Peter H. Hauschildt

Evolutionary Models for Very Low-Mass Stars and Brown Dwarfs with Dusty Atmospheres

We present evolutionary calculations for very low-mass stars and brown dwarfs based on synthetic spectra and nongray atmosphere models which include dust formation and opacity, i.e., objects with Teff 2800 K. The interior of the most massive brown dwarfs is shown to develop a conductive core after ~2 Gyr which slows down their cooling. Comparison is made in optical and infrared color-magnitude diagrams with recent late-M and L dwarf observations. The saturation in optical colors and the very red near-infrared colors of these objects are well explained by the onset of dust formation in the atmosphere. Comparison of the faintest presently observed L dwarfs with these dusty evolutionary models suggests that dynamical processes such as turbulent diffusion and gravitational settling are taking place near the photosphere. As the effective temperature decreases below Teff ≈ 1300-1400 K, the colors of these objects move to very blue near-infrared colors, a consequence of the ongoing methane absorption in the infrared. We suggest the possibility of a brown dwarf dearth in J, H, and K color-magnitude diagrams around this temperature.

Evolutionary models for cool brown dwarfs and extrasolar giant planets. The Case of HD 209458

We present evolutionary models for cool brown dwarfs and extra-solar giant planets. The models re- produce the main trends of observed methane dwarfs in near-IR color-magnitude diagrams. We also present evolutionary models for irradiated planets, coupling for the first time irradiated atmosphere profiles and inner structures. We focus on HD 209458-like systems and show that irradiation effects can substantially affect the ra- dius of sub-jovian mass giant planets. Irradiation effects, however, cannot alone explain the large observed radius of HD 209458b. Adopting assumptions which optimise irradiation effects and taking into account the extension of the outer atmospheric layers, we still find � 20% discrepancy between observed and theoretical radii. An extra source of energy seems to be required to explain the observed value of the first transit planet.

The NextGen Model Atmosphere Grid for 3000 ≤ Teff ≤ 10,000 K

We present our NextGen Model Atmosphere grid for low-mass stars for effective temperatures larger than 3000 K. These LTE models are calculated with the same basic model assumptions and input physics as the VLMS part of the NextGen grid so that the complete grid can be used, e.g., for consistent stellar evolution calculations and for internally consistent analysis of cool star spectra. This grid is also the starting point for a large grid of detailed NLTE model atmospheres for dwarfs and giants. The models were calculated from 3000 to 10,000 K (in steps of 200 K) for 3.5 ≤ log g ≤ 5.5 (in steps of 0.5) and metallicities of -4.0 ≤ [M/H] ≤ 0.0. We discuss the results of the model calculations and compare our results to the Kurucz grid. Some comparisons to standard stars like Vega and the Sun are presented and compared with detailed NLTE calculations.

Low-Temperature Opacities

Previous computations of low-temperature Rosseland and Planck mean opacities from Alexander & Ferguson areupdatedandexpanded.Thenewcomputationsincludeamorecompleteequationofstate(EOS)withmoregrain species and updated optical constants. Grains are now explicitly included in thermal equilibrium in the EOS calculation, which allows for a much wider range of grain compositions to be accurately included than was previously the case. The inclusion of high-temperature condensates such as Al2O3 and CaTiO3 significantly affects the total opacityoveranarrowrangeoftemperaturesbeforetheappearanceofthefirstsilicategrains.Thenewopacitytables are tabulated for temperatures ranging from 30,000 to 500 K with gas densities from 10 � 4 to 10 � 19 gc m � 3 .C omparisons with previous Rosseland mean opacity calculations are discussed. At high temperatures, the agreement with OPAL and Opacity Project is quite good. Comparisons at lower temperatures are more divergent as a result of differences in molecular and grain physics included in different calculations. The computation of Planck mean opacities performed with the opacity sampling method is shown to require a very large number of opacity sampling wavelength points; previously published results obtained with fewer wavelength points are shown to be significantly in error. Methods for requesting or obtaining the new tables are provided. Subject heading gs: atomic data — equation of state — methods: numerical — molecular data

THE LIMITING EFFECTS OF DUST IN BROWN DWARF MODEL ATMOSPHERES

We present opacity sampling model atmospheres, synthetic spectra, and colors for brown dwarfs and very low mass stars in the following two limiting cases of dust grain formation: (1) Inefficient gravitational settling (i.e., the dust is distributed according to the chemical equilibrium predictions) and (2) efficient gravitational settling (i.e., the dust forms and depletes refractory elements from the gas, but their opacity does not affect the thermal structure). The models include the formation of over 600 gas-phase species and 1000 liquids and crystals and the opacities of 30 different types of grains including corundum (Al2O3), the magnesium aluminum spinel MgAl2O4, iron, enstatite (MgSiO3), forsterite (Mg2SiO4), amorphous carbon, SiC, and a number of calcium silicates. The models extend from the beginning of the grain formation regime well into the condensation regime of water ice (Teff = 3000-100 K) and encompass the range of log g = 2.5-6.0 at solar metallicity. We find that silicate dust grains can form abundantly in the outer atmospheric layers of red and brown dwarfs with a spectral type later than M8. The greenhouse effects of dust opacities provide a natural explanation for the peculiarly red spectroscopic distribution of the latest M dwarfs and young brown dwarfs. The grainless (cond) models, on the other hand, correspond closely to methane brown dwarfs such as Gliese 229B. We also discover that the λλ5891, 5897 Na I D and λλ7687, 7701 K I resonance doublets play a critical role in T dwarfs, in which their red wings define the pseudocontinuum from the I to the Z bandpass.

Evolutionary models for low-mass stars and brown dwarfs: Uncertainties and limits at very young ages

We analyse pre-Main Sequence evolutionary tracks for low mass stars with masses

The NEXTGEN Model Atmosphere Grid. II. Spherically Symmetric Model Atmospheres for Giant Stars with Effective Temperatures between 3000 and 6800 K

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Model atmospheres for M (sub)dwarf stars. 1: The base model grid

based on the Baraffe et al. (1998) input physics. We also extend the recent Chabrier et al. (2000) evolutionary models based on dusty atmosphere to young brown dwarfs down to one mass of Jupiter. We analyse current theoretical uncertainties due to molecular line lists, convection and initial conditions. Simple tests on initial conditions show the high uncertainties of models at ages

Evidence for a Spectroscopic Sequence among Type Ia Supernovae

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Evolutionary stellar population synthesis at high spectral resolution: optical wavelengths

1 Myr. We find a significant sensitivity of atmosphere profiles to the treatment of convection at low gravity and