John A. Milsom

The Deuterium-Burning Mass Limit for Brown Dwarfs and Giant Planets

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 (MJ) limit for the ignition of deuterium. Here, we investigate various deuterium-burning masses for a range of models. We find that, while 13MJ 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)MJ, 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 MJ (for 3-times solar metallicity, 10% of initial deuterium burned) to �16.3 MJ (for zero metallicity, 90% of initial deuterium burned). Subject headings: radiative transfer – stars: low-mass, brown dwarfs – stars: evolution

Modeling the Formation of Clouds in Brown Dwarf Atmospheres

Because the opacity of clouds in substellar mass object (SMO) atmospheres depends on the composition and distribution of particle sizes within the cloud, a credible cloud model is essential for accurately modeling SMO spectra and colors. We present a one-dimensional model of cloud particle formation and subsequent growth based on a consideration of basic cloud microphysics. We apply this microphysical cloud model to a set of synthetic brown dwarf atmospheres spanning a broad range of surface gravities and effective temperatures (gsurf ¼ 1:78 � 10 3 3 � 10 5 cm s � 2 and Teff ¼ 600 1600 K) to obtain plausible particle sizes for several abundant species (Fe, Mg2SiO4, and Ca2Al2SiO7). At the base of the clouds, where the particles are largest, the particle sizes thus computed range from � 5 to over 300 lm in radius over the full range of atmospheric conditions considered. We show that average particle sizes decrease significantly with increasing brown dwarf surface gravity. We also find that brown dwarfs with higher effective temperatures have characteristically larger cloud particles than those with lower effective temperatures. We therefore conclude that it is unrealistic when modeling SMO spectra to apply a single particle size distribution to the entire class of objects. Subject headings: stars: atmospheres — stars: low-mass, brown dwarfs

Theoretical Spectral Models of T Dwarfs at Short Wavelengths and Their Comparison with Data

We have generated new, self-consistent spectral and atmosphere models for the effective temperature range 600-1300 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 μm. By defining spectral indices and standard colors in the optical and very near-infrared, we explore the theoretical systematics with Teff, 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 (7700 A) 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 values of Teff, in a reversal of the normal pattern, optical spectra become bluer with further decreases in Teff. Moreover, we conclude that T dwarf subtype is not a function of Teff alone but that it is a nontrivial function of gravity and metallicity as well. As do Marley and coworkers in their 2002 work, 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 cannot 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.

SPECTROSCOPIC CONSTANTS, ABUNDANCES, AND OPACITIES OF THE TiH MOLECULE

Using previous measurements and quantum chemical calculations to derive the molecular properties of the TiH molecule, we obtain new values for its rovibrational 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 ~0.52 and 0.94 μm and in the H band may be more easily measurable than heretofore thought. The recent possible identification in the L subdwarf 2MASS J0532 of the 0.94 μm 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.

Models of Neptune-Mass Exoplanets: Emergent Fluxes and Albedos

There are now many known exoplanets with Msin i within a factor of 2 of Neptunes, including the transiting planets GJ 436b 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 GJ 436b 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.

SUPER-KEPLERIAN FREQUENCIES IN ACCRETION DISKS. IMPLICATIONS FOR MASS AND SPIN MEASUREMENTS OF COMPACT OBJECTS FROM X-RAY VARIABILITY STUDIES

The detection of fast quasi-periodic variability from accreting black holes and neutron stars has been used to constrain their masses, radii, and spins. If the observed oscillations are linear modes in the accretion disks, then bounds can be placed on the properties of the central objects by assuming that these modes are locally sub-Keplerian. If, on the other hand, the observed oscillations correspond to nonlinear resonances between disk modes, then the properties of the central objects can be measured by assuming that the resonant modes are excited at the same radial annulus in the disk. In this paper, we use numerical simulations of vertically integrated, axisymmetric hydrodynamic accretion disks to provide examples of situations in which the assumptions implicit in both methods are not satisfied. We then discuss our results for the robustness of the mass and spin measurements of compact objects from variability studies.

Erratum: “Modeling the Formation of Clouds in Brown Dwarf Atmospheres” (ApJ, 586, 1320 [2003])

Because the opacity of clouds in substellar mass object (SMO) atmospheres depends on the composition and distribution of particle sizes within the cloud, a credible cloud model is essential for accurately modeling SMO spectra and colors. We present a one--dimensional model of cloud particle formation and subsequent growth based on a consideration of basic cloud microphysics. We apply this microphysical cloud model to a set of synthetic brown dwarf atmospheres spanning a broad range of surface gravities and effective temperatures (g_surf = 1.78 * 10^3 -- 3 * 10^5 cm/s^2 and T_eff = 600 -- 1600 K) to obtain plausible particle sizes for several abundant species (Fe, Mg2SiO4, and Ca2Al2SiO7). At the base of the clouds, where the particles are largest, the particle sizes thus computed range from ~5 microns to over 300 microns in radius over the full range of atmospheric conditions considered. We show that average particle sizes decrease significantly with increasing brown dwarf surface gravity. We also find that brown dwarfs with higher effective temperatures have characteristically larger cloud particles than those with lower effective temperatures. We therefore conclude that it is unrealistic when modeling SMO spectra to apply a single particle size distribution to the entire class of objects.Because the opacity of clouds in substellar mass object (SMO) atmospheres depends on the composition and distribution of particle sizes within the cloud, a credible cloud model is essential for accurately modeling SMO spectra and colors. We present a one-dimensional model of cloud particle formation and subsequent growth based on a consideration of basic cloud microphysics. We apply this microphysical cloud model to a set of synthetic brown dwarf atmospheres spanning a broad range of surface gravities and effective temperatures (gsurf = 1.78×10 – 3×10 cm s and Teff = 600 – 1600 K) to obtain plausible particle sizes for several abundant species (Fe, Mg2SiO4, and Ca2Al2SiO7). The particle sizes we have thus computed range from ∼5 μm to over 300 μm in radius over the full range of atmospheric conditions considered. We show that modal particle sizes decrease significantly with increasing brown dwarf surface gravity. We also find that brown dwarfs with higher effective temperatures have characteristically larger cloud particles than those with lower effective temperatures. We thus conclude that it is unrealistic when modeling SMO spectra to apply a single particle size distribution to the entire class of objects. Subject headings: atmospheres: clouds, condensation, grains: fundamental parameters — stars: low mass, brown dwarfs, substellar mass objects, L dwarfs, T dwarfs, spectroscopy, atmospheres, spectral