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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