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 m telescope, we confirm the pre-main-sequence status and cluster membership of the substellar candidates, through the detection of Li I, Na I consistent with low gravity, and radial velocity. Given their late spectral type (similar toM8) and the youth of the association (age similar to 10 Myr), cluster membership certifies these three objects as young brown dwarfs. One of them (2MASS 1207-3932) shows strong emission both in the hydrogen Balmer series (Halpha to Hepsilon) and in He I (4471, 5876, 6678, and 7065 Angstrom), compared with other young brown dwarfs of similar spectral type. The Halpha line is also relatively broad (10% width similar to200 km s(-1)) and asymmetric. These characteristics suggest that 2MASS 1207-3932 is a (weak) accretor. While we cannot rule out activity, comparison with a flaring field dwarf implies that such activity would have to be quite anomalous. The verification of accretion would make it 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. The close triple TWA 5A also appears to be a variable accretor, implying that long-lived disks can exist in multiple systems.