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Astrophysics

astro-ph new abstracts, Thu, 5 Jan 06 01:00:08 GMT
0601052 -- 0601086 received

astro-ph/0601052 [abs, ps, pdf, other] :

Title: The Paths of Quintessence
Authors: Eric V. Linder
Comments: 12 pages, 7 figures

The structure of the dark energy equation of state phase plane holds important information on the nature of the physics. We explain the bounds of the freezing and thawing models of scalar field dark energy in terms of the tension between the steepness of the potential vs. the Hubble drag. Additionally, we extend the phase plane structure to modified gravity theories, examine trajectories of models with certain properties, and categorize regions in terms of scalar field hierarchical parameters, showing that dark energy is generically not a slow roll phenomenon.

 

astro-ph/0601053 [abs, ps, pdf, other] :

Title: Twin-peak quasiperiodic oscillations as an internal resonance
Authors: J. Horak, V. Karas
Comments: Accepted for publication in A&A; 11 pages, 6 figures

Two inter-related peaks occur in high-frequency power spectra of X-ray lightcurves of several black-hole candidates. We further explore the idea that a non-linear resonance mechanism, operating in strong-gravity regime, is responsible for these quasi-periodic oscillations (QPOs). By extending the multiple-scales analysis of Rebusco, we construct two-dimensional phase-space sections, which enable us to identify different topologies governing the system and to follow evolutionary tracks of the twin peaks. This suggests that the original (Abramowicz and Kluzniak) parametric-resonance scheme can be viewed as an ingenuous account of the QPOs model with an internal resonance. We show an example of internal resonance in a system with up to two critical points, and we describe a general technique that permits to treat other cases in a systematical manner. A separatrix divides the phase-space sections into regions of different topology: inside the libration region the evolutionary tracks bring the observed twin-peak frequencies to an exact rational ratio, whereas in the circulation region the observed frequencies remain off resonance. Our scheme predicts the power should cyclically be exchanged between the two oscillations. Likewise the high-frequency QPOs in neutron-star binaries, also in black-hole sources one expects, as a general property of the non-linear model, that slight detuning pushes the twin-peak frequencies out of sharp resonance.

 

astro-ph/0601054 [abs, ps, pdf, other] :

Title: GRB efficiency and Possible Physical Processes Shaping the Early Afterglow
Authors: Yizhong Fan (HU and PMO), Tsvi Piran (HU)
Comments: 11 pages including 5 eps figures, submitted to MNRAS

The discovery by Swift that a good fraction of Gamma Ray Bursts (GRBs) have a slowly decaying X-ray afterglow phase led to the suggestion that energy injection into the blast wave takes place several hundred seconds after the burst. This implies that right after the burst the kinetic energy of the blast wave was very low and in turn the efficiency of production of $\gamma$-rays during the burst was extremely high, rendering the internal shocks model unlikely. We re-examine the estimates of kinetic energy in GRB afterglows and show that the efficiency of converting the kinetic energy into gamma-rays is moderate and does not challenge the standard internal shock model. We also examine several models, including in particular energy injection, suggested to interpret this slow decay phase. We show that with proper parameters, all these models give rise to a slow decline lasting several hours. However, even those models that fit all X-ray observations, and in particular the energy injection model, cannot account self-consistently for both the X-ray and the optical afterglows of well monitored GRBs such as GRB 050319 and GRB 050401. We speculate about a possible alternative resolution of this puzzle.

 

astro-ph/0601055 [abs, ps, pdf, other] :

Title: Cross correlation of Lyman-alpha absorbers with gas-rich galaxies
Authors: Emma V. Ryan-Weber (IoA, Cambridge)
Comments: 10 pages, 10 figures, accepted for publication in MNRAS

The HI Parkes All Sky Survey (HIPASS) galaxy catalogue is cross-correlated with known low redshift, low column density (N_HI <10^15 cm^-2) Lyman-alpha absorbers from the literature. The redshift-space correlation is found to be similar in strength to HIPASS galaxy self-clustering (correlation length s_0,ag=6+/-4 and s_0,gg=3.1+/-0.5 h^-1 Mpc respectively). In real-space the cross-correlation is stronger than the galaxy auto-correlation (correlation length r_0,ag=7.2+/-1.4 and r_0,gg=3.5+/-0.7 h^-1 Mpc respectively) on scales from 1-10 h^-1 Mpc, ruling out the mini-halo model for the confinement Lyman-alpha absorbers at the 99 percent confidence level. Provided that the cause of the strong cross-correlation is purely gravitational, the ratio of correlation lengths suggest that absorbers are embedded in dark matter haloes with masses log(M/Msun)=14.2 h^-1, similar to those of galaxy groups. The flattening of the cross-correlation at separations less than ~600 h^-1 kpc could correspond to the thickness of filaments in which absorbers are embedded. This work provides indirect statistical evidence for the notion that galaxy groups and large-scale filaments, particularly those that comprise gas-rich galaxies, are the dominant environments of low column density Lyman-alpha absorbers at z=0.

 

astro-ph/0601056 [abs, ps, pdf, other] :

Title: Implications of the Early X-Ray Afterglow Light Curves of Swift GRBs
Authors: Jonathan Granot, Arieh Konigl, Tsvi Piran
Comments: 31 pages, 2 figures, 2 tables; submitted to MNRAS

According to current models, gamma-ray bursts (GRBs) are produced when the energy carried by a relativistic outflow is dissipated and converted into radiation. The efficiency of this process, $\epsilon_\gamma$, is one of the critical factors in any GRB model. The X-ray afterglow light curves of Swift GRBs show an early stage of flat decay. This has been interpreted as reflecting energy injection. When combined with previous estimates, which have concluded that the kinetic energy of the late ($\gtrsim 10$ hr) afterglow is comparable to the energy emitted in gamma-rays, this interpretation implies very high values of $\epsilon_\gamma \gtrsim 0.9$. Such a high efficiency is hard to reconcile with most models, including in particular the popular internal-shocks model. We re-analyze the derivation of the kinetic energy from the afterglow X-ray flux and re-examine the resulting estimates of the efficiency. We confirm that, if the flatt decay arises from energy injection and the pre-Swift broad-band estimates of the kinetic energy are correct, then $\epsilon_\gamma \gtrsim 0.9$. We point out, however, that another interpretation of the flattish decay - a variable X-ray afterglow efficiency (e.g., due to a time dependence of afterglow shock microphysical parameters) - is possible. We also show that direct estimates of the kinetic energy from the late X-ray afterglow flux are sensitive to the assumed values of the shock microphysical parameters and suggest that broad-band afterglow fits might have underestimated the kinetic energy (e.g., by overestimating the fraction of electrons that are accelerated to relativistic energies). Either one of these possibilities implies a lower gamma-ray efficiency, and their joint effect could conceivably reduce the estimate of the typical $\epsilon_\gamma$ to $\sim 0.1-0.5$.

 

astro-ph/0601057 [abs, ps, pdf, other] :

Title: Unique Extragalactic Sightlines: Dissecting the Extreme Circumstellar Environment of Two Gamma-Ray Burst Progenitors with Echelle Spectroscopy
Authors: Jason X. Prochaska (1), Hsiao-Wen Chen (2), Joshua S. Bloom (3)
Comments: 11 pages, 9 figures. Submitted to ApJ: December 20, 2005

High-resolution spectroscopy of the afterglows for Gamma-ray bursts GRB 051111 and GRB 050730 have unveiled extreme physical conditions in the circumburst environment that are rarely seen even in the Galactic ISM. A detailed study of Fe^+ and Si^+ fine-structure transitions and atomic transitions of Mg^0 and Fe^0 yields strong constraints on the density and temperature of the gas, and the ambient radiation field. We derive, based on the large abundances of Si^+ and Fe^+ in the excited states, that the electron density exceeds 10^3 cm^-3 and argue that n_H > 10^5 cm^-3. Furthermore, the excited states are populated according to a Boltzmann distribution with excitation temperature T_Ex ~ 2000-6000K. For GRB 051111, our analysis of the neutral species and excited ions identifies two spatially distinct gaseous clouds that are responsible for producing the absorption spectrum: one that gives rise to the observed fine-structure transitions and the other that gives rise to the majority of the observed resonance transitions. The detection of strong Mg^0 absorption indicates the the majority of the gas associated with the ground-state is located far from the GRB (r> 100pc; likely outside the star-forming region hosting the GRB) otherwise it would have been ionized by the afterglow. In contrast, we argue the excited ions arise in a circumstellar medium local to the progenitor, r<10pc. Our study therefore represents the first analysis of the kinematics, chemical abundances, and gas densities in a circumburst environment. The low velocity spread of the fine-structure transitions suggest the high density environment is due to the interaction of a Wolf-Rayet wind with the ISM of the star-forming region. [abridged]

 

astro-ph/0601058 [abs, ps, pdf, other] :

Title: The influence of numerical parameters on tidally triggered bar formation
Authors: R. F. Gabbasov, M. A. Rodriguez-Meza, Jaime Klapp, Jorge L. Cervantes-Cota
Comments: 17 pages, 13 figures, 3 tables. Accepted for publication in A&A. A high resolution version of the paper is found at this http URL

The joint influence of numerical parameters such as the number of particles N, the gravitational softening length $\epsilon$ and the time-step $\Delta t$ is investigated in the context of galaxy simulations. For isolated galaxy models we have performed a convergence study and estimated the numerical parameters ranges for which the relaxed models do not deviate significantly from its initial configuration. By fixing N, we calculate the range of the mean interparticle separation $\lambda(r)$ along the disc radius. We have found that in the simulations with N=1310720 particles $\lambda$ varies by a factor of 6, and the corresponding final Toomre's parameters Q change by only about 5 per cent. By decreasing N, the $\lambda$ and Q ranges broaden. Large $\epsilon$ and small N cause an earlier bar formation. For a given set of parameters the disc heating is smaller with the Plummer softening than with the spline softening. For galaxy collision models numerical simulations indicate that the properties of the formed bars strongly depend upon the selection of N and $\epsilon$. Large values of the gravitational softening parameter and a small number of particles results in the rapid formation of a well defined, slowly rotating bar. On the other hand, small values of $\epsilon$ produce a small, rapidly rotating disc with tightly wound spiral arms, and subsequently a weak bar emerges. We have found that by increasing N, the bar properties converge and the effect of the softening parameter diminishes. Finally, in some cases short spiral arms are observed at the ends of the bar that change periodically from trailing to leading and vice-versa - the wiggle.

 

astro-ph/0601059 [abs, ps, pdf, other] :

Title: Suppressing Speckle Noise for Simultaneous Differential Extrasolar Planet Imaging (SDI) at the VLT and MMT
Authors: Beth A. Biller, Laird M. Close, Rainer Lenzen, Wolfgang Brandner, Donald McCarthy, Eric Nielsen, Stephan Kellner, Markus Hartung
Comments: 5 pages, 3 figures, 1 table. Presented at IAU Colloquium 200, Direct Imaging of Exoplanets: Science and Techniques

We discuss the instrumental and data reduction techniques used to suppress speckle noise with the Simultaneous Differential Imager (SDI) implemented at the VLT and the MMT. SDI uses a double Wollaston prism and a quad filter to take 4 identical images simultaneously at 3 wavelengths surrounding the 1.62 um methane bandhead found in the spectrum of cool brown dwarfs and gas giants. By performing a difference of images in these filters, speckle noise from the primary can be significantly attenuated, resulting in photon noise limited data past 0.5''. Non-trivial data reduction tools are necessary to pipeline the simultaneous differential imaging. Here we discuss a custom algorithm implemented in IDL to perform this reduction. The script performs basic data reduction tasks but also precisely aligns images taken in each of the filters using a custom shift and subtract routine. In our survey of nearby young stars at the VLT and MMT (see Biller et al., this conference), we achieved H band contrasts >25000 (5 sigma Delta F1(1.575 um) > 10.0 mag, Delta H > 11.5 mag for a T6 spectral type object) at a separation of 0.5" from the primary star. We believe that our SDI images are among the highest contrast astronomical images ever made from ground or space for methane rich companions.

 

astro-ph/0601060 [abs, ps, pdf, other] :

Title: A Census of the Carina Nebula. I: Cumulative Energy Input from Massive Stars
Authors: Nathan Smith
Comments: 10 pages, accepted to MNRAS

(abridged) The Carina Nebula (NGC 3372) is our richest nearby laboratory in which to study feedback through UV radiation and stellar winds from very massive stars during the formation of an OB association before supernova explosions have influenced the environment. This paper takes inventory of the combined effect from all the known massive stars that power the Carina Nebula. The present-day total ionizing photon luminosity produced by the 65 O stars and 3 WNL stars in Carina is about 1e51/s, the total bolometric luminosity of all stars earlier than B2 is 2.5e7 Lsun, and the total mechanical luminosity of stellar winds is about 1e5 Lsun. The total ionizing flux was about 25 per cent higher when eta Car was on the main sequence, before it and its companion were surrounded by its obscuring dust shell, and when the 3 WNL stars were on the main sequence. For the first 3 Myr, the net ionizing flux of the 70 O-type stars in Carina was about 150 times greater than in the Orion Nebula. About 400-500 Msun has been contributed to the region by stellar wind mass loss during the past 3 Myr. The net ionizing flux and mechanical luminosity are also given for the individual clusters Tr14, 15, and 16, and Bo10 and 11, which are relevant on smaller spatial scales.

 

astro-ph/0601061 [abs, ps, pdf, other] :

Title: Silicate Emission in the Spitzer IRS spectrum of FSC 10214+4724
Authors: H. I. Teplitz (1), L. Armus (1), B.T. Soifer (1), V. Charmandaris (2,3), J. A. Marshall (3), H. Spoon (3), C. R. Lawrence (4), L. Hao (3), S. Higdon (3), Y. Wu (3), M. Lacy (1), P. R. Eisenhardt (4), T. Herter (3), J. R. Houck (3); ((1) Spitzer Science Center, (2) University of Crete, (3) Cornell, (4) JPL)
Comments: Accepted for publication in ApJL

We present the first MIR spectrum of the z=2.2856 ultraluminous, infrared galaxy FSC 10214+4724, obtained with the Infrared Spectrograph onboard the Spitzer Space Telescope. The spectrum spans a rest wavelength range of 2.3-11.5 microns, covering a number of key diagnostic emission and absorption features. The most prominent feature in the IRS spectrum is the silicate emission at rest-frame 10 microns. We also detect an unresolved emission line at a rest wavelength of 7.65 microns which we identify with [NeVI], and a slightly resolved feature at 5.6 microns identified as a blend of [Mg VII] and [Mg V]. There are no strong PAH emission features in the FSC 10214+4724 spectrum. We place a limit of 0.1 micron on the equivalent width of 6.2 micron PAH emission but see no evidence of a corresponding 7.7 micron feature. Semi-empirical fits to the spectral energy distribution suggest about 45% of the bolometric luminosity arises from cold 50 K dust, half arises from warm (190 K) dust, and the remainder, 5%, originates from hot (640 K) dust. The hot dust is required to fit the blue end of the steep MIR spectrum. The combination of a red continuum, strong silicate emission, little or no PAH emission, and no silicate absorption, makes FSC 10214+4724 unlike most other ULIRGs or AGN observed thus far with IRS. These apparently contradictory properties may be explained by an AGN which is highly magnified by the lens, masking a (dominant) overlying starburst with unusually weak PAH emission.

 

astro-ph/0601062 [abs, ps, pdf, other] :

Title: A Survey of Close, Young Stars with the Simultaneous Differential Imager (SDI) at the VLT and MMT
Authors: Beth A. Biller, Laird M. Close, Elena Masciadri, Rainer Lenzen, Wolfgang Brandner, Donald McCarthy, Thomas Henning, Eric Nielsen, Markus Hartung
Comments: 7 pages, 3 figures, 1 table. Presented at IAU Colloquium 200, Direct Imaging of Exoplanets: Science and Techniques. For high resolution figures, see IAUC 200 conference proceedings

We discuss the preliminary results of a survey of young (<300 Myr), close (<50 pc) stars with the Simultaneous Differential Extrasolar Planet Imager (SDI) implemented at the VLT and the MMT. SDI uses a double Wollaston prism and a quad filter to take 4 identical images simultaneously at 3 wavelengths surrounding the 1.62 um methane bandhead found in the spectrum of cool brown dwarfs and gas giants. By performing a difference of images in these filters, speckle noise from the primary can be significantly attenuated, resulting in photon noise limited data. In our survey data, we achieved H band contrasts >25000 (5 sigma Delta F1(1.575 um) > 10 mag, Delta H > 11.5 mag for a T6 spectral type) at a separation of 0.5" from the primary star. With this degree of attenuation, we should be able to image (5 sigma detection) a 2-4 Jupiter mass planet at 5 AU around a 30 Myr star at 10 pc. We are currently completing our survey of young, nearby stars. We have obtained complete datasets for 35 stars in the southern sky (VLT) and 7 stars in the northern sky (MMT). We believe that our SDI images are the highest contrast astronomical images ever made from ground or space for methane rich companions.

 

astro-ph/0601063 [abs, ps, pdf, other] :

Title: Physical Interpretations of Rotating Radio Transients
Authors: Bing Zhang (UNLV), Janusz Gil (Zielona Gora Univ), Jaroslow Dyks (Nicolaus Copernicus Center)
Comments: 3 pages, submitted to ApJL

Nearly a dozen of Rotating RAdio Transients (RRATs) have been discovered recently. These objects share similar spin parameters with normal radio pulsars but distinguish themselves from normal pulsars by being quiescent most of the time and only becoming radio bright occasionally. We discuss two possible interpretations to the phenomenon. The first model suggests that these objects are pulsars slightly below the radio emission ``death line'', and become active occasionally when the conditions for pair production and coherent emission are satisfied. The second model invokes a radio emission direction reversal in normal pulsars, as has been introduced to interpret the peculiar mode changing phenomenon in PSR B1822-09. In this picture, our line of sight misses the main radio emission beam of RRATs but happens to sweep the emission beam when the radio emission direction is reversed. RRATs are therefore the other half of the normal ``nulling'' pulsars. We suggest that X-ray observations can provide clues to differentiate these two possibilities.

 

astro-ph/0601064 [abs, ps, pdf, other] :

Title: The true nature of the alleged planetary nebula W16-185
Authors: Alexandre Roman-Lopes, Zulema Abraham
Comments: 30 pages, 15 figures, 2 tables V3: minor grammatical changes. Figure 4 is available in pdf file. Accepted for publication in AJ, April / 2006

We report the discovery of a small cluster of massive stars embedded in a NIR nebula in the direction of the IRAS15411-5352 point source, which is related to the alleged planetary nebula W16-185. The majority of the stars present large NIR excess characteristic of young stellar objects and have bright counterparts in the Spitzer IRAC images; the most luminous star (IRS1) is the NIR counterpart of the IRAS source. We found very strong unresolved Brgamma emission at the IRS1 position and more diluted and extended emission across the continuum nebula. From the sizes and electron volume densities we concluded that they represent ultra-compact and compact HII regions, respectively. Comparing the Brgamma emission with the 7 mm free-free emission, we estimated that the visual extinction ranges between 14 and 20 mag. We found that only one star (IRS1) can provide the number of UV photons necessary to ionize the nebula.

 

astro-ph/0601065 [abs, ps, pdf, other] :

Title: A Detailed Observation of a LMC Supernova Remnant DEM L241 with XMM-Newton
Authors: Aya Bamba (1), Masaru Ueno (2), Hiroshi Nakajima (3), Koji Mori (4), Katsuji Koyama (3) ((1) RIKEN, (2) Tokyo Inst. of Tech., (3) Kyoto Univ., (4) Miyazaki Univ.)
Comments: 9 pages, 11 figures, A&A, in press (full-resolution version is available at this http URL)

We report on an {\it XMM-Newton} observation of the supernova remnant (SNR) \object{DEM L241} in the Large Magellanic Cloud. In the soft band image, the emission shows an elongated structure, like a killifish, with a central compact source. The compact source is point-like, and named as XMMU J053559.3$-$673509. The source spectrum is well reproduced with a power-law model with a photon index of $\Gamma = 1.57$ (1.51--1.62) and the intrinsic luminosity is $2.2\times 10^{35} \mathrm{ergs s^{-1}}$ in the 0.5--10.0 keV band, with the assumed distance of 50 kpc. The source has neither significant coherent pulsations in $2.0\times 10^{-3}$ Hz--8.0 Hz, nor time variabilities. Its luminosity and spectrum suggest that the source might be a pulsar wind nebula (PWN) in DEM L241. The spectral feature classifies this source into rather bright and hard PWN, which is similar to those in Kes 75 and B0540$-$693. The elongated diffuse structure can be divided into a ``Head'' and ``Tail'', and both have soft and line-rich spectra. Their spectra are well reproduced by a plane-parallel shock plasma ({\it vpshock}) model with a temperature of 0.3--0.4 keV and over-abundance in O and Ne and a relative under-abundance in Fe. Such an abundance pattern and the morphology imply that the emission is from the ejecta of the SNR, and that the progenitor of DEM L241 is a very massive star, more than 20 M\sun. This result is also supported by the existence of the central point source and an OB star association, LH 88. The total thermal energy and plasma mass are $\sim 4\times 10^{50}$ ergs and $\sim 200$ M\sun, respectively.

 

astro-ph/0601066 [abs, ps, pdf, other] :

Title: Spitzer Imaging of nearby ULIRGs and their Progeny: Fine-Structure Elliptical Galaxies
Authors: J.A. Surace, Z. Wang, S. Willner, H. Smith, J. Pipher, W. Forrest, G. Fazio, J. Howell, A. Evans, J. Hibbard, L. Yan, F. Marleau
Comments: 5 pages, Proceedings of the 2005 Lijiang Conference, "Extreme Starbursts: Near and Far", editors: Yu Gao & D. B. Sanders, to be published in the special issue of Pub. of PMO

We present results from two mid-infrared imaging programs of ultraluminous infrared galaxies (ULIRGs) and fine-structure elliptical galaxies. The former are known to nearly all be recent mergers between two gas-rich spiral galaxies, while the latter are also believed to be even more aged merger remnants. An examination of these two classes of objects, which may represent different stages of the same putative merger sequence, should reveal similarities in the distribution of their stellar populations and dust content consistent with that expected for time-evolution of the merger. The data reveal for the first time extended dust emission in the ULIRG galaxy bodies and along their tidal features. However, contrary to expectation, we find that the vast majority of the fine-structure elliptical galaxies lack such structured emission. This likely results from the optical selection of the elliptical sample, resulting in a population that reflects the IR-activity (or lack thereof) of optically selected interacting galaxies. Alternately, this may reflect an evolutionary process in the distribution of the dust content of the galaxy bodies.

 

astro-ph/0601067 [abs, ps, pdf, other] :

Title: Rotation in the ZAMS: Be and Bn stars
Authors: J. Zorec (IAP), Y. Frémat (ROB), C. Martayan (GEPI), L. Cidale (UNLP), A. Torres (UNLP)
Comments: 3 pages ; to appear in the proceedings of the Sapporo meeting on active OB stars ; ASP Conference Series ; eds: S. Stefl, S. Owocki and A. Okazaki

We show that Be stars belong to a high velocity tail of a single B-type star rotational velocity distribution in the MS. This implies that: 1) the number fraction N(Be)/N(Be+B) is independent of the mass; 2) Bn stars having ZAMS rotational velocities higher than a given limit might become Be stars.

 

astro-ph/0601068 [abs, ps, pdf, other] :

Title: Differential rotation in early type stars
Authors: J. Zorec (IAP), Y. Frémat (ROB), A. Domiciano De Souza (LUAN)
Comments: 3 pages ; to appear in the proceedings of the Sapporo meeting on active OB stars ; ASP Conference Series ; eds: S. Stefl, S. Owocki and A. Okazaki

Using 2D models of rotating stars, the interferometric measurements of alpha Eri and its fundamental parameters corrected for gravitational darkening effects we infer that the star might have a core rotating 2.7 times faster than the surface. We explore the consequences on spectral lines produced by surface differential rotation combined with the effects due to a kind of internal differential rotation with rotational energies higher than allowed for rigid rotation which induce geometrical deformations that do not distinguish strongly from those carried by the rigid rotation.

 

astro-ph/0601069 [abs, ps, pdf, other] :

Title: Circumstellar rings, flat and flaring discs
Authors: M.L. Arias (UNLP), J. Zorec (IAP), Y. Frémat (ROB)
Comments: 3 pages ; to appear in the proceedings of the Sapporo meeting on active OB stars ; ASP Conference Series ; eds: S. Stefl, S. Owocki and A. Okazaki

Emission lines formed in the circumstellar envelopes of several type of stars can be modeled using first principles of line formation. We present simple ways of calculating line emission profiles formed in circumstellar envelopes having different geometrical configurations. The fit of the observed line profiles with the calculated ones may give first order estimates of the physical parameters characterizing the line formation regions: opacity, size, particle density distribution, velocity fields, excitation temperature.

 

astro-ph/0601070 [abs, ps, pdf, other] :

Title: Evolution of the circumstellar disc of alpha Eri
Authors: M.M.F. Vinicius (IAGCAUS), N.V. Leister (IAGCAUS), J. Zorec (IAP), R.S. Levenhagen (IAGCAUS)
Comments: 3 pages ; to appear in the proceedings of the Sapporo meeting on active OB stars ; ASP Conference Series ; eds: S. Stefl, S. Owocki and A. Okazaki

The Halpha line emission formation region in the circumstellar disc of alpha Eri is: a) extended with a steep outward matter density decline during low H$\alpha$ emission phases; b) less extended with rather constant density distribution during the strong Halpha emission. The long-term variation of the Halpha emission has a 14-15 year cyclic B-Be phase transition. The disc formation time scales agree with the viscous decretion model. The time required for the disc dissipation is longer than expected from the viscous disc model.

 

astro-ph/0601071 [abs, ps, pdf, other] :

Title: Metallicity vs. Be phenomenon relation in the solar neighborhood
Authors: R.S. Levenhagen (IAGCAUSP), N.V. Leister (IAGCAUSP), J. Zorec (IAP), Y. Frémat (ROB)
Comments: 3 pages ; to appear in the proceedings of the Sapporo meeting on active OB stars ; ASP Conference Series ; eds: S. Stefl, S. Owocki and A. Okazaki

Fast rotation seems to be the mayor factor to trigger the Be phenomenon. Surface fast rotation can be favored by initial formation conditions, such as abundance of metals. We have observed 118 Be stars up to the apparent magnitudes V=9 mag. Models of fast rotating atmospheres and evolutionary tracks were used to interpret the stellar spectra and to determine the stellar fundamental parameters. Since the studied stars are formed in regions that are separated enough to imply some non negligible gradient of galactic metallicity, we study the effects of possible incidence of this gradient on the nature as rotators of the studied stars.

 

astro-ph/0601072 [abs, ps, pdf, other] :

Title: Nonlinear Hydromagnetic Wave Support of a Stratified Molecular Cloud II: A Parameter Study
Authors: Takahiro Kudoh (1 and 2), Shantanu Basu (1) ((1) University of Western Ontario, (2) National Astronomical Observatory of Japan)
Comments: 35 pages, 12 figures, accepted for publication in ApJ, a preprint with fine figures at this http URL

We use numerical simulations to study the effect of nonlinear MHD waves in a stratified, self-gravitating molecular cloud that is bounded by a hot and tenuous external medium. In a previous paper, we had shown the details of a standard model and studied the effect of varying the dimensionless amplitude. In this paper, we present the results of varying two other important free parameters: beta_0, the initial ratio of gas to magnetic pressure at the cloud midplane, and the dimensionless frequency of driving. Furthermore, we present the case of a temporally random driving force. Our results demonstrate that a very important consideration for the actual level of turbulent support against gravity is the ratio of driving wavelength lambda_0 to the the size of the initial non-turbulent cloud; maximum cloud expansion is achieved when this ratio is close to unity. The best consistency with the observational correlation of magnetic field strength, turbulent line width, and density is achieved by cloud models with beta_0 approx 1. We also calculate the spatial power spectra of the turbulent clouds, and show that significant power is developed on scales larger than the scale length H_0 of the initial cloud, even if the input wavelength of turbulence lambda_0 approx H_0. The cloud stratification and resulting increase of Alfven speed toward the cloud edge allows for a transfer of energy to wavelengths significantly larger than lambda_0. This explains why the relevant time scale for turbulent dissipation is the crossing time over the cloud scale rather than the crossing time over the driving scale.

 

astro-ph/0601073 [abs, ps, pdf, other] :

Title: Cosmological models with Gurzadyan-Xue dark energy
Authors: G. V. Vereshchagin, G. Yegorian

The formula for dark energy density derived by Gurzadyan and Xue provides a remarkable fit not only to the SN data but also to other basic observations, unlike numerous dark energy scenarios where the corresponding value is postulated. We consider several cosmological models based on that formula and link them to observations using two key numbers: the first acoustic peak position in the CMB power spectrum and the age of the Universe. We show that two models with varying physical constants pass these cosmological tests, being close to concordance model: de Sitter type solution with $\Omega_m=0.3$ and $\Omega_\Lambda=0.7$, and a model with dark energy equation of state $p=-2\epsilon/3$ and varying speed of light.

 

astro-ph/0601074 [abs, ps, pdf, other] :

Title: The first 8-13 micron spectra of globular cluster red giants: circumstellar silicate dust grains in 47 Tucanae (NGC 104)
Authors: Jacco Th. van Loon (Keele University, UK), Iain McDonald (Keele), Joana M. Oliveira (Keele), A. Evans (Keele), Martha L. Boyer (University of Minnesota, USA), Robert D. Gehrz (Minnesota), Elisha Polomski (Minnesota), Charles E. Woodward (Minnesota)
Comments: Accepted for publication in Astronomy and Astrophysics, 5 pages

We present 8-13 micron spectra of eight red giants in the globular cluster 47 Tucanae (NGC 104), obtained at the European Southern Observatory 3.6m telescope. These are the first mid-infrared spectra of metal-poor, low-mass stars. The spectrum of at least one of these, namely the extremely red, large-amplitude variable V1, shows direct evidence of circumstellar grains made of amorphous silicate.

 

astro-ph/0601075 [abs, ps, pdf, other] :

Title: ASCA Observations of OAO 1657-415 and its Dust-Scattered X-Ray Halo
Authors: Michael D. Audley, Fumiaki Nagase, Kazuhisa Mitsuda, Lorella Angelini, Richard L. Kelley
Comments: Accepted for publication in MNRAS

We report on two ASCA observations of the high-mass X-ray binary pulsar OAO 1657-415. A short observation near mid-eclipse caught the source in a low-intensity state, with a weak continuum and iron emission dominated by the 6.4-keV fluorescent line. A later, longer observation found the source in a high-intensity state and covered the uneclipsed through mid-eclipse phases. In the high-intensity state, the non-eclipse spectrum has an absorbed continuum component due to scattering by material near the pulsar and 80 per cent of the fluorescent iron emission comes from less than 19 lt-sec away from the pulsar. We find a dust-scattered X-ray halo whose intensity decays through the eclipse. We use this halo to estimate the distance to the source as 7.1 +/- 1.3 kpc.

 

astro-ph/0601076 [abs, ps, pdf, other] :

Title: Mass and radius determination for the neutron star in X-ray burst source 4U/MXB 1728-34
Authors: A. Majczyna, J. Madej (Warsaw University Observatory, Warsaw, Poland)
Comments: Acta Astonomica in print

We analyzed archival X-ray spectra of MXB 1728-34 obtained in 1996-99 by the Proportional Counter Array on board of the RXTE satellite. X-ray spectra were fitted to our extensive grids of model atmosphere spectra to determine the effective temperature T_eff on the neutron star surface, logarithm of surface gravity log(g), and the gravitational redshift z simultaneously. We have chosen fitting by numerical model spectra plus broad Gaussian line, modified by interstellar absorption and the absorption on dust. We arbitrarily assumed either hydrogen-helium chemical composition of a model atmosphere, or H-He-Fe mixture in solar proportion. The statistically best values of log(g), and z subsequently were used to determine mass and radius of the neutron star. We obtained the best values of the parameters for the neutron star in X-ray burst source MXB 1728-34: mass either M=0.40 or 0.63 M_sol (for H-He or H-He-Fe models, respectively), radius R=4.6 or 5.3 km, log(g)=14.6 or 14.6 and the gravitational redshift z=0.14 or 0.22. All the above parameters have very wide 1-sigma confidence limits. Their values strongly support the equation of state for strange matter in MXB 1728-34.

 

astro-ph/0601077 [abs, ps, pdf, other] :

Title: Hubble Space Telescope NICMOS Polarization Measurements of OMC-1
Authors: Janet P. Simpson (1 and 2), Sean W. J. Colgan (1), Edwin F. Erickson (1), Michael G. Burton (3), A. S. B. Schultz (2) ((1) NASA Ames Research Center, (2) SETI Institute, (3) University of New South Wales)
Comments: 41 pages, 8 figures, 4 tables, to appear in The Astrophysical Journal

We present 2micron polarization measurements of positions in the BN region of the Orion Molecular Cloud (OMC-1) made with NICMOS Camera 2 (0.2'' resolution) on HST. Our results are as follows: BN is sim 29% polarized by dichroic absorption and appears to be the illuminating source for most of the nebulosity to its north and up to sim 5'' to its south. Although the stars are probably all polarized by dichroic absorption, there are a number of compact, but non-point-source, objects that could be polarized by a combination of both dichroic absorption and local scattering of star light. We identify several candidate YSOs, including an approximately edge-on bipolar YSO 8.7'' east of BN, and a deeply-embedded variable star. Additional strongly polarized sources are IRc2-B, IRc2-D, and IRc7, all of which are obviously self-luminous at mid-infrared wavelengths and may be YSOs. None of these is a reflection nebula illuminated by a star located near radio source I, as was previously suggested. Other IRc sources are clearly reflection nebulae: IRc3 appears to be illuminated by IRc2-B or a combination of the IRc2 sources, and IRc4 and IRc5 appear to be illuminated by an unseen star in the vicinity of radio source I, or by Star n or IRc2-A. Trends in the magnetic field direction are inferred from the polarization of the 26 stars that are bright enough to be seen as NICMOS point sources. The most polarized star has a polarization position angle different from its neighbors by sim 40^o, but in agreement with the grain alignment inferred from millimeter polarization measurements of the cold dust cloud in the southern part of OMC-1.

 

astro-ph/0601078 [abs, ps, pdf, other] :

Title: Ammonia in Infrared Dark Clouds
Authors: Thushara Pillai (1), Friedrich Wyrowski (1), Sean J. Carey (2), Karl M. Menten (1) ((1)Max-Planck-Institut fuer Radioastronomie, Bonn (2)Spitzer Science Center, California Institute of Technology)
Comments: 17 pages, 11 figures, in press in Astronomy & Astrophysics

Infrared Dark Clouds appear to be the long sought population of cold and dense aggregations with the potential of harbouring the earliest stages of massive star formation. Up to now there has been no systematic study on the temperature distribution, velocity fields, chemical and physical state toward this new cloud population. Knowing these properties is crucial for understanding the presence, absence and the very potential of star formation. The present paper aims at addressing these questions. We analyse temperature structures and velocity fields and gain information on their chemical evolution. The gas emission is remarkably coextensive with the extinction seen at infrared wavelengths and with the submillimeter dust emission. Our results show that IRDCs are on average cold (T < 20 K) and have variations among the different cores. IRDC cores are in virial equilibrium, are massive (M > 100 M_sun), highly turbulent (1 -- 3 km/s) and exhibit significant velocity structure (variations around 1 -- 2 km/s over the cloud). We find an increasing trend in temperature from IRDCs with high ammonia column density to high mass protostellar objects and hot core/Ultracompact Hii regions stages of early warm high-mass star formation while linewidths of IRDCs are smaller. On the basis of this sample, we infer that while active star formation is not yet pervasive in most IRDCs, local condensations might collapse in the future or have already begun forming stars.

 

astro-ph/0601079 [abs, ps, pdf, other] :

Title: The Most Metal-Rich Intervening Quasar Absorber Known
Authors: C. Peroux, V. P. Kulkarni, J. Meiring, R. Ferlet, P. Khare, J. T. Lauroesch, G. Vladilo, D. G. York
Comments: 5 pages, 1 figure. Accepted for publication in A&A

The metallicity in portions of high-redshift galaxies has been successfully measured thanks to the gas observed in absorption in the spectra of quasars, in the Damped Lyman-alpha systems (DLAs). Surprisingly, the global mean metallicity derived from DLAs is about 1/10th solar at 0<z<4 leading to the so-called ``missing-metals problem''. In this paper, we present high-resolution observations of a sub-DLA system at z_abs=0.716 with super-solar metallicity toward SDSS J1323-0021. This is the highest metallicity intervening quasar absorber currently known, and is only the second super-solar absorber known to date. We provide a detailed study of this unique object from VLT/UVES spectroscopy. We derive [Zn/H]=+0.61, [Fe/H]=-0.51, [Cr/H]=<-0.53, [Mn/H] = -0.37, and [Ti/H] = -0.61. Observations and photoionisation models using the CLOUDY software confirm that the gas in this sub-DLA is predominantly neutral and that the abundance pattern is probably significantly different from a Solar pattern. Fe/Zn and Ti/Zn vary among the main velocity components by factors of \~ 3 and ~ 35, respectively, indicating non-uniform dust depletion. Mn/Fe is super-solar in almost all components, and varies by a factor of ~ 3 among the dominant components. It would be interesting to observe more sub-DLA systems and determine whether they might contribute significantly toward the cosmic budget of metals.

 

astro-ph/0601080 [abs, ps, pdf, other] :

Title: A Radio Census of Nuclear Activity in Nearby Galaxies
Authors: M. E. Filho, P. D. Barthel, L. C. Ho
Comments: accpeted for publication in A&A

In order to determine the incidence of black hole accretion-driven nuclear activity in nearby galaxies, as manifested by their radio emission, we have carried out a high-resolution Multi-Element Radio-Linked Interferometer Network (MERLIN) survey of LINERs and composite LINER/H{\sc ii} galaxies from a complete magnitude-limited sample of bright nearby galaxies (Palomar sample) with unknown arcsecond-scale radio properties. There are fifteen radio detections, of which three are new subarcsecond-scale radio core detections, all being candidate AGN. The detected galaxies supplement the already known low-luminosity AGN -- low-luminosity Seyferts, LINERs and composite LINER/H{\sc ii} galaxies -- in the Palomar sample. Combining all radio-detected Seyferts, LINERs and composite LINER/H{\sc ii} galaxies (LTS sources), we obtain an overall radio detection rate of 54% (22% of all bright nearby galaxies) and we estimate that at least $\sim$50% ($\sim$20% of all bright nearby galaxies) are true AGN. The radio powers of the LTS galaxies allow the construction of a local radio luminosity function. By comparing the luminosity function with those of selected moderate-redshift AGN, selected from the 2dF/NVSS survey, we find that LTS sources naturally extend the RLF of powerful AGN down to powers of about 10 times that of Sgr A*.

 

astro-ph/0601081 [abs, ps, pdf, other] :

Title: The Evolution and Structure of Pulsar Wind Nebulae
Authors: Bryan M. Gaensler, Patrick O. Slane (Harvard-Smithsonian Center for Astrophysics)
Comments: 34 pages, including 7 color figures. To appear in volume 44 of Annual Review of Astronomy & Astrophysics (2006). Version with higher resolution images available at this http URL

Pulsars steadily dissipate their rotational energy via relativistic winds. Confinement of these outflows generates luminous pulsar wind nebulae, seen across the electromagnetic spectrum in synchrotron and inverse Compton emission, and in optical emission lines when they shock the surrounding medium. These sources act as important probes of relativistic shocks, particle acceleration and of interstellar gas. We review the many recent advances in the study of pulsar wind nebulae, with particular focus on the evolutionary stages through which these objects progress as they expand into their surroundings, and on morphological structures within these nebulae which directly trace the physical processes of particle acceleration and outflow. We conclude by considering some exciting new probes of pulsar wind nebulae, including the study of TeV gamma-ray emission from these sources, and observations of pulsar winds in close binary systems.

 

astro-ph/0601082 [abs, ps, pdf, other] :

Title: Desorption rates and sticking coefficients for CO and N2 interstellar ices
Authors: S.E. Bisschop, H.J. Fraser, K.I. Oberg, E.F. van Dishoeck, S. Schlemmer
Comments: 13 pages, 7 figures, plus appendix of 2 pages containing 2 figures, Accepted by A&A

We present Temperature Programmed Desorption (TPD) experiments of CO and N2 ices in pure, layered and mixed morphologies at various ice "thicknesses" and abundance ratios as well as simultaneously taken Reflection Absorption Infrared Spectra (RAIRS) of CO. A kinetic model has been developed to constrain the binding energies of CO and N2 in both pure and mixed environments and to derive the kinetics for desorption, mixing and segregation. For mixed ices N2 desorption occurs in a single step whereas for layered ices it proceeds in two steps, one corresponding to N2 desorption from a pure N2 ice environment and one corresponding to desorption from a mixed ice environment. The latter is dominant for astrophysically relevant ice "thicknesses". The ratio of the binding energies, Rbe, for pure N2 and CO is found to be 0.936 +/- 0.03, and to be close to 1 for mixed ice fractions. The model is applied to astrophysically relevant conditions for cold pre-stellar cores and for protostars which start to heat their surroundings. The importance of treating CO desorption with zeroth rather than first order kinetics is shown. The experiments also provide lower limits of 0.87 +/- 0.05 for the sticking probabilities of CO-CO, N2-CO and N2-N2 ices at 14 K. The combined results from the desorption experiments, the kinetic model, and the sticking probability data lead to the conclusion that these solid-state processes of CO and N2 are very similar under astrophysically relevant conditions. This conclusion affects the explanations for the observed anti-correlations of gaseous CO and N2H+ in pre-stellar and protostellar cores.

 

astro-ph/0601083 [abs, ps, pdf, other] :

Title: The Initial Mass Functions of Four Embedded Stellar Clusters
Authors: A. Leistra, A. Cotera, J. Liebert
Comments: 21 pages, 16 figures, accepted to AJ

We present near-infrared $J$, $H$, and $K$ images of four embedded stellar clusters in the Galaxy. We find a significant fraction of pre-main-sequence stars present in at least one of the clusters. For the clusters dominated by main-sequence stars, we determine the initial mass function (IMF) both by using the $K$ luminosity function and a global extinction correction and by deriving individual extinction corrections for each star based on their placement in the $K$ vs. $H-K$ color-magnitude diagram. Based on our IMFs we find a significant discrepancy between the mean IMF derived via the different methods, suggesting that taking individual extinctions into account is necessary to correctly derive the IMF for an embedded cluster.

 

astro-ph/0601084 [abs, ps, pdf, other] :

Title: Stephan's Quintet: A Multi-galaxy Collision
Authors: C. Kevin Xu
Comments: Comments: 8 pages, 1 figure, invited review, in "Extreme Starburst: Near and Far", Aug. 15-19, 2005, Lijiang, China

Stephan's Quintet (SQ), discovered more than 100 years ago, is the most famous and well studied compact galaxy group. It has been observed in almost all wavebands, with the most advanced instruments including Spitzer, GALEX, HST, Chandra, VLA, and various large mm/submm telescopes/arrays such as the IRAM 30m and BIMA. The rich multi-band data reveal one of the most fascinating pictures in the universe, depicting a very complex web of interactions between member galaxies and various constituents of the intragroup medium (IGM), which in turn trigger some spectacular activities such as a 40 kpc large scale shock and a strong IGM starburst. In this talk I will give a review on these observations.

 

astro-ph/0601085 [abs, ps, pdf, other] :

Title: Photometric observations of Southern Abell Cluster Redshifts Survey Clusters: Structure of galaxies in the inner region of clusters of galaxies
Authors: V. Coenda, H. Muriel, C. J. Donzelli, H. Quintana, L. Infante, D. G. Lambas
Comments: To appear in the AJ

We analyze photometric properties of 1384 cluster galaxies as a function of the normalized distance to cluster center. These galaxies were selected in the central region ($r/r_{200} \leq$ 0.8) of 14 southern Abell clusters chosen from the Southern Abell Cluster Redshifts Survey (SARS). For 507 of these galaxies we also obtained their luminosity profiles. We have studied the morphology-clustercentric distance relation on the basis of the shape parameter $n$ of the S\'ersic's law. We also have analyzed the presence of a possible segregation in magnitude for both, the galaxy total luminosity and that of their components (i.e. the bulge and the disk).
Results show a marginal ($2\sigma$ level) decrease of the total luminosity as a function of normalized radius. However, when bulges are analyzed separately, a significant luminosity segregation is found ($3\sigma$ and $2\sigma$ for galaxies in projection and member galaxies respectively). The fraction of bulges brighter than $M_B \leq -22$ is three times larger in the core of clusters than in the outer region. Our analysis of the disk component suggests that disks are, on average, less luminous in the cluster core than at $r/r_{200} \sim 0.8$. In addition, we found that the magnitude-size relation as a function of $r/r_{200}$ indicates (at $2\sigma$ level) that disks are smaller and centrally brighter in the core of clusters. However, the Kormendy relation (the bulge magnitude-size relation) appears to be independent of environment.

 

astro-ph/0601086 [abs, ps, pdf, other] :

Title: Bright Side versus Dark Side of Star Formation -- UV and IR Views
Authors: C. Kevin Xu
Comments: 11 pages, invited review, in "When UV meets IR: a history of star formation", Moriond Meeting, March 7-11, 2005, La Thuilae, Italy

This is a review talk on the UV and infrared selected galaxies. The central question addressed is: do UV and infrared surveys see the 2 sides of star formation of the same population, or star formation of 2 different populations? We first review the literature on the UV and IR selected galaxy samples, try to quantify the difference and overlaps between these two populations of star forming galaxies. We then present some preliminary results of a GALEX/SWIRE comparison study for IR and UV selected galaxies at z=0.6, in an attempt to constrain the evolution of the dust attenuation and of stellar mass of these galaxies.