Probing Disk Accretion in Young Brown Dwarfs

We present high-resolution optical spectra of 15 objects near or below the sub-stellar limit in the Upper Scorpius and ρ Ophiuchus star-forming regions. These spectra, obtained with the HIRES instrument on the Keck I telescope, are used to investigate disk accretion, rotation and activity in young very low mass objects. We report the detection of a broad, asymmetric Hα emission line in the ρ Oph source GY 5 which is also known to harbor mid-infrared excess, consistent with the presence of an accreting disk. The Hα profiles of the Upper Sco objects suggest little or no on-going accretion. Our results imply that if most brown dwarfs are born with disks, their accretion rates decrease rapidly, at timescales comparable to or smaller than those for T Tauri disks. The Upper Sco brown dwarfs appear to be rotating faster than their somewhat younger counterparts in Taurus, consistent with spin-up due to contraction following disk unlocking. The Hα activity is comparable to saturated activity levels in field M dwarfs with similar spectral type and rotation rates. Comparison of our data with published (albeit lower-resolution) spectra of a few of the same objects from other epochs suggests possible variability in accretion/activity indicators. Subject headings: stars: low mass, brown dwarfs – stars: pre-main-sequence – circumstellar matter