groups: An Observational Perspective
Authors: Christopher J. Conselice
Comments: Invited review, 16 pages, to be published in ESO Astrophysics Symposia: "Groups of Galaxies in the Nearby Universe", eds. I. Saviane, V. Ivanov, J. Borissova

We present a pedagogical review on the formation and evolution of galaxies in groups, utilizing observational information from the Local Group to galaxies at z~6. The majority of galaxies in the nearby universe are found in groups, and galaxies at all redshifts up to z~6 tend to cluster on the scale of nearby groups (~1 Mpc). This suggests that the group environment may play a role in the formation of most galaxies. The Local Group, and other nearby groups, display a diversity in star formation and morphological properties that puts limits on how, and when, galaxies in groups formed. Effects that depend on an intragroup medium, such as ram-pressure and strangulation, are likely not major mechanisms driving group galaxy evolution. Simple dynamical friction arguments however show that galaxy mergers should be common, and a dominant process for driving evolution. While mergers between L_* galaxies are observed to be rare at z < 1, they are much more common at earlier times. This is due to the increased density of the universe, and to the fact that high mass galaxies are highly clustered on the scale of groups. We furthermore discus why the local number density environment of galaxies strongly correlates with galaxy properties, and why the group environment may be the preferred method for establishing the relationship between properties of galaxies and their local density.

 

astro-ph/0605532 [abs, ps, pdf, other] :

Title: Dark Energy: Beyond General Relativity?
Authors: David Polarski
Comments: Invited talk at Albert Einstein's Century international conference, 18-22 July 2005, Paris, France

6 pages; Subm. to AIP Conf. Proceedings

 

astro-ph/0605533 [abs, ps, pdf, other] :

Title: Wind-Blown Bubbles around Evolved Stars
Authors: S. J. Arthur
Comments: To be published as a chapter in 'Diffuse Matter from Star Forming Regions to Active Galaxies' - A volume Honouring John Dyson. Eds. T. W. Harquist, J. M. Pittard and S. A. E. G. Falle. 22 pages, 12 figures

Most stars will experience episodes of substantial mass loss at some point in their lives. For very massive stars, mass loss dominates their evolution, although the mass loss rates are not known exactly, particularly once the star has left the main sequence. Direct observations of the stellar winds of massive stars can give information on the current mass-loss rates, while studies of the ring nebulae and HI shells that surround many Wolf-Rayet (WR) and luminous blue variable (LBV) stars provide information on the previous mass-loss history. The evolution of the most massive stars, (M > 25 solar masses), essentially follows the sequence O star to LBV or red supergiant (RSG) to WR star to supernova. For stars of mass less than 25 solar masses there is no final WR stage. During the main sequence and WR stages, the mass loss takes the form of highly supersonic stellar winds, which blow bubbles in the interstellar and circumstellar medium. In this way, the mechanical luminosity of the stellar wind is converted into kinetic energy of the swept-up ambient material, which is important for the dynamics of the interstellar medium. In this review article, analytic and numerical models are used to describe the hydrodynamics and energetics of wind-blown bubbles. A brief review of observations of bubbles is given, and the degree to which theory is supported by observations is discussed.

 

astro-ph/0605534 [abs, ps, pdf, other] :

Title: On the Rayleigh--Taylor Instability of Radio Bubbles in Galaxy Clusters
Authors: Fabio Pizzolato, Noam Soker (Technion)
Comments: Sumbitted to the MNRAS

We consider the Rayleigh--Taylor instability of the rarefied radio bubbles (cavities) observed in many cooling flow clusters of galaxies. The top of a bubble becomes prone to the Rayleigh--Taylor instability as the bubble buoyantly rises through the intra--cluster medium (ICM). We show that the deceleration of the bubble--ICM interface during the bubble initial inflation phase is able to reverse the Rayleigh--Taylor instability criterion. In addition, the inflation introduces a drag effect which increases substantially the instability growth time. The combined action of these two effects considerably delays the onset of the instability. Consequently, the lifetime of the bubbles is lengthened, removing the need to invoke stabilising magnetic fields.

 

astro-ph/0605535 [abs, ps, pdf, other] :

Title: Perspectives of High Energy Neutrino Astronomy
Authors: Paolo Lipari
Comments: Proceedings of "Very Large Volume neutrino Telescopes" workshop (Catania, november 2005). 16 pages, 10 figures

This work discusses the perspectives to observe fluxes of high energy astrophysical neutrinos with the planned km3 telescopes. On the basis of the observations of GeV and TeV gamma-rays, and of ultra high energy cosmic rays, it is possible to construct well motivated predictions that indicate that the discovery of such fluxes is probable. However the range of these predictions is broad, and the very important opening of the ``neutrino window'' on the high energy universe is not guaranteed with the current design of the detectors. The problem of enlarging the detector acceptance using the same (water/ice Cherenkov) or alternative (acoustic/radio) techniques is therefore of central importance.

 

astro-ph/0605536 [abs, ps, pdf, other] :

Title: Weighing the Universe with Photometric Redshift Surveys and the Impact on Dark Energy Forecasts
Authors: Lloyd Knox (UC Davis), Yong-Seon Song (U Chicago), Hu Zhan (UC Davis)
Comments: 6 pages, submitted to ApJ

With a wariness of Occam's razor awakened by the discovery of cosmic acceleration, we abandon the usual assumption of zero mean curvature and ask how well it can be determined by planned surveys. We also explore the impact of uncertain mean curvature on forecasts for the performance of planned dark energy probes. We find that weak lensing and photometric baryon acoustic oscillation data, in combination with CMB data, can determine the mean curvature well enough that the residual uncertainty does not degrade constraints on dark energy. We also find that determinations of curvature are highly tolerant of photometric redshift errors.

 

astro-ph/0605537 [abs, ps, pdf, other] :

Title: Modeling dark energy with a top-down approach
Authors: Duane A. Dicus, Wayne W. Repko
Comments: 6 pages, 4 figures

We investigate a top-down approach for modeling the dark energy where we fit the luminosity distances directly rather than indirectly fitting the equation of state.

 

astro-ph/0605538 [abs, ps, pdf, other] :

Title: Heavy Element Abundances in Giant Stars of 47 Tuc
Authors: E. C. Wylie, P. L. Cottrell, C. A. Sneden, J. C. Lattanzio

This research forms part of an investigation into heavy element abundances in Asymptotic Giant Branch (AGB) stars in various stellar environments. Seven giant stars in the southern globular cluster 47 Tuc have been observed using the Anglo-Australian Telescope. Abundances for five s- and r- process elements have been determined: the light s-process elements, Y and Zr; the heavy s-process elements, La and Nd; and the r-process element Eu. Mean enhancements in the light s-process, [ls/Fe], of ~+0.6 dex and heavy s-process elements, [hs/Fe], of ~+0.3 dex were determined for all the giant stars. There was no statistically significant difference between the abundances determined for the Red Giant Branch (RGB) and AGB stars in this study. The results for the RGB stars differ from those obtained by a number of previous studies. However, because of the similar abundance results obtained for the AGB and RGB stars in this study we believe this provides evidence for previous enrichment of the material from which these stars formed.

 

astro-ph/0605539 [abs, ps, pdf, other] :

Title: Characterization of jovian plasma embedded dust particles
Authors: Amara L. Graps (INAF-IFSI, Rome, Italy)
Comments: 17 pages, 1 postscript figure, Planetary and Space Science (in press), Collected papers of the Dusty Rings ISSI Workshop June, 2005. For high resolution version, see: this http URL

As the data from space missions and laboratories improve, a research domain combining plasmas and charged dust is gaining in prominence. Our solar system provides many natural laboratories such as planetary rings, comet comae and tails, ejecta clouds around moons and asteroids, and Earth's noctilucent clouds for which to closely study plasma-embedded cosmic dust. One natural laboratory to study electromagnetically-controlled cosmic dust has been provided by the Jovian dust streams and the data from the instruments which were on board the Galileo spacecraft. Given the prodigious quantity of dust poured into the Jovian magnetosphere by Io and its volcanoes resulting in the dust streams, the possibility of dusty plasma conditions exist. This paper characterizes the main parameters for those interested in studying dust embedded in a plasma with a focus on the Jupiter environment. I show how to distinguish between dust-in-plasma and dusty-plasma and how the Havnes parameter P can be used to support or negate the possibility of collective behavior of the dusty plasma. The result of applying these tools to the Jovian dust streams reveals mostly dust-in-plasma behavior. In the orbits displaying the highest dust stream fluxes, portions of orbits E4, G7, G8, C21 satisfy the minimum requirements for a dusty plasma. However, the P parameter demonstrates that these mild dusty plasma conditions do not lead to collective behavior of the dust stream particles.

 

astro-ph/0605540 [abs, ps, pdf, other] :

Title: Helium enhancements in globular cluster stars from Asymptotic Giant Branch star pollution
Authors: Amanda Karakas (McMaster University), Yeshe Fenner (CfA, Harvard University), Alison Sills (McMaster University), Simon Campbell (Monash University), John Lattanzio (Monash University)
Comments: To appear in Proceedings of The VIII Torino Workshop on Nucleosynthesis in AGB stars: Constraints on AGB Nucleosynthesis from Observations, edited by C. Abia and I. Dominguez

Using a chemical evolution model we investigate the intriguing suggestion that there are populations of stars in some globular clusters (e.g. NGC 2808, omega Centauri) with enhanced levels of helium (Y from about 0.28 to 0.40) compared to the majority of the population that presumably have a primordial helium abundance. We assume that a previous generation of massive low-metallicity Asymptotic Giant Branch (AGB) stars has polluted the cluster gas via a slow stellar wind. We use two independent sets of AGB yields computed from detailed models to follow the evolution of helium, carbon, nitrogen and oxygen in the cluster gas using a Salpeter initial mass function (IMF) and a number of top-heavy IMFs. In no case were we able to fit the observational constraints, Y > 0.30 and C+N+O approximately constant. Depending on the shape of the IMF and the yields, we either obtained Y approximately greater than 0.30 and large increases in C+N+O or Y < 0.30 and C+N+O approximately constant. These results suggest that either AGB stars alone are not responsible for the large helium enrichment or that any dredge-up from this generation of stars was less than predicted by standard models.

 

astro-ph/0605541 [abs, ps, pdf, other] :

Title: Modelling galaxy spectra in presence of interstellar dust. II. From the UV to the far infrared
Authors: Lorenzo Piovan, Rosaria Tantalo, Cesare Chiosi
Comments: 28 pages, 23 figures and 1 table. Accepted by MNRAS

In this paper, we present spectrophotometric models for galaxies of different morphological type whose spectral energy distributions (SEDs) take into account the effect of dust in absorbing UV-optical light and re-emitting it in the infrared (IR). The models contain three main components: (i) the diffuse interstellar medium (ISM) composed of gas and dust, (ii) the large complexes of molecular clouds (MCs) in which new stars are formed and (iii) the stars of any age and chemical composition. The galaxy models stand on a robust model of chemical evolution taylored in such a way to match the gross properties of galaxies of different morphological type. A simple geometrical model is adopted for each type of galaxy. The total gas and star mass provided by the chemical model are distributed over the whole volume by means of suitable density profiles, one for each component and depending on the galaxy type (spheroidal, disk and disk plus bulge). The galaxy is then split in suitable volume elements to each of which the appropriate amounts of stars, MCs and ISM are assigned. We derive the SEDs of galaxies of different morphological type. First the the basic relations driving the interaction between the physical components of the galaxy are presented. Second, the main parameters are examined and their effects on the SED of three prototype galaxies (a disk, an elliptical and a starburster) are highlighted. The final part of the paper is devoted to assess the ability of our galaxy models in reproducing the SEDs of a few real galaxies of the Local Universe.

 

astro-ph/0605542 [abs, ps, pdf, other] :

Title: The Structure of Rapidly Rotating Late-Type Spirals: I. Photometry, HI and Optical Kinematics
Authors: K. Spekkens, R. Giovanelli
Comments: 26 pages, 15 figures, accepted for publication in the Astronomical Journal. For version with high resolution figures, see this http URL

We present I-band photometry, long-slit optical spectroscopy, and new aperture synthesis HI observations for 8 late-type spirals with rotation velocities in the range 243 km/s < V_{rot} < 308 km/s. The sample will be used to study the structure and angular momentum of disks at the high-mass end of the spiral galaxy population; here we discuss the basic properties of these ``fast rotators'', and derive hybrid optical/HI rotation curves for each. Despite the presence of HI warps and low-mass companions in many systems, their kinematics are regular and there is excellent agreement between optical and HI tracers near the optical radius r_{opt}. At high inclinations where projection effects are negligible, the sample galaxies exhibit flat, featureless rotation curves out to their last measured points at 1.7-3.5 r_{opt}. The intermediate inclination systems are also consistent with a constant rotation amplitude for r > 0.5 r_{opt}. We therefore find no evidence for declining rotation curves at the high-mass end of the late-type spiral galaxy population. Combining our data with the compilation of spirals with reliable outer HI kinematics from Casertano & van Gorkom (1991), we find no convincing trends between logarithmic outer rotation curve slopes and rotation amplitudes or surface brightnesses for galaxies with V_{rot} > 220 km/s, and correlations with morphological types and disk scale-lengths in this regime are marginal.

 

astro-ph/0605543 [abs, ps, pdf, other] :

Title: Cleaning up Eta Carinae: Detection of Ammonia in the Homunculus
Authors: Nathan Smith, Kate J. Brooks, Baerbel S. Koribalski, John Bally
Comments: 4 pages, accepted to ApJ letters

We report the first detection of ammonia in the Homunculus nebula around eta Carinae, which is also the first detection of emission from a polyatomic molecule in this or any other luminous blue variable (LBV) nebula. Observations of the NH3 (J,K)=(3,3) inversion transition made with the Australia Telescope Compact Array reveal emission at locations where infrared H2 emission had been detected previously, near the strongest dust emission in the core of the Homunculus. We also detect ammonia emission from the so-called ``strontium filament'' in the equatorial disk. The presence of NH3 around eta Car hints that molecular shells around some Wolf-Rayet stars could have originated in prior LBV eruptions, rather than in cool red supergiant winds or the ambient interstellar medium. Combined with the lack of any CO detection, NH3 seems to suggest that the Homunculus is nitrogen rich like the ionized ejecta around eta Car. It also indicates that the Homunculus is a unique laboratory in which to study unusual molecule and dust chemistry, as well as their rapid formation in a nitrogen-rich environment around a hot star. We encourage future observations of other transitions like NH3 (1,1) and (2,2), related molecules like N2H+, and renewed attempts to detect CO.

 

astro-ph/0605544 [abs, ps, pdf, other] :

Title: Aromatic Features in M101 HII Regions and Starburst Galaxies
Authors: Karl D. Gordon, C. W. Engelbracht, J.-D.T. Smith, G. H. Rieke, K. A. Misselt
Comments: 6 pages, To appear in the proceedings for the Spitzer Science Center 2005 Conference: Infrared Diagnostics of Galaxy Evolution, Ed. R. Chary. November, 2005, Pasadena

The behavior of the aromatic features as a function of metallicity/radiation field hardness in M101 HII regions and starburst galaxies is investigated using Spitzer/IRS spectra. These spectra reveal that the 6.2, 7.8+8.6, and 11.3 micron aromatics have different dependencies on metallicity/radiation field hardness. Specifically, the 6.2 and 7.8+8.6 micron aromatics are weak or absent at a metallicity, 12+log(O/H), of approx. 8.0 while the 11.3 micron feature remains relatively strong. These results apply to both the M101 HII regions and starbursts showing that HII regions can indeed be thought of as ``mini-starbursts.'' Comparison of this work with various candidate materials results in a good match with the annealing behavior in Quenched Carbonaceous Composite (QCC) which is an amorphous, hydrogenated, carbonaceous solid.

 

astro-ph/0605545 [abs, ps, pdf, other] :

Title: Missing Galactic PNe: [S III] Imaging Survey
Authors: J. Shiode, D. P. Clemens, K. A. Janes, A. Pinnick
Comments: 2 pages, 2 figures, to be published in Proc. IAU Symp. 234, Planetary Nebulae in our Galaxy and Beyond (3-7 April 2006), eds. M. J. Barlow & R. H. Mendez (Cambridge Univ. Press)

The total number of Galactic planetary nebulae is highly uncertain; the most inclusive current catalog contains only ~ 1,500. We will use the PRISM wide-field imager on the 1.83 m Perkins Telescope to conduct a pilot survey of the Galactic plane in search of [S III] emission from planetary nebulae obscured by dust and missed by surveys of H-alpha. We are employing the method of Jacoby & Van de Steene, who surveyed the bulge for 9532 Angstrom [S III] emission. In addition to seeing through more of the extinction, use of the [S III] emission line will a priori reject the most troublesome catalog contaminants: ultracompact H II regions.

 

astro-ph/0605546 [abs, ps, pdf, other] :

Title: The multi-frequency angular power spectrum of the epoch of reionization 21 cm signal
Authors: Kanan K. Datta, T. Roy Choudhury, Somnath Bharadwaj
Comments: 11 pages, 5 figures

Observations of redshifted 21cm radiation from HI at high redshifts is an important future probe of reionization. We consider the Multi-frequency Angular Power Spectrum (MAPS) to quantify the statistics of the HI signal as a joint function of the angular multipole l and frequency separation \Delta\nu. The signal at two different frequencies is expected to get decorrelated as \Delta\nu is increased, and quantifying this decorrelation is particularly important in deciding the frequency resolution for future HI observations. This is also expected to play a very crucial role in extracting the signal from foregrounds as the signal is expected to decorrelate much faster than the foregrounds (which are largely continuum sources) with increasing \Delta\nu. In this paper we develop formulae relating the MAPS to different components of the three dimensional HI power spectrum taking into account HI peculiar velocities. We show that the flat-sky approximation provides a very good representation over the angular scales of interest, and a final expression which is very simple to calculate and interpret. We present results considering two models for the HI distribution, namely, (i) DM: where the HI traces the dark matter and (ii) PR: where the effects of patchy reionization are incorporated through two parameters. We find that while the DM signal is largely featureless, the PR signal peaks at the angular scales of the individual bubbles, and the signal is considerably enhanced for large bubble size. For most cases of interest at l \sim 100 the signal is uncorrelated beyond \Delta\nu \sim 1 MHz or even less, whereas it occurs around \sim 0.1 MHz at l \sim 10^3. The \Delta\nu dependence also carries an imprint of the bubble size and the bias, and is expected to be an important probe of the reionization scenario (abridged).

 

astro-ph/0605547 [abs, ps, pdf, other] :

Title: Optical Monitoring of BL Lacertae Object OJ 287: a 40-Day Period?
Authors: Jianghua Wu, Xu Zhou, Xuebing Wu, Fukun Liu, Bo Peng, Jun Ma, Zhenyu Wu, Zhaoji Jiang, Jiansheng Chen
Comments: 31 pages, 8 figures, accepted by AJ

We present the results of our optical monitoring of the BL Lacertae object OJ 287 during the first half of 2005. The source did not show large-amplitude variations during this period and was in a relatively quiescent state. A possible period of 40 days was derived from its light curves in three BATC wavebands. A bluer-when-brighter chromatism was discovered, which is different from the extremely stable color during the outburst in 1994--96. The different color behaviors imply different variation mechanisms in the two states. We then re-visited the optical data on OJ 287 from the OJ-94 project and found as well a probable period of 40 days in its optical variability during the late-1994 outburst. The results suggest that two components contribute to the variability of OJ 287 during its outburst state. The first component is the normal {\sl blazar} variation. This component has an amplitude similar to that of the quiescent state and also may share a similar periodicity. The second component can be taken as a `low-frequency modulation' to the first component. It may be induced by the interaction of the assumed binary black holes in the center of this object. The 40-day period may be related to the helical structure of the magnetic field at the base of the jet, or to the orbital motion close to the central primary black hole.

 

astro-ph/0605548 [abs, ps, pdf, other] :

Title: Imprint of Gravitational Lensing by Population III Stars in Gamma Ray Burst Light Curves
Authors: Y. Hirose, M. Umemura, A. Yonehara, J. Sato
Comments: 13 pages, 13 figures, accepted for publication in ApJ

We propose a novel method to extract the imprint of gravitational lensing by Pop III stars in the light curves of Gamma Ray Bursts (GRBs). Significant portions of GRBs can originate in hypernovae of Pop III stars and be gravitationally lensed by foreground Pop III stars or their remnants. If the lens mass is on the order of $10^2-10^3M_\odot$ and the lens redshift is greater than 10, the time delay between two lensed images of a GRB is $\approx 1$s and the image separation is $\approx 10 \mu$as. Although it is difficult to resolve the two lensed images spatially with current facilities, the light curves of two images are superimposed with a delay of $\approx 1$ s. GRB light curves usually exhibit noticeable variability, where each spike is less than 1s. If a GRB is lensed, all spikes are superimposed with the same time delay. Hence, if the autocorrelation of light curve with changing time interval is calculated, it should show the resonance at the time delay of lensed images. Applying this autocorrelation method to GRB light curves which are archived as the {\it BATSE} catalogue, we demonstrate that more than half light curves can show the recognizable resonance, if they are lensed. Furthermore, in 1821 GRBs we actually find one candidate of GRB lensed by a Pop III star, which may be located at redshift 20-200. The present method is quite straightforward and therefore provides an effective tool to search for Pop III stars at redshift greater than 10. Using this method, we may find more candidates of GRBs lensed by Pop III stars in the data by the {\it Swift} satellite.

 

astro-ph/0605549 [abs, pdf] :

Title: Variable Very High Energy Gamma-ray Emission from the Microquasar LS I +61 303
Authors: MAGIC Collaboration: J. Albert, et al
Comments: 11 pages with 4 figures

Microquasars are binary star systems with relativistic radio-emitting jets. They are potential sources of cosmic rays and laboratories for elucidating the physics of relativistic jets. Here we report the detection of variable gamma-ray emission above 100 gigaelectron volts from the microquasar LS I +61 303. Six orbital cycles were recorded. Several detections occur at a similar orbital phase, suggesting the emission is periodic. The strongest gamma-ray emission is not observed when the two stars are closest to one another, implying a strong orbital modulation of the emission or the absorption processes.

 

astro-ph/0605550 [abs, ps, pdf, other] :

Title: Equation of State in Numerical Relativistic Hydrodynamics
Authors: Dongsu Ryu (1), Indranil Chattopadhyay (1), Eunwoo Choi (2) ((1) Chungnam National University, Korea, (2) Georgia State University, USA)
Comments: To appear in the ApJS September 2006, v166n1 issue. Pdf with full resolution figures can be downloaded from this http URL

Relativistic temperature of gas raises the issue of the equation of state (EoS) in relativistic hydrodynamics. We study the EoS for numerical relativistic hydrodynamics, and propose a new EoS that is simple and yet approximates very closely the EoS of the single-component perfect gas in relativistic regime. We also discuss the calculation of primitive variables from conservative ones for the EoS's considered in the paper, and present the eigenstructure of relativistic hydrodynamics for a general EoS, in a way that they can be used to build numerical codes. Tests with a code based on the Total Variation Diminishing (TVD) scheme are presented to highlight the differences induced by different EoS's.

 

astro-ph/0605551 [abs, ps, pdf, other] :

Title: Spectral analyses of eighteen hot H-deficient (pre-) white dwarfs from the Sloan Digital Sky Survey Data Release 4
Authors: S.D. Huegelmeyer, S. Dreizler, D. Homeier, J. Krzesinski, K. Werner, A. Nitta, S.J. Kleinman
Comments: 9 pages, 6 figures, accepted for publication in A&A

Context: The Sloan Digital Sky Survey Data Release 4 has provided spectra of several new PG 1159 stars and DO white dwarfs. This increase in known hot H-deficient compact objects significantly improves the statistics and helps to investigate late stages of stellar evolution. Aims: From the optical SDSS spectra, effective temperatures and surface gravities are derived in order to place the observed objects in an evolutionary context. Especially the connection between PG 1159 stars and DO white dwarfs shall be investigated. Method: Using our non-LTE model atmospheres and applying chi^2-fitting techniques, we determine stellar parameters and their errors. We derive total stellar masses for the DO white dwarfs using model evolutionary tracks. Results: We confirm three PG 1159 stars, with one showing ultra-high excitation ion features, and one sdO which we originally classified as a PG 1159 star. Additionally, we re-analysed the known PG 1159 star, PG 1424+535, with our new models. Furthermore, we present the first spectral analyses of thirteen DO white dwarfs, three of which show M-star features in their spectra, while two display ultra-high excitation ion features.

 

astro-ph/0605552 [abs, ps, pdf, other] :

Title: The planets capture model of V838 Monocerotis: conclusions for the penetration depth of the planet/s
Authors: Alon Retter, Bing Zhang, Lionel Siess, Amir Levinson
Comments: 8 pages, 2 figures, MNRAS, accepted

V838 Mon is the prototype of a new class of objects. Understanding the nature of its multi-stage outburst and similar systems is challenging. So far, several scenarios have been invoked to explain this group of stars. In this work, the planets-swallowing model for V838 Mon is further investigated, taking into account the findings that the progenitor is most likely a massive B type star. We find that the super-Eddington luminosity during the eruption can explain the fast rising times of the three peaks in the optical light curve. We used two different methods to estimate the location where the planets were consumed. There is a nice agreement between the values obtained from the luminosities of the peaks and from their rising time scale. We estimate that the planets were stopped at a typical distance of one solar radius from the center of the host giant star. The planets-devouring model seems to give a satisfying explanation to the differences in the luminosities and rising times of the three peaks in the optical light curve of V838 Mon. The peaks may be explained by the consumption of three planets or alternatively by three steps in the terminal falling process of a single planet. We argue that only the binary merger and the planets-swallowing models are consistent with the observations of the new type of stars defined by V838 Mon.

 

astro-ph/0605553 [abs, ps, pdf, other] :

Title: Asteroseismic signatures of helium gradients in late F-type stars
Authors: M. Castro, S. Vauclair
Comments: 8 pages, 11 figures, 2 tables Accepted for publication in Astronomy and Astrophysics. The official date of acceptance is 03/05/2006

Element diffusion is expected to occur in all kinds of stars : according to the relative effect of gravitation and radiative acceleration, they can fall or be pushed up in the atmospheres. Helium sinks in all cases, thereby creating a gradient at the bottom of the convective zones. This can have important consequences for the sound velocity, as has been proved in the sun with helioseismology.
We investigate signatures of helium diffusion in late F-type stars by asteroseismology.
Stellar models were computed with different physical inputs (with or without element diffusion) and iterated in order to fit close-by evolutionary tracks for each mass. The theoretical oscillation frequencies were computed and compared for pairs of models along the tracks. Various asteroseismic tests (large separations, small separations, second differences) were used and studied for the comparisons.
The results show that element diffusion leads to changes in the frequencies for masses larger than 1.2 Msun. In particular the helium gradient below the convective zone should be detectable through the second differences.

 

astro-ph/0605554 [abs, ps, pdf, other] :

Title: V4745 Sgr -- a nova above the period gap and an intermediate polar candidate
Authors: Andrej Dobrotka, Alon Retter, Alex Liu
Comments: submited to MNRAS

A period analysis of CCD unfiltered photometry of V4745 Sgr (Nova Sgr 2003 #1) performed during 23 nights in the years 2003 - 2005 is presented. The photometric data are modulated with a period of 0.20782 +- 0.00003 d (4.98768 \+- 0.00072 h). Following the shape of the phased light curve and the presence of the periodicity in all data sets with no apparent change in its value, we interpret this periodicity as orbital in nature and this is consistent with a cataclysmic variable above the period gap. We found a probable short-term periodicity of 0.017238 +- 0.000037 d (24.82272 +- 0.05328 m) which we interpret as the spin period of the white dwarf or the beat period between the orbital and spin period. Therefore, nova V4745 Sgr should be classified as an intermediate polar candidate, supporting the proposed link between transition-oscillation novae and intermediate polars. The mass-period relation for cataclysmic variables yields a secondary mass of about 0.52 +- 0.05 M_SOLAR.

 

astro-ph/0605555 [abs, ps, pdf, other] :

Title: A catalogue of ULX coincidences with FIRST radio sources
Authors: J.R. Sanchez-Sutil (1), A.J. Munoz-Arjonilla (1), J. Marti (2,1), J.L. Garrido (2,1), D. Perez-Ramirez (2,1), P. Luque-Escamilla (2,1) ((1) Grupo de Investigacion FQM-322, Universidad de Jaen, Spain, (2) Departamento de Fisica, EPS, Universidad de Jaen, Spain)
Comments: Accepted for publication in A&A; 17 pages, 9 figures

We search for ultra luminous X-ray source (ULXs) radio counterparts located in nearby galaxies in order to constrain their physical nature. Our work is based on a systematic cross-identification of the most recent and extensive available ULX catalogues and archival radio data. A catalogue of 70 positional coincidences is reported. Most of them are located within the galaxy nucleus. Among them, we find 11 new cases of non-nuclear ULX sources with possibly associated radio emission.

 

astro-ph/0605556 [abs, ps, pdf, other] :

Title: New Pulsating White Dwarfs in Cataclysmic Variables
Authors: R. Nilsson, H. Uthas, M. Ytre-Eide, J-E. Solheim, B. Warner
Comments: 5 pages, 6 figures, 2 tables, accepted for publication in MNRAS Letters

The number of discovered non-radially pulsating white dwarfs (WDs) in cataclysmic variables (CVs) is increasing rapidly by the aid of the Sloan Digital Sky Survey (SDSS). We performed photometric observations of two additional objects, SDSS J133941.11+484727.5 (SDSS 1339), independently discovered as a pulsator by Gansicke et al. (2006), and SDSS J151413.72+454911.9, which we identified as a CV/ZZ Ceti hybrid. In this letter we present the results of the remote observations of these targets performed with the Nordic Optical Telescope (NOT) during the Nordic-Baltic Research School at Moletai Observatory, and follow-up observations executed by NOT in service mode. We also present 3 candidates we found to be non-pulsating. The results of our observations show that the main pulsation frequencies agree with those found in previous CV/ZZ Ceti hybrids, but specifically for SDSS 1339 the principal period differs slightly between individual observations and also from the recent independent observation by Gansicke et al. (2006). Analysis of SDSS colour data for the small sample of pulsating and non-pulsating CV/ZZ Ceti hybrids found so far, seems to indicate that the r-i colour could be a good marker for the instability strip of this class of pulsating WDs.

 

astro-ph/0605557 [abs, ps, pdf, other] :

Title: Reconciling optical and X-ray mass estimates: the case of the elliptical galaxy NGC3379
Authors: S. Pellegrini, L. Ciotti
Comments: 8 pages, 4 figures, accepted by MNRAS

NGC3379 is a well studied nearby elliptical for which optical investigations have claimed a little dark matter content, or even no dark matter. Recently, its total mass profile M(r) has been derived by exploiting Chandra observations of its extended and X-ray emitting interstellar medium, based on the hypothesis of hydrostatic equilibrium for the hot gas. The resulting total mass within the effective radius Re has been claimed to be a few times larger than found by optical studies. Here we show that part of the discrepancy can be due to an underestimate of the optically derived mass, and the remaining discrepancy of a factor of ~2 can be explained by deviations from hydrostatic equilibrium of the hot gas. By using hydrodynamical simulations tailored to reproduce the observed hot gas properties of NGC3379, and by assuming as input for the simulations the total mass profile derived optically, we show that i) the hot gas at the present time has X-ray properties consistent with those observed only if it is outflowing over most of the galactic body, and ii) an overestimate of M of the same size found in the recent X-ray analysis is recovered when assuming hydrostatic equilibrium. We also show that the hot gas is outflowing even for a dark matter fraction within Re as large as derived with the standard X-ray procedure based on the hydrostatic equilibrium assumption, which shows the unapplicability of the method for this galaxy. Finally, we find that the whole range of dark mass amount and distribution allowed for by optical studies is compatible with a hot gas flow with the observed X-ray properties.

 

astro-ph/0605558 [abs, ps, pdf, other] :

Title: Probable detection of H2D+ in the starless core Barnard 68
Authors: M.R. Hogerheijde, P. Caselli, M. Emprechtinger, F.F.S. van der Tak, J. Alves, A. Belloche, R. Guesten, A.A. Lundgren, L-A. Nyman, N. Volgenau, M.C. Wiedner
Comments: APEX A&A special issue, accepted

The presence of H2D+ in dense cloud cores underlies ion-molecule reactions that strongly enhance the deuterium fractionation of many molecular species. We determine the H2D+ abundance in one starless core, Barnard 68, that has a particularly well established physical, chemical, and dynamical structure. We observed the ortho-H2D+ ground-state line 1_10-1_11, the N2H+ J=4-3 line, and the H13CO+ 4-3 line with the APEX telescope. We report the probable detection of the o-H2D+ line at an intensity Tmb=0.22 +- 0.08 K and exclusively thermal line width, and find only upper limits to the N2H+ 4-3 and H13CO+ 4-3 intensities. Within the uncertainties in the chemical reaction rates and the collisional excitation rates, chemical model calculations and excitation simulations reproduce the observed intensities and that of o-H2D+ in particular.

 

astro-ph/0605559 [abs, ps, pdf, other] :

Title: IGR J17544-2619: A new supergiant fast X-ray transient revealed by optical/infrared observations
Authors: L. J. Pellizza (1), S. Chaty (1), I. Negueruela (2) ((1) AIM / CEA Saclay - France, (2) Univ. de Alicante - Spain)
Comments: 7 pages, 4 figures. Accepted for publication in A&A

One of the most recent discoveries of the INTEGRAL observatory is the existence of a previously unknown population of X-ray sources in the inner arms of the Galaxy. IGR J17544-2619, IGR J16465-4507 and XTE J1739-302 are among these sources. Although the nature of these systems is still unexplained, the investigations of the optical/NIR counterparts of the two last sources, combined with high energy data, have provided evidence of them being highly absorbed high mass X-ray binaries with blue supergiant secondaries and displaying fast X-ray transient behaviour. In this work we present our optical/NIR observations of IGR J17544-2619, aimed at identifying and characterizing its counterpart. We show that the source is a high mass X-ray binary at a distance of 2-4 kpc with a strongly absorbed O9Ib secondary, and discuss the nature of the system.

 

astro-ph/0605560 [abs, ps, pdf, other] :

Title: Quantitative X-ray spectroscopy of massive stars
Authors: L.M. Oskinova, W.-R. Hamann, A. Feldmeier (Potsdam University)
Comments: 6 pages, 7 figures, in Proc. MSSL Workshop no.2 "High resolution X-ray spectroscopy: towards XEUS and Con-X", MSSL, 27 - 28 March 2006

Radiative transfer in a clumped winds is used to describe X-ray emission line profiles observed in the XMM-Newton RGS spectrum of the OI star Zeta Puppis. It is shown that this X-ray spectrum can be explained as originating from a multi-temperature collisional plasma located in the wind acceleration zone. The X-rays are attenuated in the clumped stellar wind, which gives characteristic profiles to the emergent lines. We specifically study the N VII emission line in the spectrum of Zeta Pup. Long RGS exposures reveal that the N VII line profile is structured. On the basis of our Zeta Pup atmosphere model, we rule out the presence of N VII in the cool wind component. We suggest that the detailed N VII line structure is due to self-absorption in the hot plasma. Wind clumping also affects the transfer of ionizing radiation in high-mass X-ray binaries (HMXBs). We derive analytical formulae for the ionisation parameter in dependence on the parameters of wind clumping.

 

astro-ph/0605561 [abs, ps, pdf, other] :

Title: Optical Counterparts of Ultra-Luminous X-ray Sources identified from Archival Hubble Space Telescope/WFPC2
Authors: A. Ptak (1), E. Colbert (1), R. Van der Marel (2), E. Roye (2), T. Heckman (1), B. Towne (1) ((1) JHU (2) STSci)
Comments: 35 pages with 9 figures formatted with emulateapj. Only subset of figures 1 and 2 are shown, for full version see this http URL

We present a systematic analysis of archival HST WFPC2 ``Association'' data sets that correlate with the Chandra positions of a set of 44 ultra-luminous X-ray sources (ULXs) of nearby galaxies. We have improved the Chandra-HST relative astrometry whenever possible. Disparate numbers of potential ULX counterparts are found, and in some cases none are found. The lack of or low number of counterparts in some cases may be due to insufficient depth in the WFPC2 images. Particularly in late-type galaxies, the HST image in the ULX region was often complex or crowded. We therefore address various scenarios for the nature of the ULX since it is not known which, if any, of the sources found are true counterparts. The optical luminosities of the sources are typically in the range 10^4-6 L_sun. In several cases color information is available, with the colors roughly tending to be more red in early-type galaxies. This suggests that, in general, the (potential) counterparts found in early-type galaxies are likely to be older stellar populations, and are probably globular clusters. Several early-type galaxy counterparts have blue colors, which may be due to younger stellar populations in the host galaxies, however these could also be background sources. In spiral galaxies the sources may also be due to localized structure in the disks rather than bound stellar systems. Alternatively some of the counterparts in late-type galaxies may be isolated supergiant stars. The observed X-ray/optical flux ratio is diluted by the optical emission of the cluster in cases where the system is an X-ray binary in a cluster, particularly in the case of a low-mass X-ray binaries in old cluster. (abridged)

 

astro-ph/0605562 [abs, ps, pdf, other] :

Title: Generation of the magnetic field in jets
Authors: V.Urpin
Comments: 7 pages, accepted for publication in A&A

We consider dynamo action under the combined influence of turbulence and large-scale shear in sheared jets. Shear can stretch turbulent magnetic field lines in such a way that even turbulent motions showing mirror symmetry become suitable for generation of a large-scale magnetic field. We derive the integral induction equation governing the behaviour of the mean field in jets. The main result is that sheared jets may generate a large-scale magnetic field if shear is sufficiently strong. The generated mean field is mainly concentrated in a magnetic sheath surrounding the central region of a jet, and it exhibits sign reversals in the direction of the jet axis. Typically, the magnetic field in a sheath is dominated by the component along the jet that can reach equipartition with the kinetic energy of particles, The field in the central region of jets has a more disordered structure.

 

astro-ph/0605563 [abs, ps, pdf, other] :

Title: Ammonia as a tracer of chemical equilibrium in the T7.5 dwarf Gliese 570D
Authors: D. Saumon, M.S. Marley, M.C. Cushing, S.K. Leggett, T.L. Roellig, K. Lodders, R.S. Freedman
Comments: Accepted for publication in the ApJ. 10 pages, including 3 figures

We present the first analysis of an optical to mid-infrared spectrum of the T7.5 dwarf Gliese 570D with model atmospheres, synthetic spectra, and brown dwarf evolution sequences. We obtain precise values for the basic parameters of Gl 570D: Teff=800 - 820K, log g (cm/s^2)=5.09 - 5.23, and log L/Lsun= -5.525 to -5.551. The Spitzer IRS spectrum shows prominent features of ammonia (NH3) that can only be fitted by reducing the abundance of NH3 by about one order of magnitude from the value obtained with chemical equilibrium models. We model departures from chemical equilibrium in the atmosphere of Gl 570D by considering the kinetics of nitrogen and carbon chemistry in the presence of vertical mixing. The resulting model spectrum reproduces the data very well.

 

astro-ph/0605564 [abs, ps, pdf, other] :

Title: Local Group(s)
Authors: Eva K. Grebel (U. Basel)
Comments: 17 pages, no figures. Invited review to appear in the proceedings of the ESO workshop "Groups of Galaxies in the Nearby Universe", Santiago, Dec 5-9, 2005. Eds. I. Saviane, V. Ivanov, & J. Borissova (Springer Verlag)

The properties of the galaxies of the Local Group are reviewed, followed by a brief discussion of nearby groups. The galaxy groups in our vicinity - the M81 group, the Cen A group, and the IC 342/Maffei group - are in many respects Local Group analogs: Their luminosity functions, galaxy content, fractional galaxy type distribution, crossing times, masses, and zero-velocity surface radii are similar to those of the Local Group. Also, the nearby groups usually consist of two subgroups, some of which approach each other and may ultimately merge to form a fossil group. These poor groups contrast with the less evolved, loose and extended galaxy ``clouds'' such as the Scl group and the CVn I cloud. These are characterized by long crossing times, are dominated by gas-rich, late-type galaxies, and lack gas-deficient, low luminosity early-type dwarfs. These clouds may be groups still in formation. The local Hubble flow derived from the clouds and groups is very cold.

 

astro-ph/0605565 [abs, ps, pdf, other] :

Title: Simple Magnetic Flux Balance as an Indicator of Neon VIII Doppler Velocity Partitioning in an Equatorial Coronal Hole
Authors: Scott W. McIntosh, Alisdair R. Davey, Scott W. McIntosh
Comments: To appear ApJL (June)

We present a novel investigation into the relationship between simple estimates of magnetic flux balance and the Ne VIII Doppler velocity partitioning of a large equatorial coronal hole observed by the Solar Ultraviolet Measurements of Emitted Radiation spectrometer (SUMER) on the Solar and Heliospheric Observatory (SOHO) in November 1999. We demonstrate that a considerable fraction of the large scale Doppler velocity pattern in the coronal hole can be qualitatively described by simple measures of the local magnetic field conditions, i.e., the relative unbalance of magnetic polarities and the radial distance required to balance local flux concentrations with those of opposite polarity.

 

astro-ph/0605566 [abs, ps, pdf, other] :

Title: Oscillations of longitudinally density stratified coronal loops
Authors: H. Safari, S. Nasiri, Y. Sobouti
Comments: 10 pages, 4 figures

The oscillations and wave propagation is studied in a cylindrical model of coronal loops that undergoes a longitudinal density stratification. Equations of motion are expressed by second order differential equations that are separable into radial and transverse components. The radial equation is solved in thin tube regime. The transverse equation is solved both by perturbation method, for small density scale heights, and numerically, otherwise.

 

astro-ph/0605567 [abs, ps, pdf, other] :

Title: Gravitational Waves from Light Cosmic Strings: Backgrounds and Bursts with Large Loops
Authors: Craig J. Hogan
Comments: 22 pages, Latex, 1 figure, submitted to Phys Rev D

The mean spectrum and burst statistics of gravitational waves produced by a cosmological population of cosmic string loops are estimated using analytic approximations, calibrated with earlier simulations. Formulas are derived showing the dependence of observables on the string tension, in the regime where newly-formed loops are relatively large, not very much smaller than the horizon. Large loops form earlier, are more abundant, and generate a more intense stochastic background and more frequent bursts than assumed in earlier background estimates, enabling experiments to probe lighter cosmic strings of interest to string theory. Predictions are compared with instrument noise from current and future experiments, and with confusion noise from known astrophysical gravitational wave sources such as stellar and massive black hole binaries. In these large-loop models, current data from millisecond pulsar timing already suggests that the tension is less than about $10^{-10}$, a typical value expected in strings from brane inflation. LISA will be sensitive to stochastic backgrounds created by strings as light as $G\mu\approx 10^{-15}$, at frequencies where it is limited by confusion noise of Galactic stellar populations; however, for those lightest detectable strings, bursts are rarely detectable.

 

astro-ph/0605568 [abs, ps, pdf, other] :

Title: A warped m=2 water maser disc in V778 Cyg?
Authors: Natalia Babkovskaia, Juri Poutanen, Anita M.S. Richards, Ryszard Szczerba
Comments: 7 pages, 5 figures. This paper is accepted for publication in MNRAS

The silicate carbon star V778 Cyg is a source of 22 GHz water maser emission which was recently resolved by MERLIN. Observations revealed an elongated S-like structure along which the velocities of the maser features show a linear dependence on the impact parameter. This is consistent with a doubly-warped m=2 disc observed edge-on. Water masers and silicate dust emission (detected by IRAS and ISO) have a common origin in O-rich material and are likely to be co-located in the disc. We propose a detailed self-consistent model of a masing gas-dust disc around a companion to the carbon star in a binary system, which allows us to estimate the companion mass of 1.7 +- 0.1 M_sun, the disc radius of 40 +-3 AU and the distance between companions of about 80 AU. Using a dust-gas coupling model for water masing, we calculate the maser power self-consistently, accounting for both the gas and the dust energy balances. Comparing the simulation results with the observational data, we deduce the main physical parameters of the masing disc, such as the gas and dust temperatures and their densities. We also present an analysis of the stability of the disc.

 

astro-ph/0605569 [abs, ps, pdf, other] :

Title: Image-Subtraction Photometry of Variable Stars in the Globular Clusters NGC 6388 and NGC 6441
Authors: T. M. Corwin, A. N. Sumerel (UNC at Charlotte), B. J. Pritzl (Macalester College), H. A. Smith (Michigan State), M. Catelan (PUC-Chile), A. V. Sweigart (NASA/GSFC), P. B. Stetson (DAO/HIA/NRC)
Comments: 11 pages, 10 figures. AJ, in press

We have applied Alard's image subtraction method (ISIS v2.1) to the observations of the globular clusters NGC 6388 and NGC 6441 previously analyzed using standard photometric techniques (DAOPHOT, ALLFRAME). In this reanalysis of observations obtained at CTIO, besides recovering the variables previously detected on the basis of our ground-based images, we have also been able to recover most of the RR Lyrae variables previously detected only in the analysis of Hubble Space Telescope WFPC2 observations of the inner region of NGC6441. In addition, we report five possible new variables not found in the analysis of the HST observations of NGC 6441. This dramatically illustrates the capabilities of image subtraction techniques applied to ground-based data to recover variables in extremely crowded fields. We have also detected twelve new variables and six possible variables in NGC 6388 not found in our previous ground-based studies. The revised mean period for RRab stars in NGC 6388 is 0.676 day, while the mean period of RRab stars in NGC 6441 is unchanged at 0.759 day. These values are among the largest known for any galactic globular cluster. Additional probable type II Cepheids were identified in NGC 6388, confirming its status as a metal-rich globular cluster rich in Cepheids.

 

astro-ph/0605570 [abs, ps, pdf, other] :

Title: HETE-2 Localizations and Observations of Four Short Gamma-Ray Bursts: GRBs 010326B, 040802, 051211 and 060121
Authors: T. Q. Donaghy, D. Q. Lamb, T. Sakamoto, J. P. Norris, Y. Nakagawa, J. Villasenor, J.-L. Atteia, R. Vanderspek, C. Graziani, N. Kawai, G. R. Ricker, G. B. Crew, J. Doty, G. Prigozhin, J. G. Jernigan, Y. Shirasaki, M. Suzuki, N. Butler, K. Hurley, T. Tamagawa, A. Yoshida, M. Matsuoka, E. E. Fenimore, M. Galassi, M. Boer, J.-P. Dezalay, J.-F. Olive, A. Levine, F. Martel, E. Morgan, R. Sato, S. E. Woosley, J. Braga, R. Manchanda, G. Pizzichini, K. Takagishi, M. Yamauchi
Comments: 60 pages, 19 figures, submitted to ApJ

Here we report the localizations and properties of four short-duration GRBs localized by the High Energy Transient Explorer 2 satellite (HETE-2): GRBs 010326B, 040802, 051211 and 060121, all of which were detected by the French Gamma Telescope (Fregate) and localized with the Wide-field X-ray Monitor (WXM) and/or Soft X-ray Camera (SXC) instruments. We discuss eight possible criteria for determining whether these GRBs are "short population bursts" (SPBs) or "long population bursts" (LPBs). These criteria are (1) duration, (2) pulse widths, (3) spectral hardness, (4) spectral lag, (5) energy Egamma radiated in gamma rays (or equivalently, the kinetic energy E_KE of the GRB jet), (6) existence of a long, soft bump following the burst, (7) location of the burst in the host galaxy, and (8) type of host galaxy. In particular, we have developed a likelihood method for determining the probability that a burst is an SPB or a LPB on the basis of its T90 duration alone. A striking feature of the resulting probability distribution is that the T90 duration at which a burst has an equal probability of being a SPB or a LPB is T90 = 5 s, not T90 = 2 s, as is often used. All four short-duration bursts discussed in detail in this paper have T90 durations in the Fregate 30-400 keV energy band of 1.90, 2.31, 4.25, and 1.97 sec, respectively, yielding probabilities P(S|T90) = 0.97, 0.91, 0.60, and 0.95 that these bursts are SPBs on the basis of their T90 durations alone. All four bursts also have spectral lags consistent with zero. These results provide strong evidence that all four GRBs are SPBs.