/ 05 02 15 5 v 1 8 F eb 2 00 5 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, very low mass stars and brown dwarfs, we investigate disk accretion in objects ranging from just above the hydrogen-burning limit all the way to nearly planetary masses. Our 82 targets span spectral types from M5 to M9.5, or masses from 0.15 M⊙ down to about 15 Jupiters. They are confirmed members of the ρ Ophiuchus, Taurus, Chamaeleon I, IC 348, R Coronae Australis, Upper Scorpius and TW Hydrae star-forming regions and young clusters, with ages from .1 to ∼10 Myr. The sample contains 41 brown dwarfs (spectral types ≥M6.5). We have previously presented high-resolution optical spectra for roughly half the sample; the rest are new. This is a close to complete survey of all confirmed brown dwarfs known so far in the regions examined, except in ρ Oph and IC 348 (where we are limited by a combination of extinction and distance). We find that: (1) classical T Tauri-like disk-accretion persists in the sub-stellar domain down to nearly the deuterium-burning limit; (2) while an Hα 10% width & 200 km s is our prime accretion diagnostic (following our previous work), permitted emission lines of Ca II , O I and He I are also good accretion indicators, just as in CTTs (we caution against a blind use of Hα width alone, since inclination and rotation effects on the line are especially important at the low accretion rates in these objects); (3) the Ca II 8662Å line flux is an excellent quantitative measure of the accretion rate in very low-mass stars and brown dwarfs (as in higher-mass CTTs), correlating remarkably well with the Ṁ obtained from veiling and Hα-modeling; (4) the accretion rate diminishes rapidly with mass – our measurements support previous suggestions that Ṁ ∝ M∗ (albeit with considerable scatter), and extend this correlation to the entire range of sub-stellar masses; (5) the fraction of very low-mass stellar and brown dwarf accretors decreases substantially with age, as Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138. [email protected] Department of Astronomy & Astrophysics, University of Toronto, Toronto, ON M5S 3H8, CANADA. [email protected] Astronomy Department, University of California, Berkeley, CA 94720. [email protected]