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| Astrophysics : How to understand the light curves of symbiotic stars |
| arXiv:0805.1222 (May 2008) | | How to understand the light curves of symbiotic stars | | Augustin Skopal | | Received. 08 May 2008 Last updated. 08 May 2008 | | Abstract. I introduce fundamental types of variations observed in the light curves (LC) of symbiotic stars: the orbitally-related wave-like modulation during quiescent phases, eclipses during active phases and apparent orbital changes indicated during transitions between quiescence and activity. I explain their nature with the aid of the spectral energy distribution (SED) of the composite spectrum of symbiotic stars and their simple ionization model. | | Categories. astro-ph | | Comment. 17 pages, 6 figures, J. Amer. Assoc. Var. Star Obs., in press License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/ |
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| Astrophysics : GLAST and Lorentz violation |
| arXiv:0805.1219 (May 2008) | | GLAST and Lorentz violation | | Raphael Lamon | Institut für Theoretische Physik, Universität Ulm Albert-Einstein-Allee 11 D-89069 Ulm, Germany
| | Received. 08 May 2008 Last updated. 08 May 2008 | | Abstract. We study possible Lorentz violations by means of gamma-ray bursts (GRB) with special focus on the Large Array Telescope (LAT) of GLAST. We simulate bursts with gtobssim and introduce a Lorentz violating term in the arrival times of the photons. We further perturb these arrival times and energies with a Gaussian distribution corresponding to the time resp. energy resolution of GLAST. We then vary the photon flux in gtobssim in order to derive a relation between the photon number and the standard deviation of the Lorentz violating term. We conclude with the fact that our maximum likelihood method as first developed in [1] is able to make a statement whether Nature breaks the Lorentz symmetry if the number of bursts with known redshifts is of the order of 100. | | Categories. astro-ph | | Comment. 11 pages, 7 figures and 2 tables License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/ |
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| Astrophysics : The Metal Content of Bulge Field Stars from FLAMES-GIRAFFE Spectra. I. Stellar parameters and Iron Abundances |
| arXiv:0805.1218 (May 2008) | | The Metal Content of Bulge Field Stars from FLAMES-GIRAFFE Spectra. I. Stellar parameters and Iron Abundances | | M. Zoccali, V. Hill, A. Lecureur, B. Barbuy, A. Renzini, D. Minniti, A. Gomez and S. Ortolani | P. Universidad Católica de Chile, Departamento de Astronom'i a y Astrof'i sica, Casilla 306, Santiago 22, Chile, and GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen 92190 Meudon, France, email{Vanessa.Hill@obspm.fr, Aurelie.Lecureur@obspm.fr, Ana.Gomez@obspm.fr
| | Received. 08 May 2008 Last updated. 08 May 2008 | | Abstract. We determine the iron distribution function (IDF) for bulge field stars, in three different fields along the Galactic minor axis and at latitudes b=-4 deg, b=-6 deg, and b=-12 deg. A fourth field including NGC6553 is also included in the discussion. About 800 bulge field K giants were observed with the GIRAFFE spectrograph of FLAMES@VLT at spectral resolution R~20,000. Several of them were observed again with UVES at R~45,000 to insure the accuracy of the measurements. The LTE abundance analysis yielded stellar parameters and iron abundances that allowed us to construct an IDF for the bulge that, for the first time, is based on high-resolution spectroscopy for each individual star. The IDF derived here is centered on solar metallicity, and extends from [Fe/H]~ -1.5 to [Fe/H]~ +0.5. The distribution is asymmetric, with a sharper cutoff on the high-metallicity side, and it is narrower than previously measured. A variation in the mean metallicity along the bulge minor axis is clearly between b=-4 deg and b=-6 deg ([Fe/H] decreasing by ~ 0.6 dex per kpc). The field at b=-12 deg is consistent with the presence of a gradient, but its quantification is complicated by the higher disk/bulge fraction in this field. Our findings support a scenario in which both infall and outflow were important during the bulge formation, and then suggest the presence of a radial gradient, which poses some challenges to the scenario in which the bulge would result solely from the vertical heating of the bar. | | Categories. astro-ph | | Comment. A&A in press License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/ |
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| Astrophysics : The Arecibo Arp 220 Spectral Census I: Discovery of the Pre-Biotic Molecule Methanimine and New Cm-wavelength Transitions of Other Molecules |
| arXiv:0805.1205 (May 2008) | | The Arecibo Arp 220 Spectral Census I: Discovery of the Pre-Biotic Molecule Methanimine and New Cm-wavelength Transitions of Other Molecules | | C. J. Salter, T. Ghosh, B. Catinella, M. Lebron, M. S. Lerner, R. Minchin and E. Momjian | Arecibo Observatory, HC 03 Box 53995, Arecibo, PR 00612
| | Received. 08 May 2008 Last updated. 08 May 2008 | | Abstract. An on-going Arecibo line search between 1.1 and 10 GHz of the prototypical starburst/megamaser galaxy, Arp 220, has revealed a spectrum rich in molecular transitions. These include the "pre-biotic" molecules: methanimine (CH$_{2}$NH) in emission, three $v_{2}=1$ direct l-type absorption lines of HCN, and an absorption feature likely to be from either $^{18}$OH or formic acid (HCOOH). In addition, we report the detection of two, possibly three, transitions of $lambda$4-cm excited OH not previously detected in Arp~220 which are seen in absorption, and a possible absorption feature from the 6.668-GHz line of methanol. This marks the first distant extragalactic detection of methanimine, a pre-biotic molecule. Also, if confirmed, the possible methanol absorption line presented here would represent the first extragalactic detection of methanol at a distance further than 10 Mpc. In addition, the strong, previously undetected, cm-wave HCN $v_{2}=1$ direct l-type lines will aid the study of dense molecular gas and active star-forming regions in this starburst galaxy. | | Categories. astro-ph | | Comment. 25 pages, 10 figures. Accepted for publication in AJ License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/ |
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| Astrophysics : The Transparency of Galaxy Clusters |
| arXiv:0805.1200 (May 2008) | | The Transparency of Galaxy Clusters | | Jo Bovy, David W. Hogg and John Moustakas | | Received. 08 May 2008 Last updated. 08 May 2008 | | Abstract. If galaxy clusters contain intracluster dust, the spectra of galaxies lying behind clusters should show attenuation by dust absorption. We compare the optical (3500 - 7200 Å) spectra of 60,267 luminous, early-type galaxies selected from the Sloan Digital Sky Survey to search for the signatures of intracluster dust. We select massive, quiescent (i.e., non-star-forming) galaxies using an EW(H$alpha$) $leq 2$ Å cut and consider galaxies in three bins of velocity dispersion, ranging from 150 to 300 km s$^{-1}$. The uniformity of early-type galaxy spectra in the optical allows us to construct luminosity-weighted composite spectra with high signal-to-noise ratio (ranging from $10^2-10^4$). We compare the composite spectra of galaxies that lie behind and adjacent to galaxy clusters and find no convincing evidence of dust attenuation on scales $sim 0.15-2$ Mpc; we derive a generic limit of $E(B-V) esssim 5 imes 10^{-3}$ mag on scales $sim 1-2$ Mpc using conservative jackknife error bars, corresponding to a dust mass $lesssim 10^8$ $M_{ødot}$. On scales smaller than 1 Mpc this limit is slightly weaker, e.g. E(B-V) $lesssim 10^{-2}$ mag. | | Categories. astro-ph | | Comment. 23 pages, 6 figures. Submitted to ApJ License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/ |
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| Astrophysics : Antares: Towards a Large Underwater Neutrino Experiment |
| arXiv:0805.1191 (May 2008) | | Antares: Towards a Large Underwater Neutrino Experiment | | M. Spurio and the ANTARES Collaboration | | Received. 08 May 2008 Last updated. 08 May 2008 | | Abstract. After a long R&D phase to validate its detector concept, the ANTARES (Astronomy with a Neutrino Telescope and Abyss environmental RESearch) collaboration is operating the largest neutrino telescope in the Northern hemisphere, which is close to completion. It is located in the Mediterranean Sea, offshore from Toulon in France at a depth of 2500 m of water which provide a shield from cosmic rays. The detector design is based on the reconstruction of events produced by neutrino interactions. The expected angular resolution for high energy muon neutrinos (E>10 TeV) is less than 0.3 deg. To achieve this good angular resolution, severe requirements on the time resolution of the detected photons and on the determination of the relative position of the detection devices must be reached. The full 12-line detector is planned to be fully operational during this year. At present (April 2008) there are 10 lines taking data plus an instrumented line deployed at the edge of the detector to monitor environmental sea parameters. This paper describes the design of the detector as well as some results obtained during the 2007 5-line run (from March to December). | | Categories. astro-ph | | Comment. Contribution to the Rencontres de Physique, La Thuile, 24/2-1/3 2008. 16 pages and 8 figures License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/ |
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| Astrophysics : L dwarfs in the Hyades |
| arXiv:0805.1189 (May 2008) | | L dwarfs in the Hyades | | E. Hogan, R. F. Jameson, S. L. Casewell, S. L. Osbourne and N. C. Hambly | | Received. 08 May 2008 Last updated. 08 May 2008 | | Abstract. We present the results of a proper motion survey of the Hyades to search for brown dwarfs, based on UKIDSS and 2MASS data. This survey covers ~275 square degrees to depth of K~15 mag, equivalent to a mass of 0.05 solar masses assuming a cluster age of 625 Myr. The discovery of 12 L dwarf Hyades members is reported. These members are also brown dwarfs, with masses between 0.05 < M < 0.075 solar masses. A high proportion of these L dwarfs appear to be photometric binaries. | | Categories. astro-ph | | Comment. 7 pages, 4 figures, 2 tables. Accepted for publication in MNRAS License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/ |
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| Astrophysics : Early spectroscopic identification of SN 2008D |
| arXiv:0805.1188 (May 2008) | | Early spectroscopic identification of SN 2008D | | D. Malesani, J. P. U. Fynbo, J. Hjorth, G. Leloudas, J. Sollerman, M. D. Stritzinger, P. M. Vreeswijk, D. J. Watson, J. Gorosabel, M. J. Micha{l}owski, C. C. Thöne, T. Augusteijn, D. Bersier, P. Jakobsson, A. O. Jaunsen, C. Ledoux, A. J. Levan, B. Milvang-Jensen, E. Rol, N. R. Tanvir, K. Wiersema, D. Xu, L. Albert, M. Bayliss, C. Gall, L. F. Grove, B. P. Koester, E. Leitet, T. Pursimo and I. Skillen | | Received. 08 May 2008 Last updated. 08 May 2008 | | Abstract. SN 2008D was discovered while following up a luminous, unusual X-ray transient in the nearby spiral galaxy NGC 2770. We present early spectra of the optical counterpart (1.77 days after the X-ray transient) which first allowed to identify the object as a supernova at redshift z = 0.007. These spectra were acquired during the initial declining phase of the light curve, likely due to the shock breakout. They show a relatively flat continuum with broad undulations, and a strong, W-shaped feature with minima at 3980 and 4190 AA (rest frame). We also present extensive spectroscopic and photometric monitoring of the supernova covering the photospheric phase. Unlike supernovae associated with gamma-ray bursts, SN 2008D displayed prominent He features and is therefore of Type Ib. | | Categories. astro-ph | | Comment. 6 pages, 5 figures, 4 tables; submitted to ApJ Letters License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/ |
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| Astrophysics : Accretion-driven core collapse and the collisional formation of massive stars |
| arXiv:0805.1176 (May 2008) | | Accretion-driven core collapse and the collisional formation of massive stars | | C. Clarke I. Bonnell | | Received. 08 May 2008 Last updated. 08 May 2008 | | Abstract. We consider the conditions required for a cluster core to shrink, by adiabatic accretion of gas from the surrounding cluster, to densities such that stellar collisions are a likely outcome. We show that the maximum densities attained, and hence the viability of collisions, depends on a competition between core shrinkage (driven by accretion) and core puffing up (driven by relaxation effects). The expected number of collisions scales as $N_{core}^{5/3} ilde v^2$ where $N_{core}$ is the number of stars in the cluster core and $ ilde v$ is the free fall velocity of the parent cluster (gas reservoir). Thus whereas collisions are very unlikely in a relatively low mass, low internal velocity system such as the Orion Nebula Cluster, they become considerably more important at the mass and velocity scale characteristic of globular clusters. Thus stellar collisions in response to accretion induced core shrinkage remains a viable prospect in more massive clusters, and may contribute to the production of intermediate mass black holes in these systems. | | Categories. astro-ph | | Comment. MNRAS accepted License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/ |
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| Astrophysics : Aspect ratio dependence in magnetorotational instability shearing box simulations |
| arXiv:0805.1172 (May 2008) | | Aspect ratio dependence in magnetorotational instability shearing box simulations | | G. Bodo, A. Mignone, F. Cattaneo, P. Rossi and A. Ferrari | INAF, Osservatorio Astronomico di Torino, Strada Osservatorio 20, Pino Torinese, Italy and Dipartimento di Fisica Generale, Universitá di Torino, Via Pietro Giuria 1, 10125 Torino, Italy and Department of Astronomy and Astrophysics, The University of Chicago, 5640 S. Ellis ave., Chicago IL 60637, USA
| | Received. 08 May 2008 Last updated. 08 May 2008 | | Abstract. Aims: We study the changes in the properties of turbulence driven by the magnetorotational instability in a shearing box, as the computational domain size in the radial direction is varied relative to the height Methods: We perform 3D simulations in the shearing box approximation, with a net magnetic flux, and we consider computational domains with different aspect ratios Results: We find that in boxes of aspect ratio unity the transport of angular momentum is strongly intermittent and dominated by channel solutions in agreement with previous work. In contrast, in boxes with larger aspect ratio, the channel solutions and the associated intermittent behavior disappear. Conclusions: There is strong evidence that, as the aspect ratio becomes larger, the characteristics of the solution become aspect ratio independent. We conclude that shearing box calculations with aspect ratio unity or near unity may introduce spurious effects. | | Categories. astro-ph | | Comment. 5 pages, 6 figures, Astronomy and Astrophysics accepted License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/ |
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