Magellan Echelle Spectroscopy of Tw Hydrae Brown Dwarfs

We present high-resolution optical spectroscopy of four candidate members of the nearby TW Hydrae young association including three brown dwarfs (2MASS 1207-3932, 2MASS 1139-3159 and TWA 5B) and one T Tauri multiple star (TWA 5A). Using echelle spectra from the Magellan Baade 6.5-meter telescope, we confirm the pre-main sequence status and cluster membership of the substellar candidates , through the detection of LiI, NaI consistent with low gravity, and radial velocity. Given their late spectral type (∼M8) and the youth of the association (age ∼ 10Myr), cluster membership certifies these three objects as very low-mass young brown dwarfs. One of them (2MASS 1207-3932) shows strong emission both in the Hydrogen Balmer series (Hα to Hǫ) and in HeI (4471, 5876, 6678 and 7065Å), compared to other young brown dwarfs of similar spectral type. The Hα line is also relatively broad (10% width ∼ 200 km s −1) and asymmetric. These characteristics suggest that 2MASS 1207-3932 is a (weak) accretor. While we cannot rule out activity, comparison to a flaring field dwarf implies that such activity would have to be quite anomalous. The verification of accretion would make 2MASS 1207-3932 the oldest actively accreting brown dwarf known to date, suggesting that inner disk lifetimes in substellar objects can be comparable to those in stars, consistent with a similar formation mechanism. Finally, TWA 5A appears to be a variable accretor: observations separated by two days show broad accretion-like Hα (10% width ∼270 km s −1), with significant changes in the Hα profile, as well as in the strengths of HeI, Na D and [OI6300]. TWA 5A is known to be a close triple; thus, our result implies that long-lived disks can exist even in multiple systems. – 2 – Subject headings: stars: low mass, brown dwarfs — stars: pre-main-sequence — circumstellar matter — open clusters and associations: individual (TW Hydrae)