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 (LMTTs) and brown dwarfs in Taurus-Auriga. The observed Li I 6708 Angstrom 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<30 km s(-1)), unlike low-mass objects in Orion. Many of the slowly rotating, nonaccreting stars and brown dwarfs exhibit prominent Hα emission (equivalent widths of 3-36 &ANGS;), indicative of active chromospheres. We demonstrate empirically that the full width 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(-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 proportional to 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.