Discovery of a Second L Subdwarf in the Two Micron All Sky Survey

I report the discovery of the second L subdwarf identified in the Two Micron All Sky Survey, 2MASS J16262034+3925190. This high proper motion object (mu = 1.27+/-0.03 "/yr) exhibits near-infrared spectral features indicative of a subsolar metallicity L dwarf, including strong metal hydride and H2O absorption bands, pressure-broadened alkali lines, and blue near-infrared colors caused by enhanced collision-induced H2 absorption. This object is of later type than any of the known M subdwarfs, but does not appear to be as cool as the apparently late-type sdL 2MASS 0532+8246. The radial velocity (Vrad = -260+/-35 km/s) and estimated tangential velocity (Vtan ~ 90-210 km/s) of 2MASS 1626+3925 indicate membership in the Galactic halo, and this source is likely near or below the hydrogen burning minimum mass for a metal-poor star. L subdwarfs such as 2MASS 1626+3925 are useful probes of gas and condensate chemistry in low-temperature stellar and brown dwarf atmospheres, but more examples are needed to study these objects as a population as well as to define a rigorous classification scheme.Comment: 11 pages, 3 figures, accepted for publication ApJ Letters, v. 614 October 200

An Improbable Solution to the Underluminosity of 2M1207B: A Hot Protoplanet Collision Afterglow

We introduce an alternative hypothesis to explain the very low luminosity of the cool (L-type) companion to the ~25 M_Jup ~8 Myr-old brown dwarf 2M1207A. Recently, Mohanty et al. (2007) found that effective temperature estimates for 2M1207B (1600 +- 100 K) are grossly inconsistent with its lying on the same isochrone as the primary, being a factor of ~10 underluminous at all bands between I (0.8 um) and L' (3.6 um). Mohanty et al. explain this discrepency by suggesting that 2M1207B is an 8 M_Jup object surrounded by an edge-on disk comprised of large dust grains producing 2.5^m of achromatic extinction. We offer an alternative explanation: the apparent flux reflects the actual source luminosity. Given the temperature, we infer a small radius (~49,000 km), and for a range of plausible densities, we estimate a mass < M_Jup. We suggest that 2M1207B is a hot protoplanet collision afterglow and show that the radiative timescale for such an object is >~1% the age of the system. If our hypothesis is correct, the surface gravity of 2M1207B should be an order of magnitude lower than predicted by Mohanty et al. (2007).Comment: ApJ Letters, in press (11 pages

L and T Dwarf Models and the L to T Transition

Using a model for refractory clouds, a novel algorithm for handling them, and the latest gas-phase molecular opacities, we have produced a new series of L and T dwarf spectral and atmosphere models as a function of gravity and metallicity, spanning the \teff range from 2200 K to 700 K. The correspondence with observed spectra and infrared colors for early- and mid-L dwarfs and for mid- to late-T dwarfs is good. We find that the width in infrared color-magnitude diagrams of both the T and L dwarf branches is naturally explained by reasonable variations in gravity and, therefore, that gravity is the "second parameter" of the L/T dwarf sequence. We investigate the dependence of theoretical dwarf spectra and color-magnitude diagrams upon various cloud properties, such as particle size and cloud spatial distribution. In the region of the L$\to$T transition, we find that no one cloud-particle-size and gravity combination can be made to fit all the observed data. Furthermore, we note that the new, lower solar oxygen abundances of Allende-Prieto, Lambert, & Asplund (2002) produce better fits to brown dwarf data than do the older values. Finally, we discuss various issues in cloud physics and modeling and speculate on how a better correspondence between theory and observation in the problematic L$\to$T transition region might be achieved.Comment: accepted to the Astrophysical Journal, 21 figures (20 in color); spectral models in electronic form available at http://zenith.as.arizona.edu/~burrow

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 two limiting case of dust grain formation: 1) inefficient gravitational settling i.e. the dust is distributed according to the chemical equilibrium predictions, 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 (Al$_2$O$_3$), the magnesium aluminum spinel MgAl$_2$O$_4$, iron, enstatite (MgSiO$_3$), forsterite (Mg$_2$SiO$_4$), 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 encompasses 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 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 recover that the $\lambda$5891,5897\AA Na I D and $\lambda$7687,7701\AA K I resonance doublets plays a critical role in T dwarfs where their red wing define the pseudo-continuum from the $I$ to the $Z$ bandpass.Comment: 49 pages, ApJ, in press. 22 figures (included). Corrected nasty typos. Also available at http:/phoenix.physast.uga.ed

A Possible Bifurcation in Atmospheres of Strongly Irradiated Stars and Planets

We show that under certain circumstances the differences between the absorption mean and Planck mean opacities can lead to multiple solutions for an LTE atmospheric structure. Since the absorption and Planck mean opacities are not expected to differ significantly in the usual case of radiative equilibrium, non-irradiated atmospheres, the most interesting situations where the effect may play a role are strongly irradiated stars and planets, and also possibly structures where there is a significant deposition of mechanical energy, such as stellar chromospheres and accretion disks. We have presented an illustrative example of a strongly irradiated giant planet where the bifurcation effect is predicted to occur for a certain range of distances from the star.Comment: 22 pages, 6 figures, submitted to Ap

Photophoretic Structuring of Circumstellar Dust Disks

We study dust accumulation by photophoresis in optically thin gas disks. Using formulae of the photophoretic force that are applicable for the free molecular regime and for the slip-flow regime, we calculate dust accumulation distances as a function of the particle size. It is found that photophoresis pushes particles (smaller than 10 cm) outward. For a Sun-like star, these particles are transported to 0.1-100 AU, depending on the particle size, and forms an inner disk. Radiation pressure pushes out small particles (< 1 mm) further and forms an extended outer disk. Consequently, an inner hole opens inside ~0.1 AU. The radius of the inner hole is determined by the condition that the mean free path of the gas molecules equals the maximum size of the particles that photophoresis effectively works on (100 micron - 10 cm, depending on the dust property). The dust disk structure formed by photophoresis can be distinguished from the structure of gas-free dust disk models, because the particle sizes of the outer disks are larger, and the inner hole radius depends on the gas density.Comment: 15 pages, 9 figures, Accepted by ApJ; corrected a typo in the author nam

The Origin of Primordial Dwarf Stars and Baryonic Dark Matter

I present a scenario for the production of low mass, degenerate dwarfs of mass $>0.1 M_{\odot}$ via the mechanism of Lenzuni, Chernoff & Salpeter (1992). Such objects meet the mass limit requirements for halo dark matter from microlensing surveys while circumventing the chemical evolution constraints on normal white dwarf stars. I describe methods to observationally constrain this scenario and suggest that such objects may originate in small clusters formed from the thermal instability of shocked, heated gas in dark matter haloes, such as suggested by Fall & Rees (1985) for globular clusters.Comment: TeX, 4 pages plus 2 postscript figures. To appear in Astrophysical Journal Letter

Sub-Pixel Response Measurement of Near-Infrared Sensors

Wide-field survey instruments are used to efficiently observe large regions of the sky. To achieve the necessary field of view, and to provide a higher signal-to-noise ratio for faint sources, many modern instruments are undersampled. However, precision photometry with undersampled imagers requires a detailed understanding of the sensitivity variations on a scale much smaller than a pixel. To address this, a near-infrared spot projection system has been developed to precisely characterize near-infrared focal plane arrays and to study the effect of sub-pixel non uniformity on precision photometry. Measurements of large format near-infrared detectors demonstrate the power of this system for understanding sub-pixel response.Comment: 9 pages, 13 figures, submitted to PAS

Beyond the T Dwarfs: Theoretical Spectra, Colors, and Detectability of the Coolest Brown Dwarfs

We explore the spectral and atmospheric properties of brown dwarfs cooler than the latest known T dwarfs. Our focus is on the yet-to-be-discovered free-floating brown dwarfs in the \teff range from $\sim$800 K to $\sim$130 K and with masses from 25 to 1 \mj. This study is in anticipation of the new characterization capabilities enabled by the launch of SIRTF and the eventual launch of JWST. We provide spectra from $\sim$0.4 \mic to 30 \mic, highlight the evolution and mass dependence of the dominant H$_2$O, CH$_4$, and NH$_3$ molecular bands, consider the formation and effects of water-ice clouds, and compare our theoretical flux densities with the sensitivities of the instruments on board SIRTF and JWST. The latter can be used to determine the detection ranges from space of cool brown dwarfs. In the process, we determine the reversal point of the blueward trend in the near-infrared colors with decreasing \teff, the \teffs at which water and ammonia clouds appear, the strengths of gas-phase ammonia and methane bands, the masses and ages of the objects for which the neutral alkali metal lines are muted, and the increasing role as \teff decreases of the mid-infrared fluxes longward of 4 \mic. These changes suggest physical reasons to expect the emergence of at least one new stellar class beyond the T dwarfs. Our spectral models populate, with cooler brown dwarfs having progressively more planet-like features, the theoretical gap between the known T dwarfs and the known giant planets. Such objects likely inhabit the galaxy, but their numbers are as yet unknown.Comment: Includes 14 figures, most in color; accepted to the Astrophysical Journa

Rosseland and Planck mean opacities for primordial matter

We present newly calculated low-temperature opacities for gas with a primordial chemical composition. In contrast to earlier calculations which took a pure metal-free Hydrogen/Helium mixture, we take into account the small fractions of Deuterium and Lithium as resulting from Standard Big Bang Nucleosynthesis. Our opacity tables cover the density range -16 < log rho [g cm^{-3}] < -2 and temperature range of 1.8 < T [K] < 4.6, while previous tables were usually restricted to T > 10^3 K. We find that, while the presence of Deuterium does not significantly alter the opacity values, the presence of Lithium gives rise to major modifications of the opacities, at some points increasing it by approximately 2 orders of magnitude relative to pure Hydrogen/Helium opacities.Comment: 16 pages, 8 figures, submitted to MNRAS, all figures in grey-scale and at reduced resolution, for high-res colour PDF see http://www.ita.uni-heidelberg.de/~mm/publications/MayerDuschl-2.pd