1,722,171 research outputs found
AA manuscript no.
No full textWe present a long (150 ks elapsed time) X-ray observation of the dM3.5e star EV Lac, performed with the ASCA observatory in July 1998, during which an exceptionally intense flaring event (lasting approximately 12 ks) was observed; at the flare's peak, the X-ray count rate in the ASCA GIS detectors was ' 300 times the quiescent value. The physical parameters of the flaring region have been derived by analyzing the decay, using both a "classic" quasi-static approach and an approach based on hydrodynamic simulations of decaying flaring loops. Notwithstanding the large peak X-ray luminosity, this second method shows that the flare's decay is compatible with its being produced in a relatively compact region of semilength L ' 1:3 10 cm (' 0:5 R ), large but not exceptional even by solar standards. The flare decays is fast (with a measured e-folding time for the light curve of 2 ks), but nevertheless the hydrodynamic-based analysis shows strong evidence for sustained heating, with the shape of the light curve dominated by the time evolution of the heating rather than by the natural cooling of the flaring plasma. As a consequence, the quasi-static method yields a much larger estimate of the loop's length (L ' 2 R ). The event shows (similarly to some other well-studied large stellar flares) a light curve characterized by two separate decay time constants (with the initial decay being faster) and a significant enhancement in the plasma metal abundance at the peak of the flare. The energetics of the event are exceptional, with the peak X-ray luminosity of the event reaching up to ' 25% of the bolometric luminosity of the star, making this the largest X-ray flare (in relative terms) observed to date on a main-sequence star
AA manuscript no.
No full textROSAT X-ray (PSPC) and EUV (WFC) observations of a sample of 12 nearby Halo and Old-Disk low mass stars have been analyzed to determine their emission levels, properties of their coronal spectra and to characterize their temporal variations on time scales from hours to ten years. The light curves of the old-disk stars of our sample with the highest count statistics, show that variability of a factor two on a time scale of a few thousands of seconds is a common property of old-disk stars. The coronal emission of GJ 191, the only halo star with enough counts to make the time analysis feasible, shows significant variations on a time scale of six months
Astronomy Astrophysics manuscript no.
No full textWe have mapped the large-scale structure of the Serpens cloud core using moderately optically thick ( CO(1-0) and CS(2-1)) and optically thin tracers (C S(2-1), and N2H (1-0)), using the 16-element focal plane array operating at a wavelength of 3mm at the Five College Radio Astronomy Observatory. Our main goal was to study the large-scale distribution of the molecular gas in the Serpens region and to understand its relation with the denser gas in the cloud cores, previously studied at high angular resolution. All our molecular tracers show two main gas condensations, or sub-clumps, roughly corresponding to the North-West and SouthEast clusters of submillimeter continuum sources. We also carried out a kinematical study of the Serpens cloud
A new electron-methanol molecule pseudopotential and its application for the solvated electron in methanol
Full text linkA new electron–methanol molecule pseudopotential is developed and tested in the present paper. The formal development of the potential is based on quantum mechanical calculations on the electron-methanol molecule model in the static exchange approximation. The computational model includes a steep confining potential that keeps the otherwise unbound excess electron in the vicinity of the methanol molecule. Using the Phillips-Kleinman theorem we introduce a smooth pseudo-wave function of the excess electron with the exact eigenenergy and correct asymptotic behavior. The non-local potential energy operator of the model Hamiltonian is then replaced to a local potential that reproduces the ground-state properties of the excess electron satisfactorily. The pseudopotential is then optimized in an analytically simple functional form to fit this approximate local potential in conjunction with the point charges and the geometry of a classical, all-site methanol-methanol interaction potential. Of the adjustable parameters, the parameters for the carbon and the methyl hydrogen atoms are optimized, while those for the oxygen and the hydroxyl hydrogen are taken from a previous electron-water molecule pseudopotential. A polarization term is added to the potential a posteriori. The polarization parameters are chosen to reproduce the experimental position of the optical absorption spectrum of an excess electron in mixed quantum-classical molecular dynamics simulations. The energetic, structural and spectroscopic properties of the solvated electron in a methanol bath are simulated at 300 K, and compared to previous solvated electron simulations and available experimental data
AA manuscript no.
No full textNGC 3532 is a very rich southern open cluster of age 200 \Gamma 350 Myr; it is therefore a good candidate to investigate the X-ray activity--age--rotation relationship at ages intermediate between the Pleiades and the Hyades, where, to our knowledge, X-rays studies exist for only one cluster (NGC 6475). We have performed an X-ray study of NGC 3532 using HRI observations retrieved from the ROSAT archive. The observations have a limiting sensitivity L x 4 \Theta 10 in the center of the field. We detected 50 X-ray sources above a 4oe threshold, half of which have a known optical counterpart within 10 arcsec; 15 of the X-ray sources have at least one cluster member as optical counterpart
Astronomy Astrophysics manuscript no.
No full textA finite element method for solving the resonance line transfer problem in moving media is presented. The algorithm works in three spatial dimensions on unstructured grids which are adaptively refined by means of an a posteriori error indicator
manuscripta mathematica manuscript No.
No full textLet Hn (z) be the function of a complex variable z defined by Hn (z) = G(ia1 ian )e iz(b 1 possible plus and minus sign combinations, the same sign combination being used in both the argument of G and in the exponent. The numbers a1 , a2 , a3 , . . . and b1 , b2 , b3 , . . . are assumed to be positive, and G is an entire function of genus 0 or 1 that is real on the real axis and has only real zeros. Then the exponential sum Hn (z) has only real zeros. 1
AA manuscript no.
No full textIt was recently shown (Masciadri et al. 2000, Masciadri 2001a), using a numerical simulation (Meso-Nh model), that the hypothesis of considering horizontally uniform C N can have severe consequences. For example, the integration of the C N along lines of sight different from the zenith gives variations that can be larger than 0.50 . In this paper we try to validate this result following two approaches. (a) numerical approach: we compare measured and simulated C N profiles. The first ones were obtained with a Generalized Scidar (GS) and the latter are simulated with the Meso-Nh model at the same azimuth and elevation as those of the observations. Further measurements (DIMM, mast and balloons) are considered in order to support the comparison. (b) experimental approach: we compare C N profiles measured (almost simultaneously) by a GS along different lines of sight. The results of this study show that (1) the horizontal size of the turbulent layers can be finite, (2) the simulations and the measurements are well correlated and (3) for the first time, we show that the model can reproduce observed seeing values that vary as much as 0.50 during the same night. This definitely shows that the numerical simulations are a useful tool in the context of the turbulence characterization for astronomical applications. Finally, we discuss the implications that a finite horizontal size of the turbulent layers could have on the new adaptive optics techniques, particularly in applications to the extremely large size telescopes
Astronomy Astrophysics manuscript no.
No full textWe discuss the spectral energy distribution of three very low mass objects in Chamaeleon I for which ground-based spectroscopy and photometry as well as ISO measurements in the mid-infrared are available (Comer'on et al. 2000; Persi et al. 2000). One of these stars (Cha Hff1) is a bona-fide brown dwarf, with mass 0.04--0.05 M fi . We show that the observed emission is very well described by models of circumstellar disks identical to those associated to T Tauri stars, scaled down to keep the ratio of the disk-to-star mass constant and to the appropriate stellar parameters. This result provides a first indication that the formation mechanism of T Tauri stars (via core contraction and formation of an accretion disk) extends to objects in the brown dwarf mass range
AA manuscript no.
No full textLandstreet & Mathys (2000) have discovered that almost all the magnetic Ap stars having rotation periods longer than about one month have their magnetic and rotation axes fairly closely aligned, in contrast to the more common magnetic Ap stars of shorter period, in which the two axes are usually inclined to one another at a large angle. Furthermore, as shown earlier by Mathys et al. (1997) and by Hubrig et al. (2000), these most slowly rotating magnetic Ap stars have also magnetic fields several times larger than are typical for shorter period magnetic Ap stars, and generally have masses below 3 M
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