Hydrogen model atmospheres for white dwarf stars

We present a detailed calculation of model atmospheres for DA white dwarfs. Our atmosphere code solves the atmosphere structure in local thermodynamic equilibrium with a standard partial linearization technique, which takes into account the energy transfer by radiation and convection. This code incorporates recent improved and extended data base of collision induced absorption by molecular hydrogen. We analyse the thermodynamic structure and emergent flux of atmospheres in a range 2500 < Teff < 60000$ K and 6.5 < log g < 9.0. Bolometric correction and colour indices are provided for a subsample of the model grid. Comparison of the colours is made with published observational material and results of other recent model calculations. Motivated by the increasing interest on helium core white dwarfs, we analyse the photometric characteristics of these stars during their cooling, using evolutionary models recently available. Effective temperatures, surface gravities, masses and ages have been determined for some helium core white dwarf candidates, and their possible binary nature is briefly discussed.Comment: 12 pages, 13 figures. Accepted for publication in MNRA

The Free-Free Opacity in Warm, Dense, and Weakly Ionized Helium

We investigate the ionization and the opacity of warm, dense helium under conditions found in the atmospheres of cool white dwarf stars. Our particular interest is in densities up to $\rm 3 g/cm^{3}$ and temperatures from 1000K to 10000K. For these physical conditions various approaches for modeling the ionization equilibrium predict ionization fractions that differ by orders of magnitudes. Furthermore, estimates of the density at which helium pressure-ionizes vary from $\rm 0.3$ to $\rm 14 g/cm^{3}$. In this context, the value of the electron-atom inverse bremsstrahlung absorption is highly uncertain. We present new results obtained from a non-ideal chemical model for the ionization equilibrium, from Quantum Molecular Dynamics (QMD) simulations, and from the analysis of experimental data to better understand the ionization fraction in fluid helium in the weak ionization limit.Comment: 4 pages, 3 figures, 1 table. Accepted for publication in the Proceedings of the 14th APS Topical Conference on Shock Compression of Condensed Matter, Baltimore, M

Spectroscopic Detection of Carbon Monoxide in Two Late-type T Dwarfs

M band spectra of two late-type T dwarfs, 2MASS J09373487+2931409, and Gliese 570D, confirm evidence from photometry that photospheric CO is present at abundance levels far in excess of those predicted from chemical equilibrium. These new and unambiguous detections of CO, together with an earlier spectroscopic detection of CO in Gliese 229B and existing M band photometry of a large selection of T dwarfs, suggest that vertical mixing in the photosphere drives the CO abundance out of chemical equilibrium and is a common, and likely universal feature of mid-to-late type T dwarfs. The M band spectra allow determinations of the time scale of vertical mixing in the atmosphere of each object, the first such measurements of this important parameter in late T dwarfs. A detailed analysis of the spectral energy distribution of 2MASS J09373487+2931409 results in the following values for metallicity, temperature, surface gravity, and luminosity: [M/H]~-0.3, T_eff=925-975K, log g=5.20-5.47, log L/L_sun=-5.308 +/- 0.027. The age is 3-10 Gyr and the mass is in the range 45-69 M_Jup.Comment: 36 pages incl. 12 figures and 3 tables, accepted by Ap

Near-UV absorption in very cool DA white dwarfs

The atmospheres of very cool, hydrogen-rich white dwarfs (Teff <6000 K) are challenging to models because of the increased complexity of the equation of state, chemical equilibrium, and opacity sources in a low-temperature, weakly ionized dense gas. In particular, many models that assume relatively simple models for the broadening of atomic levels and mostly ideal gas physics overestimate the flux in the blue part of their spectra. A solution to this problem that has met with some success is that additional opacity at short wavelengths comes for the extreme broadening of the Lyman alpha line of atomic H by collisions primarily with H2. For the purpose of validating this model more rigorously, we acquired Hubble Space Telescope STIS spectra of 8 very cool white dwarfs (5 DA and 3 DC stars). Combined with their known parallaxes, BVRIJHK and Spitzer IRAC photometry, we analyze their entire spectral energy distribution (from 0.24 to 9.3 micron) with a large grid of model atmospheres and synthetic spectra. We find that the red wing of the Lyman alpha line reproduces the rapidly decreasing near-UV flux of these very cool stars very well. We determine better constrained values of Teff and gravity as well as upper limits to the helium abundance in their atmospheres.Comment: 41 pages, 9 figures. Accepted for publication in the Ap

SDSS J141624.08+134826.7: Blue L Dwarfs and Non-Equilibrium Chemistry

We present an analysis of the recently discovered blue L dwarf SDSS J141624.08+134826.7. We extend the spectral coverage of its published spectrum to ~4 microns by obtaining a low-resolution L band spectrum with SpeX on the NASA IRTF. The spectrum exhibits a tentative weak CH4 absorption feature at 3.3 microns but is otherwise featureless. We derive the atmospheric parameters of SDSS J141624.08+134826.7 by comparing its 0.7-4.0 micron spectrum to the atmospheric models of Marley and Saumon which include the effects of both condensate cloud formation and non-equilibrium chemistry due to vertical mixing and find the best fitting model has Teff=1700 K, log g=5.5 [cm s-2], fsed=4, and Kzz=10^4 cm2 s-1. The derived effective temperature is significantly cooler than previously estimated but we confirm the suggestion by Bowler et al. that the peculiar spectrum of SDSS J141624.08+134826.7 is primarily a result of thin condensate clouds. In addition, we find strong evidence of vertical mixing in the atmosphere of SDSS J141624.08+134826.7 based on the absence of the deep 3.3 micron CH4 absorption band predicted by models computed in chemical equilibrium. This result suggests that observations of blue L dwarfs are an appealing way to quantitatively estimate the vigor of mixing in the atmospheres of L dwarfs because of the dramatic impact such mixing has on the strength of the 3.3 micron CH4 band in the emergent spectra of L dwarfs with thin condensate clouds.Comment: Accepted for publication in the Astronomical Journa