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

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

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

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

Acesso, equidade e coesão social: avaliação de estratégias intersetoriais para a população em situação de rua

Objective To understand and evaluate the work of intersectoral assistance on the insertion and the flow of people in situation of street with severe mental illness in public services of Mental Health. Method A case study developed from ten visits to a night shelter between March and April 2012. For data collection, the participant observation and semi-structured interviews were carried out with four sheltered individuals, as well as non-directive group interviews with five technicians of the social-assistance services. Results Were analyzed using Content Analysis and developing a Logic Model validated with the professionals involved. Conclusion The social assistance services are the main entry of this clientele in the public network of assistance services, and the Mental Health services have difficulty in responding to the specificities of the same clientele and in establishing intersectoral work.
Objetivo Compreender e avaliar o trabalho de assistência intersetorial sobre a inserção e o fluxo de pessoas em situação de rua, com transtorno mental grave, nos serviços públicos de Saúde Mental. Método Estudo de caso, desenvolvido a partir de 10 visitas a um albergue, entre março e abril de 2012. Para a coleta de dados foi realizada a observação participante e entrevistas semiestruturadas com quatro albergados, além de entrevistas não diretivas em grupo, com cinco técnicos dos serviços socioassistenciais. Resultados Foram analisados por meio da Análise de Conteúdo e da elaboração de Modelo Lógico, e validados junto aos profissionais envolvidos. Conclusão Os serviços socioassistenciais são a principal entrada dessa clientela à rede pública de assistência, e que os serviços de Saúde Mental apresentam dificuldades em responder às especificidades dessa mesma clientela e estabelecer trabalho intersetorial.
Objetivo Comprender y evaluar el trabajo de asistencia intersectorial acerca de la inserción y el flujo de personas en situación de calle, con trastorno mental severo, en los servicios públicos de Salud Mental. Método Estudio de caso, desarrollado a partir de 10 visitas a un albergue, entre marzo y abril de 2012. Para la recolección de datos fue realizada la observación participante y entrevistas semiestructuradas con cuatro albergados, además de entrevistas no directivas en grupo, con cinco técnicos de los servicios socioasistenciales. Resultados Fueron validados mediante el Análisis de Contenido y la confección del Modelo Lógico, y validados junto a los profesionales involucrados. Conclusión Los servicios socioasistenciales son la principal forma de ingreso de esa clientela a la red pública de asistencia, y que los servicios de Salud Mental presentan dificultades de responder a las especificidades de dicha clientela y establecer trabajo intersectorial.Universidade de São Paulo Faculty of MedicineUniversidade Federal de São Paulo (UNIFESP)UNIFESPSciEL

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

Fragmentation and the formation of primordial protostars: the possible role of Collision Induced Emission

The mechanisms which could lead to chemo-thermal instabilities and fragmentation during the formation of primordial protostars are investigated analytically. We introduce approximations for H2 cooling rates bridging the optically thin and thick regimes. These allow us to discuss instabilities up to densities when protostars become optically thick to continuum radiation (n~10^16 cm^-3). During the collapse, instability arises at two different stages: at low density (n~10^8-10^11 cm^-3), it is due to fast 3-body reactions converting H into H2; at high density (n>10^13 cm^-3), it is due to Collisional Induced Emission (CIE). In agreement with the 3D simulations, we find that the instability at low densities cannot lead to fragmentation, because fluctuations do not survive turbulent mixing, and because their growth is slow. The situation at high density is similar. The CIE-induced instability is as weak as the low density one, with similar ratios of growth and dynamical time scales. Fluctuation growth time is longer than free fall time, and fragmentation seems unlikely. One then expects the first stars to be massive, not to form binaries nor harbour planets. Nevertheless, full 3D simulations are required. They could become possible using simplified estimates of radiative transfer effects, which we show to work very well in the 1D case. This indicates that the effects of radiative transfer during the initial stages of formation of primordial protostars can be treated as local corrections to cooling. (Abridged)Comment: 17 pages, 9 figures; accepted for publication in MNRA

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

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

The Planetary Mass Companion 2MASS1207-3932 B: Temperature, Mass and Evidence for an Edge-On Disk

We present J-band imaging and H+K-band low-resolution spectroscopy of 2MASS1207-3932 AB, obtained with VLT NACO. For the putative planetary mass secondary, we find J = 20.0+/-0.2 mag. The HK spectra of both components imply low gravity, and a dusty atmosphere for the secondary. Comparisons to synthetic spectra yield Teff_A ~ 2550+/-150K, and Teff_B ~ 1600+/-100K, consistent with their late-M and mid-to-late L types. For these Teff, and an age of 5-10 Myrs, evolutionary models imply M_A ~ 24+/-6 M_Jup and M_B ~ 8+/-2 M_Jup. Independent comparisons of these models to the observed colors, spanning ~I to L', also yield the same masses and temperatures. Our primary mass agrees with other recent analyses; however, our secondary mass, while still in the planetary regime, is 2-3 times larger than claimed previously. This discrepancy can be traced to the luminosities: while the absolute photometry and Mbol of the primary agree with theoretical predictions, the secondary is ~ 2.5+/-0.5 mag fainter than expected in all bands from I to L' and in Mbol. This accounts for the much lower secondary mass (and temperature) derived earlier. We argue that this effect is highly unlikely to result from a variety of model-related problems, and is instead real. This conclusion is bolstered by the absence of any luminosity problems in either the primary, or in AB Pic B which we also analyse. We therefore suggest grey extinction in 2M1207B, due to occlusion by an edge-on circum-secondary disk. This is consistent with the observed properties of edge-on disks around T Tauri stars, and with the known presence of a high-inclination evolved disk around the primary. Finally, the system's implied mass ratio of ~0.3 suggests a binary-like formation scenario. (abridged)Comment: Accepted by The Astrophysical Journal, 43 pages text + 16 figs + 1 tabl