244 research outputs found
The role of non-gray model atmospheres in the evolution of low mass metal poor stars
Gray model atmospheres are generally considered a reasonable approximation to make upon stars of mass greater than about 0.6 M-circle dot. Here we show that non-gray atmospheres can significantly affect evolutionary models, with masses up to 0.9 M-circle dot. The effect of including a non-gray atmosphere is strongest in the pre-main and post-main Sequence. This may have implications for the ages of the oldest globular clusters
The Effect of the Electron Donor H⁺₃ on the Pre-Main-Sequence and Main-Sequence Evolution of Low-Mass, Zero-Metallicity Stars
H₃⁺ has been shown (1991 work of Lenzuni and coworkers) to be the dominant positive ion in a zero-metallicity gas at low temperature and intermediate to high density. It therefore affects both the number of free electrons and the opacity of the gas. The most recent H₃⁺ partition function (1995 work of Neale & Tennyson) is an order of magnitude larger at 4000 K than all previous partition functions, implying that H₃⁺ is a more important electron donor than previously thought. Here we present new Rosseland mean opacities for a hydrogen-helium gas of 1000 K ≤ T ≤ 9000 K and -14 ≤ log₁₀ [ρ (g cm⁻³)] ≤ -2. In the calculation of these opacities, we have made use of the latest collision-induced absorption data as well as the most recent H₃⁺ partition function and line opacity data. It is shown that these updated and new sources of opacity give rise to a Rosseland mean opacity for a hydrogen-helium gas that is, in general, greater than that calculated in earlier works. The new opacity data are then used to model the evolution of low-mass (0.15-0.8 M_{☉}), zero-metallicity stars, from pre-main-sequence collapse to main-sequence turnoff. To investigate the effect of H₃⁺ on the evolution of low-mass, zero-metallicity stars, we repeat our calculations neglecting H₃⁺ as a source of electrons and line opacity. We find that H₃⁺ can have an effect on the structure and evolution of stars of mass ~0.5 M_{☉} or less. A gray atmosphere is used for the calculation, which is sufficient to demonstrate that H₃⁺ affects the evolution of very low mass stars to a greater degree than previously believed
A variationally computed line list for hot NH3
We present 'BYTe', a comprehensive 'hot' line list for the ro-vibrational
transitions of ammonia, 14NH3, in its ground electronic state. This line list
has been computed variationally using the program suite TROVE, a new
spectroscopically-determined potential energy surface and an ab initio dipole
moment surface. BYTe, is designed to be used at all temperatures up to 1500K.
It comprises 1137650964 transitions in the frequency range from 0 to 12000
cm-1, constructed from 1366519 energy levels below 18000 cm-1 having J values
below 36. Comparisons with laboratory data confirm the accuracy of the line
list which is suitable for modelling a variety of astrophysical problems
including the atmospheres of extrasolar planets and brown dwarfs.Comment: the paper has been submitted to MNRA
Investigating the properties of granulation in the red giants observed by Kepler
More than 1000 red giants have been observed by NASA/Kepler mission during a
nearly continuous period of ~ 13 months. The resulting high-frequency
resolution (< 0.03 muHz) allows us to study the granulation parameters of these
stars. The granulation pattern results from the convection motions leading to
upward flows of hot plasma and downward flows of cooler plasma. We fitted
Harvey-like functions to the power spectra, to retrieve the timescale and
amplitude of granulation. We show that there is an anti-correlation between
both of these parameters and the position of maximum power of acoustic modes,
while we also find a correlation with the radius, which agrees with the theory.
We finally compare our results with 3D models of the convection.Comment: 4 pages, 1 figure. To appear in the ASP proceedings of "The 61st
Fujihara seminar: Progress in solar/stellar physics with helio- and
asteroseismology", 13th-17th March 2011, Hakone, Japa
Subaru and Swift observations of V652 Herculis: resolving the photospheric pulsation
High-resolution spectroscopy with the Subaru High Dispersion Spectrograph, and Swift ultraviolet photometry are presented for the pulsating extreme helium star V652 Her. Swift provides the best relative ultraviolet photometry obtained to date, but shows no direct evidence for a shock at ultraviolet or X-ray wavelengths. Subaru has provided high spectral and high temporal resolution spectroscopy over six pulsation cycles (and eight radius minima).
These data have enabled a line-by-line analysis of the entire pulsation cycle and provided a description of the pulsating photosphere as a function of optical depth. They show that the photosphere is compressed radially by a factor of at least 2 at minimum radius, that the phase of radius minimum is a function of optical depth and the pulse speed through the photosphere is between 141 and 239 km s−1 (depending how measured) and at least 10 times the local sound speed. The strong acceleration at minimum radius is demonstrated in individual line profiles; those formed deepest in the photosphere show a jump discontinuity of over 70 kms−1 on a time-scale of 150 s. The pulse speed and line profile jumps imply a shock is present at minimum radius. These empirical results provide input for hydrodynamical modelling of the pulsation and hydrodynamical plus radiative transfer modelling of the dynamical spectra
Detection of Photometric Variations in the sdBV Star JL 166
We report the discovery of oscillations in the hot subdwarf B star JL 166
from time-series photometry using the Goodman Spectrograph on the 4.1-m
Southern Astrophysical Research Telescope. Previous spectroscopic and
photometric observations place the star near the hot end of the empirical sdB
instability strip and imply the presence of a cool companion. Amplitude spectra
of the stellar light curve reveal at least 10 independent pulsation modes with
periods ranging from 97 to 178 s and amplitudes from 0.9 to 4 mma. We adopt
atmospheric parameters of T_eff = 34350 K and log g = 5.75 from a model
atmosphere analysis of our time-averaged, medium-resolution spectrum.Comment: 18 pages, 6 figures, accepted for publication in the Astronomical
Journa
A high accuracy computed water line list
A computed list of HO infra-red transition frequencies and
intensities is presented. The list, BT2, was produced using a discrete variable
representation two-step approach for solving the rotation-vibration nuclear
motions. It is the most complete water line list in existence, comprising over
500 million transitions (65% more than any other list) and it is also the most
accurate (over 90% of all known experimental energy levels are within 0.3
cm of the BT2 values). Its accuracy has been confirmed by extensive
testing against astronomical and laboratory data.
The line list has been used to identify individual water lines in a variety
of objects including: comets, sunspots, a brown dwarf and the nova-like object
V838 Mon. Comparison of the observed intensities with those generated by BT2
enables physical values to be derived for these objects. The line list can also
be used to provide an opacity for models of the atmospheres of M-dwarf stars
and assign previously unknown water lines in laboratory spectra.Comment: 8 Pages, zero figures. Submitted to MNRAS. On line data at:
ftp://cdsarc.u-strasbg.fr/cats/VI/11
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