There is no universally acknowledged criterion to distinguish brown dwarfs from planets. Numerous studies have used or suggested a definition based on an object's mass, taking the ∼13-Jupiter mass (M J ) limit for the ignition of deuterium. Here, we investigate various deuterium-burning masses for a range of models. We find that, while 13M J is generally a reasonable rule of thumb, the deuterium fusion mass depends on the helium abundance, the initial deuterium abundance, the metallicity of the model, and on what fraction of an object's initial deuterium abundance must combust in order for the object to qualify as having burned deuterium. Even though, for most proto-brown dwarf conditions, 50% of the initial deuterium will burn if the object's mass is ∼(13.0 ± 0.8)M J , the full range of possibilities is significantly broader. For models ranging from zero-metallicity to more than three times solar metallicity, the deuterium burning mass ranges from ∼11.0 M J (for 3-times solar metallicity, 10% of initial deuterium burned) to ∼16.3 M J (for zero metallicity, 90% of initial deuterium burned).Subject headings: radiative transfer -stars: low-mass, brown dwarfs -stars: evolution 100 10 M J 11 M J 12 M J 12.5 M J 13 M J 100 10 M J 11 M J 12 M J 12.5 M J 13 M J

The cloud model of Cooper et al. (2003) estimates to first-order accuracy the cloud particle sizes typically found in brown dwarfs and planetary atmospheres. This model, which is one-dimensional, is based on microphysical considerations and incorporates the results of the theories of homogeneous and heterogeneous particle nucleation.We have posted the source code for this cloud model for public use as a tool for the intercomparison of planetary radiation transport models attempting to incorporate the physics of cloud condensation. Follow the 'Computational Models' link from the URL above (Theoretical Astrophysics Program -University of Arizona) for download instructions, source code, and additional documentation.

We have generated new, self-consistent spectral and atmosphere models for the effective temperature range 600 K to 1300 K thought to encompass the known T dwarfs. For the first time, theoretical models are compared with a family of measured T dwarf spectra at wavelengths shortward of ∼1.0 micron. By defining spectral indices and standard colors in the optical and very near-infrared, we explore the theoretical systematics with T eff , gravity, and metallicity. We conclude that the short-wavelength range is rich in diagnostics that complement those in the near-infrared now used for spectral subtyping. We also conclude that the wings of the Na D and K I (7700Å) resonance lines and aggressive rainout of heavy metals (with the resulting enhancement of the sodium and potassium abundances at altitude) are required to fit the new data shortward of 1.0 µm . Furthermore, we find that the water bands weaken with increasing gravity, that modest decreases in metallicity enhance the effect in the optical of the sodium and potassium lines, and that at low T eff s , in a reversal of the normal pattern, optical spectra become bluer with further decreases in T eff . Moreover, we conclude that T dwarf subtype is not a function of T eff alone, but that it is a non-trivial function of gravity and metallicity as well. As do Marley et al. (2001), we see evidence in early T dwarf atmospheres of a residual effect of clouds. With cloudless models, we obtain spectral fits to the two late T dwarfs with known parallaxes, but a residual effect of clouds on the emergent spectra of even late T dwarfs can not yet be discounted. However, our focus is not on detailed fits to individual objects, but on the interpretation of the overall spectral and color trends of the entire class of T dwarfs, as seen at shorter wavelengths.

Using previous measurements and quantum chemical calculations to derive the molecular properties of the TiH molecule, we obtain new values for its ro-vibrational constants, thermochemical data, spectral line lists, line strengths, and absorption opacities. Furthermore, we calculate the abundance of TiH in M and L dwarf atmospheres and conclude that it is much higher than previously thought. We find that the TiH/TiO ratio increases strongly with decreasing metallicity, and at high temperatures can exceed unity. We suggest that, particularly for subdwarf L and M dwarfs, spectral features of TiH near $\sim$0.52 \mic, 0.94 \mic, and in the $H$ band may be more easily measureable than heretofore thought. The recent possible identification in the L subdwarf 2MASS J0532 of the 0.94 \mic feature of TiH is in keeping with this expectation. We speculate that looking for TiH in other dwarfs and subdwarfs will shed light on the distinctive titanium chemistry of the atmospheres of substellar-mass objects and the dimmest stars.Comment: 37 pages, including 4 figures and 13 tables, accepted to the Astrophysical Journa

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In this paper, we calculate new line lists and opacities for the 12 bands of the A$^6\Sigma^{+}$ -- X$^6\Sigma^{+}$ transitions of the CrH molecule. Identified in objects of the new L dwarf spectroscopic class (many of which are brown dwarfs), as well as in sunspots, the CrH molecule plays an important role in the diagnosis of low-temperature atmospheres. As a tentative first application of these opacities, we employ our new theoretical CrH data in an atmospheres code to obtain a CrH/H$_2$ number ratio for the skin of the L5 dwarf 2MASSI J1507038-151648 of $\sim 2-4\times 10^{-9}$, in rough agreement with chemical equilibrium expectations. Since in previous compilations the oscillator strength was off by more than an order of magnitude, this agreement represents a modest advance. However, in order to determine the CrH abundance in an L dwarf atmosphere, silicate clouds need to be incorporated into the model, and cloud modeling is still in a primitive stage of development. Nevertheless, one important step in L dwarf modeling is a reliable CrH opacity and this is what we have here attempted to provide.Comment: Includes 3 color figures (in .gif format). Accepted to the Astrophysical Journa

There are now many known exoplanets with M sin i within a factor of two of Neptune's, including the transiting planets GJ436b and HAT-P-11b. Planets in this mass-range are different from their more massive cousins in several ways that are relevant to their radiative properties and thermal structures. By analogy with Neptune and Uranus, they are likely to have metal abundances that are an order of magnitude or more greater than those of larger, more massive planets. This increases their opacity, decreases Rayleigh scattering, and changes their equation of state. Furthermore, their smaller radii mean that fluxes from these planets are roughly an order of magnitude lower than those of otherwise identical gas giant planets. Here, we compute a range of plausible radiative equilibrium models of GJ436b and HAT-P-11b. In addition, we explore the dependence of generic Neptune-mass planets on a range of physical properties, including their distance from their host stars, their metallicity, the spectral type of their stars, the redistribution of heat in their atmospheres, and the possible presence of additional optical opacity in their upper atmospheres.Subject headings: equation of state -line: profiles -planetary systems -radiative transfer -stars: individual GJ436, HAT-P-11 -astrochemistry

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