Surface gravity

Abstract: We analyse pre-Main Sequence evolutionary tracks for low mass stars with masses $m \\le 1.4 \\msol$ based on the Baraffe et al. (1998) input physics. We also extend the recent Chabrier et al. (2000) evolutionary models based on dusty atmosphere to young brown dwarfs down to one mass of Jupiter. We analyse current theoretical uncertainties due to molecular line lists, convection and initial conditions. Simple tests on initial conditions show the high uncertainties of models at ages $\\simle$ 1 Myr. We find a significant sensitivity of atmosphere profiles to the treatment of convection at low gravity and $\\te < 4000$ K, whereas it vanishes as gravity increases. This effect adds another source of uncertainty on evolutionary tracks at very early phases. We show that at low surface gravity ($\\log g \\simle 3.5$,) the common picture of vertical Hayashi lines with constant $\\te$ is oversimplified. The effect of a variation of initial deuterium abundance is studied. We compare our models with evolutionary tracks available in the literature and discuss the main differences. We finally analyse to which extent current observations of young systems provide a good test for pre-Main Sequence tracks.

Abstract: We present new JHK photometry on the MKO-NIR system and JHK spectroscopy for a large sample of L and T dwarfs. Photometry has been obtained for 71 dwarfs, and spectroscopy for 56. The sample comprises newly identified very red objects from the Sloan Digital Sky Survey (SDSS) and known dwarfs from the SDSS and the Two Micron All Sky Survey (2MASS). Spectral classification has been carried out using four previously defined indices from Geballe et al. that measure the strengths of the near infrared water and methane bands. We identify nine new L8?9.5 dwarfs and 14 new T dwarfs from SDSS, including the latest yet found by SDSS, the T7 dwarf SDSS J175805.46+463311.9. We classify 2MASS J04151954-0935066 as T9, the latest and coolest dwarf found to date. We combine the new results with our previously published data to produce a sample of 59 L dwarfs and 42 T dwarfs with imaging data on a single photometric system and with uniform spectroscopic classification. We compare the near-infrared colors and absolute magnitudes of brown dwarfs near the L?T transition with predictions made by models of the distribution and evolution of photospheric condensates. There is some scatter in the Geballe et al. spectral indices for L dwarfs, suggesting that these indices are probing different levels of the atmosphere and are affected by the location of the condensate cloud layer. The near-infrared colors of the L dwarfs also show scatter within a given spectral type, which is likely due to variations in the altitudes, spatial distributions, and thicknesses of the clouds. We have identified a small group of late-L dwarfs that are relatively blue for their spectral type and that have enhanced FeH, H2O, and K I absorption, possibly due to an unusually small amount of condensates. The scatter seen in the H-K color for late-T dwarfs can be reproduced by models with a range in surface gravity. The variation is probably due to the effect on the K-band flux of pressure-induced H2 opacity. The correlation of H-K color with gravity is supported by the observed strengths of the J-band K I doublet. Gravity is closely related to mass for field T dwarfs with ages greater than108 yr and the gravities implied by the H-K colors indicate that the T dwarfs in our sample have masses in the range 15?75MJupiter. One of the SDSS dwarfs, SDSS J111010.01+011613.1, is possibly a very low mass object, with log g ~ 4.2?4.5 and mass ~ 10?15MJupiter.

Abstract: We present high-resolution optical spectra obtained with the HIRES spectrograph on the W. M. Keck I Telescope of seven low-mass T Tauri stars (LMTTs) and brown dwarfs in Taurus-Auriga. The observed Li I 6708 A absorption, low surface gravity signatures, and radial velocities confirm that all are members of the Taurus star-forming region; no new spectroscopic binaries are identified. Four of the seven targets observed appear to be T Tauri brown dwarfs. Of particular interest is the previously classified "continuum T Tauri star" GM Tau, which has a spectral type of M6.5 and possibly a substellar mass. These spectra, in combination with previous high-resolution spectra of LMTTs, are used to understand the formation and early evolution of objects in Taurus-Auriga with masses near and below the stellar/substellar boundary. None of the LMTTs in Taurus are rapidly rotating (v sin i 270 km s-1 are classical T Tauri stars (i.e., accreting), independent of stellar spectral type. Although LMTTs can have accretion rates comparable to that of more typical, higher mass T Tauri stars (e.g., K7-M0 spectral types), the average mass accretion rate appears to decrease with decreasing mass. A functional form of ∝ M is consistent with the available data, but the dependence is difficult to establish because of both selection biases in observed samples and the decreasing frequency of active accretion disks at low masses (M < 0.2 M☉). The diminished frequency of accretion disks for LMTTs, in conjunction with their lower, on average, mass accretion rates, implies that they are formed with less massive disks than higher mass T Tauri stars. The radial velocities, circumstellar properties, and known binaries do not support the suggestion that many of the lowest mass members of Taurus have been ejected from higher stellar density regions within the cloud. Instead, LMTTs appear to have formed and are evolving in the same way as higher mass T Tauri stars, but with smaller disks and shorter disk lifetimes.

Abstract: We have embarked on a project, under the aegis of the Nearby Stars (NStars)/Space Interferometry Mission Preparatory Science Program, to obtain spectra, spectral types, and, where feasible, basic physical parameters for the 3600 dwarf and giant stars earlier than M0 within 40 pc of the Sun. In this paper, we report on the results of this project for the first 664 stars in the northern hemisphere. These results include precise, homogeneous spectral types, basic physical parameters (including the effective temperature, surface gravity, and overall metallicity [M/H]), and measures of the chromospheric activity of our program stars. Observed and derived data presented in this paper are also available on the project's Web site.