Probing Disk Accretion in Young Brown Dwarfs

We present high-resolution optical spectra of 15 objects near or below thesub-stellar limit in the Upper Scorpius and $\rho$ Ophiuchus star-formingregions. These spectra, obtained with the HIRES instrument on the Keck Itelescope, are used to investigate disk accretion, rotation and activity inyoung very low mass objects. We report the detection of a broad, asymmetricH$\alpha$ emission line in the $\rho$ Oph source GY 5 which is also known toharbor mid-infrared excess, consistent with the presence of an accreting disk.The H$\alpha$ profiles of the Upper Sco objects suggest little or no on-goingaccretion. Our results imply that if most brown dwarfs are born with disks,their accretion rates decrease rapidly, at timescales comparable to or smallerthan those for T Tauri disks. The Upper Sco brown dwarfs appear to be rotatingfaster than their somewhat younger counterparts in Taurus, consistent withspin-up due to contraction following disk unlocking. The H$\alpha$ activity iscomparable to saturated activity levels in field M dwarfs with similar spectraltype and rotation rates. Comparison of our data with published (albeitlower-resolution) spectra of a few of the same objects from other epochssuggests possible variability in accretion/activity indicators.Comment: to appear in The Astrophysical Journal Letter