arXiv.org > astro-ph

Astrophysics

astro-ph new abstracts, Wed, 15 Mar 06 01:00:11 GMT
0603323 -- 0603371 received

astro-ph/0603323 [abs, ps, pdf, other] :

Title: A Shot in the Dark: A Technique for Locating the Stellar Counterparts of Damped Lyman Alpha Absorbers
Authors: John M. O'Meara (1), Hsiao-Wen Chen (2), David L. Kaplan (1) ((1) M. I. T., (2) University of Chicago)
Comments: 11 pages, 4 figures, Accepted for publication in ApJL

We present initial results from a Magellan telescope program to image galaxies that give rise to damped Lyman absorbers (DLAs) at 1.63 < z_DLA < 2.37. Our program differs from previous efforts in that we target quasars with intervening Lyman limit systems (LLS) along the line of sight at redshift z_LLS > 3.5. The higher-redshift LLS is applied as a blocking filter to remove the glare of the background quasar at the rest-frame ultraviolet wavelengths of the foreground galaxy. The complete absence of quasar light offers an unimpeded view along the sightline to the redshift of the LLS, allowing an exhaustive search for the DLA galaxy to the sensitivity limit of the imaging data (at or better than 0.25L*). In both of our pilot fields (PKS2000-330, z_DLA=2.033 and SDSS0322-0558, z_DLA=1.69), we identify an L* galaxy within 5" of the sightline which has optical colors consistent with star-forming galaxies at z~2. We examine the correlation between absorption-line properties and galaxy luminosity and impact distance, and compare the high-redshift galaxy and absorber pairs with those known at z<1.

 

astro-ph/0603324 [abs, ps, pdf, other] :

Title: The effect of mass-segregation on gravitational wave sources near massive black holes
Authors: Clovis Hopman, Tal Alexander (Weizmann)
Comments: Submitted to ApJL

Gravitational waves (GWs) from the inspiral of compact remnants (CRs) into massive black holes (MBHs) will be observable to cosmological distances. While a CR spirals in, 2-body scattering by field stars may cause it to fall into the MBH before reaching a short period orbit that would give an observable signal. As a result, only CRs very near (~0.01 pc) the MBH can spiral in successfully. In a multi-mass stellar population, the heaviest objects sink to the center, where they are more likely to slowly spiral into the MBH without being swallowed prematurely. We study how mass-segregation modifies the stellar distribution and the rate of GW events. We find that the inspiral rate per galaxy for white dwarfs is 30 per Gyr, for neutron stars 6 per Gyr, and for stellar black holes (SBHs) 250 per Gyr. The high rate for SBHs is due to their extremely steep density profile, n_{BH}(r)\propto r^{-2}. The GW detection rate will be dominated by SBHs.

 

astro-ph/0603325 [abs, ps, pdf, other] :

Title: Simultaneous photometry and echelle-spectroscopy of the dwarf nova BZ Ursae Majoris during the 2005 January Outburst
Authors: V. V. Neustroev (1), S. V. Zharikov (2), R. Michel (2) ((1) National University of Ireland, Galway, (2) OAN IA UNAM, Mexico)
Comments: 8 pages, 8 figures (12 eps files). Accepted for publication in MNRAS. The paper with high resolution colour images can be downloaded from this http URL

We report simultaneous photometric and echelle-spectroscopic observations of the dwarf nova BZ UMa during which we were lucky to catch the system at the onset of an outburst, the development of which we traced in detail from quiescence to early decline. The outburst had a precursor, and was of a short duration (~5 days) with a highly asymmetrical light curve. On the rise we observed a `jump' during which the brightness almost doubled over the course of half an hour. Power spectra analysis revealed well-defined oscillations with period of ~42 minutes. Using Doppler tomography we found that the unusual emission distribution detected in quiescence held during the outburst. After the maximum a new emission source arose, from the inner hemisphere of the secondary star, which became the brightest at that time. We analyse this outburst in terms of `inside-out' and `outside-in' types, in order to determine which of these types occured in BZ UMa.

 

astro-ph/0603326 [abs, ps, pdf, other] :

Title: The hot stars in orbit around the M31 central supermassive black hole: are they young or old?
Authors: Pierre Demarque, Shanil Virani (Yale University)
Comments: 9 pages, 2 figures, submitted to New Astronomy for publication

The cluster of hot stars observed in orbit around the central black hole of M31 has been interpreted by Bender et al (2005) as a 200 Myr starburst. The formation of a population of young stars in close proximity to a massive black hole presents a difficult challenge to star formation theory. We point out that in a high stellar density environment, the course of stellar evolution is modified by frequent collisions and mergers. An old stellar population of evolved blue horizontal-branch stars and of merger products cannot be ruled out on the basis of the available data. Observations are suggested that would help distinguish between a ``young'' and ``old'' stellar population interpretation of the observations.

 

astro-ph/0603327 [abs, ps, pdf, other] :

Title: First Results from the CHARA Array VII: Long-Baseline Interferometric Measurements of Vega Consistent with a Pole-On, Rapidly Rotating Star
Authors: J. P. Aufdenberg, A. Merand, V. Coude du Foresto, O. Absil, E. Di Folco, P. Kervella, S. T. Ridgway, D. H. Berger, T. A. ten Brummelaar, H. A. McAlister, J. Sturmann, L. Sturmann, N. H. Turner
Comments: 16 pages, 9 figures, accepted by ApJ

We have obtained high-precision interferometric measurements of Vega with the CHARA Array and FLUOR beam combiner in the K' band at projected baselines between 103m and 273m. The measured visibility amplitudes beyond the first lobe are significantly weaker than expected for a slowly rotating star characterized by a single effective temperature and surface gravity. Our measurements, when compared to synthetic visibilities and synthetic spectrophotometry from a Roche-von Zeipel gravity-darkened model atmosphere, provide strong evidence for the model of Vega as a rapidly rotating star viewed very nearly pole-on. Our best fitting model indicates that Vega is rotating at ~91% of its angular break-up rate with an equatorial velocity of 275 km/s. Together with the measured vsin(i), this velocity yields an inclination for the rotation axis of 5 degrees. For this model the pole-to-equator effective temperature difference is 2250 K, a value much larger than previously derived from spectral line analyses. The derived equatorial T_eff of 7900 K indicates Vega's equatorial atmosphere may be convective and provides a possible explanation for the discrepancy. The model has a luminosity of ~37 Lsun, a value 35% lower than Vega's apparent luminosity based on its bolometric flux and parallax, assuming a slowly rotating star. The model luminosity is consistent with the mean absolute magnitude of A0V stars. Our model predicts the spectral energy distribution of Vega as viewed from its equatorial plane; a model which may be employed in radiative models for the surrounding debris disk.

 

astro-ph/0603328 [abs, ps, pdf, other] :

Title: Neutron-capture elements in the metal-poor globular cluster M15
Authors: Kaori Otsuki, Satoshi Honda, Wako Aoki, Toshitaka Kajino, Grant J. Mathews
Comments: Accepted to ApJL

We report on observations of six giants in the globular cluster M15 (NGC 7078) using the Subaru Telescope to measure neutron-capture elemental abundances. Our abundance analyses based on high-quality blue spectra confirm the star-to-star scatter in the abundances of heavy neutron-capture elements (e.g., Eu), and no significant s-process contribution to them, as was found in previous studies. We have found, for the first time, that there are anti-correlations between the abundance ratios of light to heavy neutron-capture elements ([Y/Eu] and [Zr/Eu]) and heavy ones (e.g., Eu). This indicates that light neutron-capture elements in these stars cannot be explained by only a single r-process. Another process that has significantly contributed to the light neutron-capture elements is required to have occurred in M15. Our results suggest a complicated enrichment history for M15 and its progenitor.

 

astro-ph/0603329 [abs, pdf] :

Title: Sulfur Volcanism on Io
Authors: Kandis Lea Jessup, John R. Spencer, Roger Yelle
Comments: pages:44 figures: 7 tables:3

In February 2003, March 2003 and January 2004 Pele plume transmission spectra were obtained during Jupiter transit with Hubble's Space Telescope Imaging Spectrograph (STIS), using the 0.1 arcsec long slit and the G230LB grating. The STIS spectra covered the 2100-3100 A wavelength region and extended spatially along Io's limb both northward of Pele. The S2 and SO2 absorption signatures evident in the these data indicate that the gas signature at Pele was temporally variable, and that an S2 absorption signature was present ~ 12 deg. from the Pele vent near 6+/-5 S and 264 +/-15 W, suggesting the presence of another S2 bearing plume on Io. Contemporaneous with the spectral data, UV and visible-wavelength images of the plume were obtained in reflected sunlight with the Advanced Camera for Surveys (ACS) prior to Jupiter transit. The dust scattering recorded in these data provide an additional qualitative measure of plume activity on Io, indicating that the degree of dust scattering over Pele varied as a function of the date of observation, and that there were several other dust bearing plumes active just prior to Jupiter transit. We present constraints on the composition and variability of the gas abundances of the Pele plume as well as the plumes detected by ACS and recorded within the STIS data, as a function of time. We discuss the implications of these results for thermochemical conditions at the plume vents, and our understanding of plume eruption styles active on Io.

 

astro-ph/0603330 [abs, ps, pdf, other] :

Title: High Mass Star Formation. II. The Mass Function of Submillimeter Clumps in M17
Authors: M. A. Reid, C. D. Wilson
Comments: 33 pages, 6 figures, 3 tables. Accepted for publication in the Astrophysical Journal

We have mapped an approximately 5.5 by 5.5 pc portion of the M17 massive star-forming region in both 850 and 450 micron dust continuum emission using the Submillimeter Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT). The maps reveal more than 100 dusty clumps with deconvolved linear sizes of 0.05--0.2 pc and masses of 0.8--120 solar masses, most of which are not associated with known mid-infrared point sources. Fitting the clump mass function with a double power law gives a mean power law exponent of alpha_high = -2.4 +/- 0.3 for the high-mass power law, consistent with the exponent of the Salpeter stellar mass function. We show that a lognormal clump mass distribution with a peak at about 4 solar masses produces as good a fit to the clump mass function as does a double power law. This 4 solar mass peak mass is well above the peak masses of both the stellar initial mass function and the mass function of clumps in low-mass star-forming regions. Despite the difference in intrinsic mass scale, the shape of the M17 clump mass function appears to be consistent with the shape of the core mass function in low-mass star-forming regions. Thus, we suggest that the clump mass function in high-mass star-forming regions may be a scaled-up version of that in low-mass regions, instead of its extension to higher masses.

 

astro-ph/0603331 [abs, ps, pdf, other] :

Title: The Disk and Extraplanar Environment of NGC 247
Authors: T. J. Davidge
Comments: Includes 16 eps figures; to appear in the Astrophysical Journal

The stellar content of the spiral galaxy NGC 247 is investigated. The main sequence turn-off (MSTO) in the inner 12 kpc of the disk corresponds to an age of 6 Myr. A mean star formation rate (SFR) of 0.1 solar masses per year during the past 16 Myr is computed from star counts. The color of the red supergiant plume does not change with radius, suggesting that the mean metallicity of young stars does not vary by more than 0.1 dex. The number of bright main sequence stars per local stellar mass density climbs towards larger radii out to a distance of 12 kpc; the scale lengths that characterize the radial distributions of young and old stars in the disk thus differ. The density of bright main sequence stars with respect to projected HI mass gradually drops with increasing radius. The population of very young stars disappears in the outer disk; the MSTO at galactocentric radii between 12 and 15 kpc corresponds to 16 Myr, while between 15 and 18 kpc the age is > 40 Myr. Red giant branch (RGB) stars are resolved at a projected minor axis galactocentric distance of 12 kpc. There is a broad spread in metallicity among the RGB stars, with a mean [M/H] = -1.2. The RGB-tip occurs at i' = 24.5 +/- 0.1, indicating that the distance modulus is 27.9 +/- 0.1. Luminous AGB stars with an age 3 Gyr are also seen in this field.

 

astro-ph/0603332 [abs, ps, pdf, other] :

Title: VLT/UVES Observations of Interstellar Molecules and Diffuse Bands in the Magellanic Clouds
Authors: D. E. Welty (1), S. R. Federman (2), R. Gredel (3), J. A. Thorburn (1), D. L. Lambert (4) ((1) Univ. of Chicago, (2) Univ. of Toledo, (3) Max-Planck-Inst. fur Astron., (4) Univ. of Texas)
Comments: 59 pages, 15 figures, 10 tables; aastex; accepted to ApJS

We discuss the abundances of interstellar CH, CH+, and CN in the Magellanic Clouds (MC), derived from spectra of 7 SMC and 13 LMC stars obtained (mostly) with the VLT/UVES. CH and/or CH+ are detected toward 3 SMC and 9 LMC stars; CN is detected toward 2 stars. In the MC, the CH/H2 ratio is comparable to that found for diffuse Galactic molecular clouds in some sight lines, but is lower by factors up to 10-15 in others. The abundance of CH in the MC thus appears to depend on local physical conditions -- and not just on metallicity. The observed relationships between the column density of CH and those of CN, CH+, Na I, and K I in the MC are generally consistent with the trends observed in our Galaxy. Using existing data for the rotational populations of H2, we estimate temperatures, radiation field strengths, and local hydrogen densities for the diffuse molecular gas. Densities estimated from N(CH), assuming that CH is produced via steady-state gas-phase reactions, are considerably higher; much better agreement is found by assuming that the CH is made via the (still undetermined) process(es) responsible for the observed CH+. The UVES spectra also reveal absorption from the diffuse interstellar bands at 5780, 5797, and 6284 A in the MC. On average, the three DIBs are weaker by factors of 7-9 (LMC) and about 20 (SMC), compared to those observed in Galactic sight lines with similar N(H I), and by factors of order 2-6, relative to E(B-V), N(Na I), and N(K I). The detection of several of the ``C2 DIBs'', with strengths similar to those in comparable Galactic sight lines, however, indicates that no single, uniform scaling factor (e.g., one related to metallicity) applies to all DIBs (or all sight lines) in the MC. (abstract abridged)

 

astro-ph/0603333 [abs, ps, pdf, other] :

Title: Properties of High Redshift Quasars-II: What does the quasar luminosity function tell us about super-massive black-hole evolution?
Authors: Stuart Wyithe, T. Padmanabhan
Comments: 12 pages, 6 figures. Submitted to MNRAS

In the local universe, the masses of Super-Massive Black-Holes (SMBH) appear to correlate with the physical properties of their hosts, including the mass of the dark-matter halo. Using these clues as a starting point many studies have produced models that can explain phenomena like the quasar luminosity function. The shortcoming of this approach is that working models are not unique, and as a result it is not always clear what input physics is being constrained. Here we take a different approach. We identify critical parameters that describe the evolution of SMBHs at high redshift, and constrain their parameter space based on observations of high redshift quasars from the Sloan Digital Sky Survey. We find that the luminosity function taken in isolation is somewhat limited in its ability to constrain SMBH evolution due to some strong degeneracies. This explains the presence in the literature of a range of equally successful models based on different physical hypotheses. Including the constraint of the local SMBH to halo mass ratio breaks some of the degeneracies, and our results suggest halo masses at z~4.8 of 10^{12.5+/-0.3}M_solar (with 90% confidence), with a SMBH to halo mass ratio that decreases with time (>99%). We also find a quasar luminosity to halo mass ratio that increases with halo mass (>99%). These features need to be incorporated in all successful models of SMBH evolution. On the other hand current observations do not permit any conclusions regarding the evolution of quasar lifetime, or the SMBH occupation fraction in dark matter halos.

 

astro-ph/0603334 [abs, ps, pdf, other] :

Title: Primordial Helium Abundance: A Reanalysis of the Izotov-Thuan Spectroscopic Sample
Authors: Masataka Fukugita, Masahiro Kawasaki
Comments: 12 pages, 3 figures

A reanalysis is made for the helium abundance determination for the Izotov-Thuan (2004) spectroscopic sample of extragalactic H II regions. We find that the effect of underlying stellar absorption of the He I lines, which is more important for metal poor systems, affects significantly the inferred primordial helium abundance $Y_p$ obtained in the zero metallicity limit and the slope of linear extrapolation, $dY/dZ$. This brings $Y_p$ from $0.234\pm0.004$ to $0.250\pm 0.004$ and $dY/dZ=4.7\pm 1.0$ to $1.1\pm 1.4$. Conservatively, this indicates the importance of the proper understanding of underlying stellar absorption for accurate determinations of the primordial helium abundance to the error of $\delta Y_p\simeq 0.002-0.004$.

 

astro-ph/0603335 [abs, ps, pdf, other] :

Title: High eccentricity planets from the Anglo-Australian Planet Search
Authors: Hugh R. A. Jones, R. Paul Butler, C.G. Tinney, Geoffrey W. Marcy, Brad D. Carter, Alan J. Penny, Chris McCarthy, Jeremy Bailey
Comments: to appear in MNRAS

We report Doppler measurements of the stars HD187085 and HD20782 which indicate two high eccentricity low-mass companions to the stars. We find HD187085 has a Jupiter-mass companion with a ~1000d orbit. Our formal `best fit' solution suggests an eccentricity of 0.47, however, it does not sample the periastron passage of the companion and we find that orbital solutions with eccentricities between 0.1 and 0.8 give only slightly poorer fits (based on RMS and chi^2) and are thus plausible. Observations made during periastron passage in 2007 June should allow for the reliable determination of the orbital eccentricity for the companion to HD187085. Our dataset for HD20782 does sample periastron and so the orbit for its companion can be more reliably determined. We find the companion to HD20782 has M sin i=1.77+/-0.22M_JUP, an orbital period of 595.86+/-0.03d and an orbit with an eccentricity of 0.92+/-0.03. The detection of such high-eccentricity (and relatively low velocity amplitude) exoplanets appears to be facilitated by the long-term precision of the Anglo-Australian Planet Search. Looking at exoplanet detections as a whole, we find that those with higher eccentricity seem to have relatively higher velocity amplitudes indicating higher mass planets and/or an observational bias against the detection of high eccentricity systems.

 

astro-ph/0603336 [abs, ps, pdf, other] :

Title: A postmortem investigation of the Type IIb supernova 2001ig
Authors: Stuart D. Ryder (1), Clair E. Murrowood (1,2), Raylee A. Stathakis (1) ((1) Anglo-Australian Observatory, (2) University of Tasmania)
Comments: 5 pages, 3 figures. Accepted for publication in MNRAS Letters. Fig. 1 resolution degraded to meet size limitations; full resolution version available from this http URL

We present images taken with the GMOS instrument on Gemini-South, in excellent (<0.5 arcsec) seeing, of SN 2001ig in NGC 7424, ~1000 days after explosion. A point source seen at the site of the SN is shown to have colours inconsistent with being an H II region or a SN 1993J-like remnant, but can be matched to a late-B through late-F supergiant with A_V<1. We believe this object is the massive binary companion responsible for periodic modulation in mass loss material around the Wolf-Rayet progenitor which gave rise to significant structure in the SN radio light curve.

 

astro-ph/0603337 [abs, ps, pdf, other] :

Title: A Dust Emission Model for Very Young Galaxies: Expected Properties and Far Infrared Diagnostics
Authors: Tsutomu T. Takeuchi
Comments: 8 pages, 4 figures, to appear in the proceedings of "Infrared Diagnostics of Galaxy Evolution"

Dust plays crucial roles in galaxy formation and evolution. In the early epoch of galaxy evolution dust is only supplied by supernovae (SNe). With the aid of a new physical model of dust production by SNe, we constructed a model of dust emission from forming galaxies. We show the evolution of the spectral energy distribution (SED). Then we adopt this model to a local starbursting dwarf galaxy SBS 0335$-$052. Further we discuss the SEDs of high redshift galaxies, and consider their observational feasibility.

 

astro-ph/0603338 [abs, ps, pdf, other] :

Title: Investigating on the nuclear obscuration in two types of Seyfert 2 galaxies
Authors: X. W. Shu, J. X. Wang, P. Jiang, L. L. Fan, T. G. Wang (USTC)
Comments: 16 pages, including 3 figures and 2 tables, ApJ submitted, full version of the tables will be available after addressing the referee report

We build a large sample of Seyfert 2 galaxies (Sy2s) with both optical spectropolarimetric and X-ray data available, in which 41 Sy2s with the detection of polarized broad emission line (PBL) and 32 without. We found that for luminous Sy2s (with L_2 - 10keV > 10^42 erg/s), most (92%) of the Compton-thin sources show PBL, while only 53% Compton-thick sources do. Our results, for the first time with high statistical confidence, show that at least for luminous Sy2s, the nuclear obscuration plays an important role in the visibility of PBL in Sy2s. These results put constraints on the geometry of the obscuring material/electron scattering region in the framework of the unified model. We can reach these results in the unified model if: a) the absorption column density is higher at large inclinations and b) the scattering region is obscured at large inclinations. Consistent with previous studies, we also found a much lower detection rate of PBL in less luminous Sy2s (23%), indicating the detectability of PBL also depends on the central AGN activities.

 

astro-ph/0603339 [abs, ps, pdf, other] :

Title: On the X-ray Baldwin effect for narrow Fe Kalpha emission line
Authors: P. Jiang, J. X. Wang, T. G. Wang (USTC)
Comments: 21 pages, including 3 tables and 7 figures, ApJ 2006, v644 in press

Most Active Galactic Nuclei (AGN) exhibit a narrow Fe Kalpha line at ~ 6.4 keV in the X-ray spectra, due to the fluorescent emission from cold material far from the inner accretion disk. Using XMM-Newton observations, Page et al. found that the equivalent width (EW) of the narrow Fe Kalpha line decreases with increasing luminosity (EW ~ L^-0.17pm0.08), suggesting a decrease in the covering factor of the material emitting the line (presumably the torus). By combining the archival Chandra HETG observations of 34 type 1 AGNs with XMM observations in literature, we build a much large sample with 101 AGNs. We find a similar X-ray Baldwin effect in the sample (EW ~ L^-0.2015pm0.0426), however, we note that the anti-correlation is dominated by the radio loud AGN in the sample, whose X-ray spectra might be contaminated by the relativistic jet. Excluding the radio loud AGN, we find a much weaker anti-correlation (EW ~ L^-0.1019pm0.0524). We present Monte-Carlo simulations showing that such a weak anti-correlation can be attributed to the relative short time scale variations of the X-ray continuum.

 

astro-ph/0603340 [abs, ps, pdf, other] :

Title: Electron-ion recombination measurements motivated by AGN X-ray absorption features: Fe XIV forming Fe XIII
Authors: E. W. Schmidt, S. Schippers, A. Mueller, M. Lestinsky, F. Sprenger, M. Grieser, R. Repnow, A. Wolf, C. Brandau, D. Lukic, M. Schnell, D. W. Savin
Comments: 4 pages, 4 figures, 1 table submitted to ApJ

Recent spectroscopic models of active galactic nuclei (AGN) have indicated that the recommended electron-ion recombination rate coefficients for iron ions with partially filled M-shells are incorrect in the temperature range where these ions form in photoionized plasmas. We have investigated this experimentally for Fe XIV forming Fe XIII. The recombination rate coefficient was measured employing the electron-ion merged beams method at the Heidelberg heavy-ion storage-ring TSR. The measured energy range of 0-260 eV encompassed all dielectronic recombination (DR) 1s2 2s2 2p6 3l 3l' 3l'' nl''' resonances associated with the 3p1/2 -> 3p3/2, 3s -> 3p, 3p -> 3d and 3s -> 3d core excitations within the M-shell of the Fe XIV 1s2 2s2 2p6 3s2 3p parent ion. This range also includes the 1s2 2s2 2p6 3l 3l' 4l'' nl''' resonances associated with 3s -> 4l'' and 3p -> 4l'' core excitations. We find that in the temperature range 2--14 eV, where Fe XIV is expected to form in a photoionized plasma, the Fe XIV recombination rate coefficient is orders of magnitude larger than previously calculated values.

 

astro-ph/0603341 [abs, ps, pdf, other] :

Title: Detectability of dirty dust grains in brown dwarf atmospheres
Authors: CH. Helling, W.-F. Thi, P. Woitke, M. Fridlund
Comments: A&A Letter, accepted

Dust clouds influence the atmospheric structure of brown dwarfs, and they affect the heat transfer and change the gas-phase chemistry. However, the physics of their formation and evolution is not well understood. In this letter, we predict dust signatures and propose a potential observational test of the physics of dust formation in brown dwarf atmosphere based on the spectral features of the different solid components predicted by dust formation theory. A momentum method for the formation of dirty dust grains (nucleation, growth, evaporation, drift) is used in application to a static brown dwarf atmosphere structure to compute the dust grain properties, in particular the heterogeneous grain composition and the grain size. Effective medium and Mie theory are used to compute the extinction of these spherical grains. Dust formation results in grains whose composition differs from that of grains formed at equilibrium. Our kinetic model predicts that solid amorphous SiO2[s]
(silica) is one of the most abundant solid component followed by amorphous
MgSiO4[s] and MgSiO3[s], while SiO2[s] is absent in equilibrium models because it is a metastable solid. Solid amorphous SiO2[s] possesses a strong broad absorption feature centered at 8.7mum, while amorphous Mg2SiO4[s]/MgSiO3[s] absorb at 9.7mum beside other absorption features at longer wavelength. Those features at lambda < 15mum are detectable in absorption if grains are small (radius < 0.2mum) in the upper atmosphere as suggested by our model. We suggest that the detection of a feature at 8.7mum in deep infrared spectra could provide evidence for non-equilibrium dust formation that yields grains composed of metastable solids in brown dwarf atmospheres. This feature will shift towards 10mum and broaden if silicates (e.g. fosterite) are much more abundant.

 

astro-ph/0603342 [abs, ps, pdf, other] :

Title: A hierarchy of cosmic compact objects - without black holes
Authors: Johan Hansson

We make the case for the existence of a, hitherto unknown and unobserved, hierarchy of ever more compact cosmic objects in the universe. This hypothesis is based on i) the assumption of "elementary" particle sub-constituents on several levels below the presently known, inspired by Glashow's "blooming desert", ii) the existence of nearly scale-invariant density fluctuations in the early universe, e.g. as predicted by inflationary models, iii) our own previous theoretical work showing that a class of objects considerably more compact than previously thought possible in astrophysics can exist. We also give several independent arguments strongly pointing towards the non-existence of black holes. Some brief suggestions on observational signals due to the hierarchy, both in collected astronomical data and in possible future observations, concludes the paper.

 

astro-ph/0603343 [abs, ps, pdf, other] :

Title: Radiation from an expanding cocoon as an explanation of the steep decay observed in GRB early afterglow light curves
Authors: Asaf Pe'er, Peter Mészáros, Martin J. Rees
Comments: 7 pages, 6 figures; submitted for publication in Ap.J

Observations of early afterglow emission from gamma-ray bursts (GRB's) with the Swift satellite show steep decay of the X-ray light curve, F_\nu(t) ~ t^{-\alpha} with \alpha ~ 2.5 - 4 at ~300-500 s after the burst trigger. The spectrum in this time interval is consistent with a spectrum F_\nu ~ \nu^{-\beta} with \beta ~1. Here, we show that these results can be explained as due to emission from the hot plasma "cocoon" associated with the jet, which expands relativistically after the jet has broken through the stellar envelope, if a substantial fraction of the coccon kinetic energy is dissipated at scattering optical depths \tau_T ~ 10^2-10^3. This results in the bulk of the coccon photons being observed at X-ray energies, after a delay of few hundreds of seconds relative to the gamma-ray photons from the jet. Multiple Compton scattering inside the cocoon causes a spread in the arrival times of the X-ray photons. We calculate numerically the observed light curve of photons emerging from the cocoon, and show that it exhibits a steep decay, which resembles that observed in many GRB afterglows. During the adiabatic expansion that follows the dissipation phase, photons lose energy to the expanding plasma, and as a result, the emerging photon energy distribution differs from a black-body spectrum, and can be approximated as a power law in the Swift XRT band. Comparison of the numerical results with the Swift XRT data of GRB050315 and GRB050421 shows good agreement between the light curves and spectra during the initial steep decay phase.

 

astro-ph/0603344 [abs, ps, pdf, other] :

Title: First Two-Micron Imaging Polarimetry of Beta Pictoris
Authors: Motohide Tamura, Misato Fukagawa, Hiroshi Kimura, Tetsuo Yamamoto, Hiroshi Suto, Lyu Abe
Comments: 18 pages, 6 figures, accepted for JpJ

High-resolution K band imaging polarimetry of the beta Pic dust disk has been conducted with adaptive optics and a coronagraph using the Subaru 8.2-m telescope. Polarization of ~10 % is detected out to r ~ 120 AU with a centro-symmetric vector pattern around the central star, confirming that the disk is seen as an infrared reflection nebula. We have modeled our near-infrared and previous optical polarization results in terms of dust scattering in the disk and have found that both the degrees of polarization and the radial intensity profiles are well reproduced. We argue that the observed characteristics of the disk dust are consistent with the presence of ice-filled fluffy aggregates consisting of submicron grains in the beta Pic system. There is a gap around 100 AU in both the intensity and polarization profiles, which suggests a paucity of planetesimals in this region. The radial intensity profile also shows ripple-like structures, which are indicative of the presence of multiple planetesimal belts, as in the case of the M-type Vega-like star AU Mic.

 

astro-ph/0603345 [abs, ps, pdf, other] :

Title: The host galaxy/AGN connection in nearby early-type galaxies. A new view of the origin of the radio-quiet/radio-loud dichotomy?
Authors: Alessandro Capetti (1) Barbara Balmaverde (2) ((1)INAF - Osservatorio Astronomico di Torino, Italy, (2) Universita' di Torino, Torino, Italy)
Comments: Accepted for publication in A&A

[ABRIDGED] This is the third in a series of three papers exploring the connection between the multiwavelength properties of AGN in nearby early-type galaxies and the characteristics of their hosts. We selected 116 AGN candidates requiring a radio flux of 1 mJy. We classified the objects with HST images into ``core'' and ``power-law'' galaxies, on the basis of the nuclear slope of their brightness profiles. We used HST and Chandra data to isolate their nuclear emission to study the multiwavelength behaviour of their nuclei. The properties of the nuclei hosted by the 29 core galaxies were presented in Paper II. Core galaxies invariably host a radio-loud nucleus, with a median radio-loudness of Log R = 3.6 and an X-ray based radio loudness parameter of Log R,X = -1.3. Here we discuss the properties of the nuclei of the 22 ``power-law'' galaxies. They show a substantial excess of optical and X-ray emission with respect to core galaxies at the same level of radio luminosity. Conversely, their radio-loudness parameters, Log R ~ 1.6 and Log R,X ~ -3.3, are similar to those measured in Seyfert galaxies. Thus the radio-loudness of AGN hosted by early-type galaxies appears to be univocally related to the host's brightness profile: radio-loud AGN are only hosted by core galaxies, while radio-quiet AGN are found only in power-law galaxies. The brightness profile is determined by the galaxy's evolution, through its merger history; our results suggest that the same process sets the AGN flavour.

 

astro-ph/0603346 [abs, ps, pdf, other] :

Title: Extrasolar Transiting Planet Search with Subaru Suprime-Cam
Authors: Seitaro Urakawa, Toru Yamada, Yasushi Suto, Edwin. L. Turner, Yoichi Itoh, Tadashi Mukai, Motohide Tamura, Yiping Wang
Comments: 14pages, 27figures

We report the results of a prototype photometric search for transiting extrasolar planets using Subaru Suprime-Cam. Out of about 100,000 stars monitored around the Galactic plane (l=90 degree,b=0 degree), we find that 7,700 (27,000) stars show the photometric precision below 1% (3%) for 60 second exposures which is required to detect extrasolar planets by the transit method. Thus Suprime-Cam has the photometric stability and accuracy required for a transiting planet survey. During this observing run, we detected three transiting planetary candidates (i'-band magnitude around 18.5) which exhibit a single full transit-like light curve with a fractional depth of <5%. While future photometric and/or spectroscopic follow-ups remain to be done, the estimated parameters for the three systems are consistent with planetary companions around main-sequence stars. We also found two eclipsing binary candidates and eleven variable stars exhibiting W UMa-like light curves.

 

astro-ph/0603347 [abs, ps, pdf, other] :

Title: The X-ray properties of the magnetic Cataclysmic Variable UU Col
Authors: D. de Martino, G. Matt, K. Mukai, J.-M. Bonnet-Bidaud, V. Burwitz, B. T. Gaensicke, F. Haberl, M. Mouchet
Comments: 9 pages, 8 figures plus aa.cls style file, Accepted for publication in Astronomy & Astrophysics Main Journal

XMM-Newton observations aimed at determining for the first time the broad-band X-ray properties of the faint high galactic latitude Intermediate Polar UU Col are presented. We performed X-ray timing analysis in different energy ranges of the EPIC cameras which reveals the dominance of the 863s white dwarf rotational period. The spin pulse is strongly energy dependent. Weak variabilities at the beat 935s and at the 3.5hr orbital periods are also observed, but the orbital modulation is detected only below 0.5keV. Simultaneous UV and optical photometry shows that the spin pulse is anti-phased with respect to the hard X-rays. Analysis of the EPIC and RGS spectra reveals the complexity of the X-ray emission, being composed of a soft 50eV black--body component and two optically thin emission components at 0.2keV and 11keV strongly absorbed by dense material with an equivalent hydrogen column density of 10**(23)cm**(-2) partially (50%) covering the X-ray source. The complex X-ray and UV/optical temporal behaviour indicates that accretion occurs predominantly (80%) via a disc with a partial contribution (20%) directly from the stream. The main accreting pole dominates at high energies whilst the secondary pole mainly contributes in the soft X-rays and at lower energies. The bolometric flux ratio of the soft-to-hard X-ray emissions is found to be consistent with the prediction of the standard accretion shock model. We find the white dwarf in UU Col accretes at a low rate and possesses a low magnetic moment. It is therefore unlikely that UU Col will evolve into a moderate field strength Polar, which leaves the soft X-ray Intermediate Polars a still enigmatic small group of magnetic Cataclysmic Variables.

 

astro-ph/0603348 [abs, ps, pdf, other] :

Title: On the mean field dynamo with Hall effect
Authors: A. Kandus, M.J. Vasconcelos, A.H. Cerqueira (LATO-DCET-UESC, Brazil)
Comments: 8 pages, 4 figures. Accepted by A&A

We study in the present paper how Hall effect modifies the quenching process of the electromotive force (e.m.f.) in Mean Field Dynamo (MFD) theories. We write down the evolution equations for the e.m.f. and for the large and small scale magnetic helicity, treat Hall effect as a perturbation and integrate the resulting equations assuming boundary conditions such that the total divergencies vanish. For force-free large scale magnetic fields, Hall effect acts by coupling the small scale velocity and magnetic fields. For the range of parameters considered, the overall effect is a stronger quenching of the e.m.f. than in standard MHD and a damping of the inverse cascade of magnetic helicity. In astrophysical environments characterized by the parameters considered here, Hall effect would produce an earlier quenching of the e.m.f. and consequently a weaker large scale magnetic field.

 

astro-ph/0603349 [abs, ps, pdf, other] :

Title: Large-scale study of the NGC 1399 globular cluster system in Fornax
Authors: L. P. Bassino (1), F. R. Faifer (1), J. C. Forte (1), B. Dirsch (2), T. Richtler (2), D. Geisler (2), Y. Schuberth (3) ((1) Facultad de Ciencias Astronomicas y Geofisicas, Universidad Nacional de La Plata, Argentina, (2) Universidad de Concepcion, Chile, (3) Sternwarte der Univ. Bonn, Germany)
Comments: 9 pages, 9 figures. Accepted for publication in A&A

We present a Washington C and Kron-Cousins R photometric study of the globular cluster system of NGC 1399, the central galaxy of the Fornax cluster. A large areal coverage of 1 square degree around NGC 1399 is achieved with three adjoining fields of the MOSAIC II Imager at the CTIO 4-m telescope. Working on such a large field, we can perform the first indicative determination of the total size of the NGC 1399 globular cluster system. The estimated angular extent, measured from the NGC 1399 centre and up to a limiting radius where the areal density of blue globular clusters falls to 30 per cent of the background level, is 45 +/- 5 arcmin, which corresponds to 220 - 275 kpc at the Fornax distance. The bimodal colour distribution of this globular cluster system, as well as the different radial distribution of blue and red clusters, up to these large distances from the parent galaxy, are confirmed. The azimuthal globular cluster distribution exhibits asymmetries that might be understood in terms of tidal stripping of globulars from NGC 1387, a nearby galaxy. The good agreement between the areal density profile of blue clusters and a projected dark-matter NFW density profile is emphasized.

 

astro-ph/0603350 [abs, pdf] :

Title: Cold molecular gas in the Perseus cluster core - Association with X-ray cavity, Halpha filaments and cooling flow -
Authors: P. Salome, F. Combes, A. C. Edge, C. Crawford, M. Erlund, A. C. Fabian, N. A. Hatch, R. M. Johnstone, J. S. Sanders, R. J. Wilman
Comments: Accepted in A&A, 12 pages

Cold molecular gas has been recently detected in several cooling flow clusters of galaxies where huge optical nebulosities often stand. These optical filaments are tightly linked to the cooling flow and to the related phenomena, like the rising bubbles of relativistic plasma, fed by the radio jets. We present here a map in the CO(2-1) rotational line of the cold molecular gas associated with some Halpha filaments surrounding the central galaxy of the Perseus cluster: NGC 1275. The map, extending to about 50 kpc (135 arcsec) from the center of the galaxy, has been made with the 18-receiver array HERA, at the focus of the IRAM 30m telescope. Although most of the cold gas is concentrated to the center of the galaxy, the CO emission is also clearly associated to the extended filaments conspicuous in ionised gas and could trace a possible reservoir fueling the star formation there. Some of the CO emission is also found where the X-ray gas could cool down more efficiently: at the rims of the central X-ray cavity (where the hot gas is thought to have been pushed out and compressed by the central AGN expanding radio lobes). The CO global kinematics does not show any rotation in NGC 1275. The cold gas is probably a mixture of gas falling down on the central galaxy and of uplifted gas dragged out by a rising bubble in the intracluster medium. As recenlty suggested in other cluster cores, the cold gas peculiar morphology and kinematics argue for the picture of an intermittent cooling flow scenario where the central AGN plays an important role.

 

astro-ph/0603351 [abs, ps, pdf, other] :

Title: Thermal Stability of White Dwarfs Accreting Hydrogen-rich Matter and Progenitors of Type Ia Supernovae
Authors: Ken'ichi Nomoto (U. Tokyo), Hideyuki Saio (Tohoku U.), Mariko Kato (Keio U.), Izumi Hachisu (U. Tokyo)
Comments: submitted to the Astrophysical Journal, 18 pages including 6 figures

We revisit the properties of white dwarfs accreting hydrogen-rich matter by constructing steady-state models, in which hydrogen shell burning consumes hydrogen at the same rate as the white dwarf accretes it. We obtain such steady-state models for various accretion rates and white dwarf masses. We confirm that these steady models are thermally stable only when the accretion rate is higher than ~1 E-7 Msun/yr. We show that recently claimed ``quiescent burning'' in the ``surface hydrogen burning'' models at a much wider range of accretion rates results from the too large zone mass in the outer part of the models; hydrogen burning must occur in a much more superficial layer. A comparison of the positions on the HR diagram suggests that most of the luminous supersoft X-ray sources are white dwarfs accreting matter at rates high enough that the hydrogen burning shell is thermally stable. Implications on the progenitors of Type Ia supernovae are discussed.

 

astro-ph/0603352 [abs, ps, pdf, other] :

Title: What can we learn from long term monitoring of X-ray bursters?
Authors: Andrew Cumming (McGill University)
Comments: 5 pages, 2 figures, to be published in: "The Transient Milky Way: a perspective for MIRAX", eds. F. D'Amico, J. Braga & R. Rothschild, AIP Conf. Proc

The last few years have seen the discovery of a number of new aspects of Type I X-ray bursts: the extremely energetic and long duration superbursts, intermediate duration bursts at low luminosities, mHz QPOs, and burst oscillations. These discoveries promise a new understanding of nuclear burning on accreting neutron stars, and offer a chance to use observations to probe neutron star properties. I discuss what we can learn from future long term monitoring with MIRAX.

 

astro-ph/0603353 [abs, ps, pdf, other] :

Title: Observational constraints on self-accelerating cosmology
Authors: Roy Maartens, Elisabetta Majerotto (Portsmouth)
Comments: 5 pages, 4 figs

The DGP brane-world model provides a simple alternative to the standard LCDM cosmology, with the same number of parameters. There is no dark energy - the late universe self-accelerates due to an infrared modification of gravity. We compute the joint constraints on the DGP model from supernovae redshifts, the cosmic microwave background shift parameter, and the baryon oscillation peak in the SDSS luminous red galaxy sample. Flat DGP models are within the 1 sigma contour, but the flat LCDM model provides a better fit to the data .

 

astro-ph/0603354 [abs, ps, pdf, other] :

Title: High-resolution mapping of interstellar clouds with near-infrared scattered light
Authors: M. Juvela (1), V.-M. Pelkonen (1), P. Padoan (2), K. Mattila (1) ((1) Helsinki University Observatory, (2) CASS/UCSD)
Comments: 20 pages, submitted to A&A

With current wide-field near-infrared (NIR) instruments the scattered light in the near-infrared can be mapped over large areas. Below A_V ~ 10 the surface brightness is directly proportional to the column density, and at slightly higher column densities the saturation of the intensity values can be corrected using the ratios of the intensity in different NIR bands. NIR scattered light provides a promising new method for the mapping of quiescent interstellar clouds. We develop a method to convert the observed near-infrared surface brightness into estimates of the column density. We study the effect of possible error sources on the accuracy of such estimates. We also propose to reduce systematic errors by combining surface brightness data with extinction measurements derived from the near-infrared colour excess of background stars. Our study is based on a set of three-dimensional magnetohydrodynamic turbulence simulations. Maps of near-infrared scattered light are obtained with radiative transfer calculations, and the maps are converted back into column density estimates using the proposed method. The results are compared with the true column densities. Extinction measurements are simulated using the same turbulence simulations, and are used as a complementary column density tracer. We find that NIR intensities can be converted into a reliable estimate of the column density in regions with A_V up to almost 20mag.

 

astro-ph/0603355 [abs, ps, pdf, other] :

Title: Integrated-light VRI imaging photometry of globular clusters in the Magellanic clouds
Authors: Paul Goudfrooij (1), Diane Gilmore (1), Markus Kissler-Patig (2), Claudia Maraston (3) ((1) STScI, (2) ESO, (3) University of Oxford)
Comments: 9 pages, 2 figures, accepted for publication in MNRAS

We present accurate integrated-light photometry in Johnson/Cousins V, R, and I for a sample of 28 globular clusters in the Magellanic Clouds. The majority of the clusters in our sample have reliable age and metallicity estimates available in the literature. The sample encompasses ages between 50 Myr and 7 Gyr, and metallicities ([Fe/H]) between -1.5 and 0.0 dex. The sample is dominated by clusters of ages between roughly 0.5 and 2 Gyr, an age range during which the bolometric luminosity of simple stellar populations is dominated by evolved red giant branch stars and thermally pulsing asymptotic giant branch (TP-AGB) stars whose theoretical colours are rather uncertain. The VRI colours presented in this paper have been used to calibrate stellar population synthesis model predictions.

 

astro-ph/0603356 [abs, ps, pdf, other] :

Title: HST/FOS Spectral Mapping of V2051 Ophiuchi in a Low State
Authors: R. K. Saito, R. Baptista
Comments: 20 pages, 11 figures, accept to publication in The Astronomical Journal (issue 131, April 2006)

We report a study of the spectra and structure of the accretion disk of the dwarf nova V2051 Ophiuchi while the star was in an unusual faint brightness state during 1996. The differences between the UV resonant lines and continuum disk surface brightness distributions indicate a vertically extended disk with the emission from these lines arising from the upper atmospheric layers. Distinct emission along the stream trajectory suggests the occurrence of gas stream overflow. Spatially resolved spectra show that the lines are in emission at all disk radii. The Balmer decrement becomes shallower with increasing radius. The FWHMs of the emission lines show differences with respect to that expected for a gas in Keplerian rotation and the line intensities drop with a radial dependency of $I\propto R^{-1.78}$. The uneclipsed light contributes about 5 - 10 per cent of the total flux, and its spectrum is dominated by a Balmer jump and strong lines in emission. Broad absorption bands, possibly due to Fe II, are present in the spectra of the disk side farther away from the secondary star, suggesting it arises from absorption by a extended gas region above the disk; the differences between the spectra of the hemisphere farther from and nearer to the secondary star are interpreted in terms of chromospheric emission from a disc with a non-negligible opening angle (limb brightening effect). Stellar atmosphere model fits to the extracted white dwarf spectrum lead to a temperature $T_{WD}= 9500 ^{+2900}_{-1900} K$ and a distance of $d = 67 ^{+22}_{-25} pc$ if the inner disk is opaque, or $d = 92 ^{+30}_{-35} pc$ if the inner disk is optically thin.

 

astro-ph/0603357 [abs, ps, pdf, other] :

Title: Molecular Cloud Turbulence and Star Formation
Authors: J. Ballesteros-Paredes (CRyA-UNAM), R.S. Klessen (AIP), M.-M. Mac Low (AMNH), E. Vazquez-Semadeni (CRyA-UNAM)
Comments: Review chapter in "Protostars and Planets V". 18 pages, 4 figures

We review the properties of turbulent molecular clouds (MCs), focusing on the physical processes that influence star formation (SF). MC formation appears to occur during large-scale compression of the diffuse ISM driven by supernovae, magnetorotational instability, or gravitational instability in galactic disks of stars and gas. The compressions generate turbulence that can accelerate molecule production and produce the observed morphology. We then review the properties of MC turbulence, including density enhancements observed as clumps and cores, magnetic field structure, driving scales, the relation to observed scaling relations, and the interaction with gas thermodynamics. We argue that MC cores are dynamical, not quasistatic, objects with relatively short lifetimes not exceeding a few megayears. We review their morphology, magnetic fields, density and velocity profiles, and virial budget. Next, we discuss how MC turbulence controls SF. On global scales turbulence prevents monolithic collapse of the clouds; on small scales it promotes local collapse. We discuss its effects on the SF efficiency, and critically examine the possible relation between the clump mass distribution and the initial mass function, and then turn to the redistribution of angular momentum during collapse and how it determines the multiplicity of stellar systems. Finally, we discuss the importance of dynamical interactions between protostars in dense clusters, and the effect of the ionization and winds from those protostars on the surrounding cloud. We conclude that the interaction of self-gravity and turbulence controls MC formation and behavior, as well as the core and star formation processes within them.

 

astro-ph/0603358 [abs, ps, pdf, other] :

Title: The Chemical Evolution of Protoplanetary Disks
Authors: E. A. Bergin, Y. Aikawa, G. A. Blake, E. F. van Dishoeck
Comments: 16 pages, 5 figures. To appear in Protostars and Planets V. Manuscript with original (non-reduced) figures can be found at this http URL

In this review we re-evaluate our observational and theoretical understanding of the chemical evolution of protoplanetary disks. We discuss how improved observational capabilities have enabled the detection of numerous molecules exposing an active disk chemistry that appears to be in disequilibrium. We outline the primary facets of static and dynamical theoretical chemical models. Such models have demonstrated that the observed disk chemistry arises from warm surface layers that are irradiated by X-ray and FUV emission from the central accreting star. Key emphasis is placed on reviewing areas where disk chemistry and physics are linked: including the deuterium chemistry, gas temperature structure, disk viscous evolution (mixing), ionization fraction, and the beginnings of planet formation.

 

astro-ph/0603359 [abs, ps, pdf, other] :

Title: Infrared Constraints on AGN Tori Models
Authors: E. Hatziminaoglou, J. Fritz, I. Perez-Fournon, A. Franceschini, A. Hernan-Caballero, A. Afonso-Luis, C. Lonsdale, F. Fang, S. Oliver, M. Rowan-Robinson, D. Shupe, H. Smith, J. Surace, E. Gonzales-Solares, the SWIRE Team
Comments: 4 pages, 4 figures, to appear in "Infrared Diagnostics of Galaxy Evolution", ASP Conference Series, Pasadena, 14-16 November 2005

This work focuses on the properties of dusty tori in active galactic nuclei (AGN) derived from the comparison of SDSS type 1 quasars with mid-Infrared (MIR) counterparts and a new, detailed torus model. The infrared data were taken by the Spitzer Wide-area InfraRed Extragalactic (SWIRE) Survey. Basic model parameters are constraint, such as the density law of the graphite and silicate grains, the torus size and its opening angle. A whole variety of optical depths is supported. The favoured models are those with decreasing density with distance from the centre, while there is no clear tendency as to the covering factor, ie small, medium and large covering factors are almost equally distributed. Based on the models that better describe the observed SEDs, properties such as the accretion luminosity, the mass of dust, the inner to outer radius ratio and the hydrogen column density are computed.

 

astro-ph/0603360 [abs, ps, pdf, other] :

Title: First Light
Authors: Abraham Loeb (Harvard)
Comments: 158 pages, lecture notes for the SAAS-Fee Winter School, April 2006 (to be published by Springer Verlag); this http URL

The first dwarf galaxies, which constitute the building blocks of the collapsed objects we find today in the Universe, had formed hundreds of millions of years after the big bang. This pedagogical review describes the early growth of their small-amplitude seed fluctuations from the epoch of inflation through dark matter decoupling and matter-radiation equality, to the final collapse and fragmentation of the dark matter on all mass scales above \~10^{-4} solar masses. The condensation of baryons into halos in the mass range of ~10^5-10^{10} solar masses led to the formation of the first stars and the re-ionization of the cold hydrogen gas, left over from the big bang. The production of heavy elements by the first stars started the metal enrichment process that eventually led to the formation of rocky planets and life. A wide variety of instruments currently under design [including large-aperture infrared telescopes on the ground or in space (JWST), and low-frequency arrays for the detection of redshifted 21cm radiation], will establish better understanding of the first sources of light during an epoch in cosmic history that was largely unexplored so far. Numerical simulations of reionization are computationally challenging, as they require radiative transfer across large cosmological volumes as well as sufficently high resolution to identify the sources of the ionizing radiation. The technological challenges for observations and the computational challenges for numerical simulations, will motivate intense work in this field over the coming decade.

 

astro-ph/0603361 [abs, ps, pdf, other] :

Title: Identification of the Broad Solar Emission Features Near 117 nm
Authors: Eugene H. Avrett, Robert L. Kurucz, Rudolf Loeser
Comments: 5 pages, 4 figures, 1 table (Accepted for Astronomy & Astrophysics)

Wilhelm et al. have recently called attention to the unidentified broad emission features near 117 nm in the solar spectrum. They discuss the observed properties of these features in detail but do not identify the source of this emission.
We show that the broad autoionizing transitions of neutral sulfur are responsible for these emission features. Autoionizing lines of \ion{S}{i} occur throughout the spectrum between Lyman alpha and the Lyman limit. Sulfur is a normal contributor to stellar spectra.
We use non-LTE chromospheric model calculations with line data from the Kurucz 2004 \ion{S}{i} line list to simulate the solar spectrum in the range 116 to 118 nm. We compare the results with SUMER disk-center observations from Curdt et al. and limb observations from Wilhelm et al.
Our calculations generally agree with the SUMER observations of the broad autoionizing \ion{S}{i} emission features, the narrow \ion{S}{i} emission lines, and the continuum in this wavelength region, and agree with basic characteristics of the center-to-limb observations. In addition to modeling the average spectrum, we show that a change of $\pm$ 200 K in the temperature distribution causes the intensity to change by a factor of 4. This exceeds the observed intensity variations 1) with time in quiet regions at these wavelengths, and 2) with position from cell centers to bright network. These results do not seem compatible with current dynamical models that have temporal variations of 1000 K or more in the low chromosphere.

 

astro-ph/0603362 [abs, ps, pdf, other] :

Title: Stochastic particle acceleration and synchrotron self--Compton radiation in TeV blazars
Authors: K. Katarzynski, G. Ghisellini, A. Mastichiadis, F. Tavecchio, L. Maraschi
Comments: 11 pages, 4 figures, accepted for publication in A&A

We analyse the influence of the stochastic particle acceleration for the evolution of the electron spectrum. We assume that all investigated spectra are generated inside a spherical, homogeneous source and also analyse the synchrotron and inverse Compton emission generated by such an object. The stochastic acceleration is treated as the diffusion of the particle momentum and is described by the momentum-diffusion equation. We investigate the stationary and time dependent solutions of the equation for several different evolutionary scenarios. The scenarios are divided into two general classes. First, we analyse a few cases without injection or escape of the particles during the evolution. Then we investigate the scenarios where we assume continuous injection and simultaneous escape of the particles. In the case of no injection and escape the acceleration process, competing with the radiative cooling, only modifies the initial particle spectrum. The competition leads to a thermal or quasi-thermal distribution of the particle energy. In the case of the injection and simultaneous escape the resulting spectra depend mostly on the energy distribution of the injected particles. In the simplest case, where the particles are injected at the lowest possible energies, the competition between the acceleration and the escape forms a power-law energy distribution. We apply our modeling to the high energy activity of the blazar Mrk 501 observed in April 1997. Calculating the evolution of the electron spectrum self-consistently we can reproduce the observed spectra well with a number of free parameters that is comparable to or less than in the "classic stationary" one--zone synchrotron self-Compton scenario.

 

astro-ph/0603363 [abs, ps, pdf, other] :

Title: Cosmic-Ray Acceleration at Ultrarelativistic Shock Waves: Effects of Downstream Short-Wave Turbulence
Authors: Jacek Niemiec (1 and 2), Michal Ostrowski (3), Martin Pohl (1) ((1) Department of Physics and Astronomy, Iowa State University, Ames, USA, (2) Institute of Nuclear Physics PAS, Krakow, Poland, (3) Astronomical Observatory, Jagiellonian University, Krakow, Poland)
Comments: 9 pages, 8 figures, submitted to ApJ

The present paper is the last of a series studying the first-order Fermi acceleration processes at relativistic shock waves with the method of Monte Carlo simulations applied to shocks propagating in realistically modeled turbulent magnetic fields. The model of the background magnetic field structure of Niemiec & Ostrowski (2004, 2006) has been augmented here by a large-amplitude short-wave downstream component, imitating that generated by plasma instabilities at the shock front. Following Niemiec & Ostrowski (2006), we have considered ultrarelativistic shocks with the mean magnetic field oriented both oblique and parallel to the shock normal. For both cases simulations have been performed for different choices of magnetic field perturbations, represented by various wave power spectra within a wide wavevector range. The results show that the introduction of the short-wave component downstream of the shock is not sufficient to produce power-law particle spectra with the "universal" spectral index 4.2. On the contrary, concave spectra with cutoffs are preferentially formed, the curvature and cutoff energy being dependent on the properties of turbulence. Our results suggest that the electromagnetic emission observed from astrophysical sites with relativistic jets, e.g. AGN and GRBs, is likely generated by particles accelerated in processes other than the widely invoked first-order Fermi mechanism.

 

astro-ph/0603364 [abs, ps, pdf, other] :

Title: X-ray emission from the blazar AO 0235+16: the XMM-Newton and Chandra point of view
Authors: C. M. Raiteri, M. Villata, M. Kadler, T. P. Krichbaum, M. Boettcher, L. Fuhrmann, M. Orio
Comments: 12 pages, 16 PNG figures, in press for A&A

In this paper we analyse five observations of the BL Lac object AO 0235+16 performed with the Chandra and XMM-Newton satellites during the years 2000-2005. In the February 2002 observation the source is found in a bright state and presents a steep X-ray spectrum, while in all the other epochs it is faint and the spectrum is hard. The soft X-ray spectrum appears to be strongly absorbed, likely by the intervening system at z=0.524, which also absorbs the optical-UV radiation. We find that models that consider spectral curvature are superior to single power law ones in fitting the X-ray spectrum. In particular, we favour a double power law model, which agrees with the assumption of a superposition of two different components in the X-ray domain. Both in the Chandra and in one of the XMM-Newton observations, a tentative detection of the redshifted Fe Kalpha emission line may suggest its origin from the inner part of an accretion disc. Thermal emission from this accretion disc might explain the UV-soft-X-ray bump that appears in the spectral energy distributions, when the X-ray spectra are complemented with the optical-UV data from the Optical Monitor onboard XMM-Newton. More likely, the bump can be interpreted in terms of an additional synchrotron component emitted from an inner region of the jet with respect to that where the lower-energy emission comes from. An inspection of the X-ray light curves reveals that intraday variability occurs only when the source is in a bright state.

 

astro-ph/0603365 [abs, ps, pdf, other] :

Title: Matched Filters for Source Detection in the Poissonian Noise Regime
Authors: Ian Stewart

A procedure is described for estimating an optimum kernel for the detection by convolution of signals among Poissonian noise. The technique is applied to the detection of x-ray point sources in XMM-Newton data, and is shown to yield an improvement in detection sensitivity of up to 60% over the sliding-box method used in the creation of the 1XMM catalog.

 

astro-ph/0603366 [abs, ps, pdf, other] :

Title: Chaplygin Inflation
Authors: O. Bertolami, V. Duvvuri
Comments: Revtex4, 4 pages

We discuss chaotic inflation in the context of a modified gravity model inspired in the Chaplygin equation of state. We find that all observational constraints can be satisfied provided the Chaplygin scale is smaller than $7 \times 10^{-3} M$ and the inflaton mass is smaller than $7 \times 10^{-6} M$, where $M^2\equiv(8 \pi G)^{-1} is the reduced Planck mass.

 

astro-ph/0603367 [abs, ps, pdf, other] :

Title: Global universe anisotropy probed by the alignment of structures in the cosmic microwave background
Authors: Y. Wiaux (1), P. Vielva (2), E. Martinez-Gonzalez (2), P. Vandergheynst (1) ((1) Swiss Federal Institute of Technology, Lausanne, Switzerland, (2) Instituto de Fisica de Cantabria (CSIC-UC), Santander, Spain)
Comments: 4 pages, 2 figures, Revtex4, Submitted to Phys. Rev. Lett

We question the global universe isotropy by probing the alignment of local structures in the cosmic microwave background (CMB) radiation. The original method proposed relies on a steerable wavelet decomposition of the CMB signal on the sphere. The analysis of the first-year WMAP data identifies a mean preferred plane with a normal direction close to the CMB dipole axis, and a mean preferred direction in this plane, very close to the ecliptic poles axis. Previous statistical anisotropy results are thereby synthesized, but further analyses are still required to establish their origin.

 

astro-ph/0603368 [abs, ps, pdf, other] :

Title: Restrictions on parameters of sterile neutrino dark matter from observations of galaxy clusters
Authors: Alexey Boyarsky, Andrey Neronov, Oleg Ruchayskiy, Mikhail Shaposhnikov
Comments: 10 pages

We find restrictions on the mass and mixing angle of the dark matter sterile neutrinos using X-ray observations of Coma and Virgo galaxy clusters.

 

astro-ph/0603369 [abs, ps, pdf, other] :

Title: CMB multipole measurements in the presence of foregrounds
Authors: Angelica de Oliveira-Costa (MIT), Max Tegmark (MIT)
Comments: 11 PRD pages, 6 figures

Most analysis of Cosmic Microwave Background spherical harmonic coefficients a_lm has focused on estimating the power spectrum C_l=<|a_lm|^2> rather than the coefficients themselves. We present a minimum-variance method for measuring a_lm given anisotropic noise, incomplete sky coverage and foreground contamination, and apply it to the WMAP data. Our method is shown to constitute lossless data compression in the sense that the widely used quadratic estimators of the power spectrum C_l can be computed directly from our a_lm-estimators. As the Galactic cut is increased, the error bars Delta-a_lm on low multipoles go from being dominated by foregrounds to being dominated by sample variance from other multipoles, with the intervening minimum defining the optimal cut. Applying our method to the WMAP quadrupole and octopole, we find that their previously reported "axis of evil" alignment appears to be rather robust to Galactic cut and foreground contamination.

 

astro-ph/0603370 [abs, ps, pdf, other] :

Title: Weak-line T Tauri Star Disks I. Initial Spitzer Results from the Cores to Disks Legacy Project
Authors: D. L. Padgett, L. Cieza, K. R. Stapelfeldt, N. J. Evans, D. Koerner, A. Sargent, M. Fukagawa, E. F. van Dishoek, J. Augereau, L. Allen, G. Blake, T. Brooke, N. Chapman, P. Harvey, A. Porras, S. Lai, L. Mundy, P. C. Myers, W. Spiesman, Z. Wahhaj
Comments: 29 pages, 9 figures, accepted for publication in ApJ

Using the Spitzer Space Telescope, we have observed 90 weak-line and classical T Tauri stars in the vicinity of the Ophiuchus, Lupus, Chamaeleon, and Taurus star-forming regions as part of the Cores to Disks (c2d) Spitzer Legacy project. In addition to the Spitzer data, we have obtained contemporaneous optical photometry to assist in constructing spectral energy distributions. These objects were specifically chosen as solar-type young stars with low levels of H alpha emission, strong X-ray emission, and lithium absorption i.e. weak-line T Tauri stars, most of which were undetected in the mid-to-far IR by the IRAS survey. Weak-line T Tauri stars are potentially extremely important objects in determining the timescale over which disk evolution may take place. Our objective is to determine whether these young stars are diskless or have remnant disks which are below the detection threshold of previous infrared missions. We find that only 5/83 weak-line T Tauri stars have detectable excess emission between 3.6 and 70 micron which would indicate the presence of dust from the inner few tenths of an AU out to the planet-forming regions a few tens of AU from the star. Of these sources, two have small excesses at 24 microns consistent with optically thin disks; the others have optically thick disks already detected by previous IR surveys. All of the seven classical T Tauri stars show excess emission at 24 and 70 micron, although their properties vary at the shorter wavelengths. Our initial results show that disks are rare among young stars selected for their weak H alpha emission.

 

astro-ph/0603371 [abs, ps, pdf, other] :

Title: The Expansion Asymmetry and Age of the Cassiopeia A Supernova Remnant
Authors: R. A. Fesen (1), M. C. Hammell (1), J. Morse (2), R. A. Chevalier (3), K. J. Borkowski (4), M. A. Dopita (5), C. L. Gerardy (6), S. S. Lawrence (7), J. C. Raymond (8), S. van den Bergh (9) ((1) Dartmouth, (2) Arizona State Univ., (3) Univ. of Virginia, (4) North Carolina State Univ., (6) Australian National Univ., (7) Hofstra Univ., (8) Harvard-SAO, (9) DAO)
Comments: 13 pages, 5 figures, ApJ, in press

HST ACS images of the young SN remnant Cas A are used to explore the expansion and spatial distribution of its highest velocity debris. Proper motions of over 1800 outlying ejecta knots are reported. The distribution of transverse expansion velocities for these knots shows a striking bipolar asymmetry with the highest velocity knots confined to nearly opposing northeast and southwest `jets'. The jets appear kinematically and chemically distinct with respect to the remnant's highest velocity debris seen in other directions. Significant gaps in the spatial distribution of outlying ejecta lie in directions which are approximately perpendicular to the jets. Extrapolations of 9 month proper motions for all outer ejecta knots and a subsample of 72 bright and compact knots suggest explosion dates (assuming no knot deceleration) of 1662 +/- 27 and 1672 +/- 18, respectively. We find some evidence for non-uniform deceleration in different directions with knots located along the northwestern limb among the least decelerated ejecta suggesting a convergence date of 1681 +/-19. The remnant's central X-ray point source lies some $7''$ to the southeast of the estimated expansion center (PA = 169 deg) indicating a projected motion of ~350 km/s toward the middle of the broad southern outer ejecta knot gap.