Evidence for a T Tauri Phase in Young Brown Dwarfs

As part of a multifaceted program to investigate the origin and early evolution of substellar objects, we present high-resolution Keck optical spectra of 14 very low mass sources in the IC 348 young cluster and the Taurus star-forming cloud. All of our targets, which span a range of spectral types from M5 to M8, exhibit moderate to very strong Hα emission. In half of the IC 348 objects, the Hα profiles are broad and asymmetric, indicative of ongoing accretion. Of these, IC 348-355 (M8) is the lowest mass object to date to show accretion-like Hα. Three of our ~M6 IC 348 targets with broad Hα also harbor broad O I (8446 Å) and Ca II (8662 Å) emission, and one shows broad He I (6678 Å) emission; these features are usually seen in strongly accreting classical T Tauri stars. We find that in very low mass accretors, the Hα profile may be somewhat narrower than that in higher mass stars. We propose that low accretion rates combined with small infall velocities at very low masses can conspire to produce this effect. In the nonaccretors in our sample, Hα emission is commensurate with, or higher than, saturated levels in field M dwarfs of similar spectral type. Our results constitute the most compelling evidence to date that young brown dwarfs undergo a T Tauri-like accretion phase similar to that in stars. This is consistent with a common origin for most low-mass stars, brown dwarfs, and isolated planetary mass objects.