The T Tauri Phase Down to Nearly Planetary Masses: Echelle Spectra of 82 Very Low Mass Stars and Brown Dwarfs

Using the largest high-resolution spectroscopic sample to date of young, verylow mass stars (VLMS) and brown dwarfs (BDs), we investigate disk accretion inobjects ranging from just above the hydrogen-burning limit all the way tonearly planetary masses. Our 82 targets span spectral types from M5 to M9.5, ormasses from 0.15 Msun down to ~15 Jupiters. They are confirmed members of therho Oph, Taurus, Cha I, IC 348, R CrA, Upper Sco and TW Hydrae regions, withages <1 to ~10 Myr. The sample contains 41 BDs (spectral types >= M6.5). Wefind that: (1) classical T Tauri-like disk-accretion persists in the BD domaindown to nearly the deuterium-burning limit; (2) in addition to H-alpha,permitted emission lines of CaII, OI and HeI are also good accretionindicators, as in CTTs; (3) the CaII 8662A flux is an excellent quantitativemeasure of the accretion rate (Mdot) in VLMS and BDs(as in CTTs); (4) Mdotdiminishes as M^2 -- our measurements support previous findings of thiscorrelation, and extend it to the entire range of sub-stellar masses; (5) theaccretor fraction among VLMS and BDs decreases substantially with age, as inhigher-mass stars; (6) at any given age, the VLMS and BD accretor fraction iscomparable to that in higher-mass stars; and (7) a number of sources with IRdisk excesses do not evince measurable accretion, with the incidence of such amismatch increasing with age: this implies that disks in the low mass regimecan persist beyond the main accretion phase, and parallels the transition fromthe classical to post-T Tauri stage in more massive stars. These strongsimilarities at young ages, between higher-mass stars and low-mass bodies closeto and below the hydrogen-burning limit, are consistent with a common formationmechanism in the two mass regimes. (abridged)Comment: 64 pages, 7 figures. ApJ accepte