Very Low Mass Stars and Brown Dwarfs in Taurus-Auriga

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 and brown dwarfs (LMTTs) in Taurus-Auriga. The observed Li I 6708 Å 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 a mass just below the stellar/substellar boundary. 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 (vsini < 30 km/s), unlike low mass objects in Orion. Many of the slowly rotating, non-accreting stars and brown dwarfs exhibit prominent Hα emission (equivalent widths of 3 36 Å), indicative of active chromospheres. We demonstrate empirically that the fullwidth at 10% of the Hα emission profile peak is a more practical and possibly more accurate indicator of accretion than either the equivalent width of Hα or optical veiling: 10%-widths > 270 km/s 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 Ṁ ∝ 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 McDonald Observatory, R.L.M. Hall 15.308, Austin, TX 78712-1083 Present Address: Department of Astronomy, California Institute of Technology, MS 105-24, Pasadena, CA 91125 Department of Astronomy, University of California at Berkeley, Berkeley, CA 94720-3411