Measuring accretion in young substellar objects:: Approaching the planetary mass regime

We present observations of H alpha emission-line profiles taken at Magellan Observatory for a sample of 39 young low-mass stars and brown dwarfs in the Taurus and Chamaeleon I star-forming regions. We have identified 11 new substellar accretors, more than tripling the number of known brown dwarfs with measurable accretion activity. These include the lowest-mass objects yet seen with accretion, with masses down to similar to 0.015 M-circle dot. Using models of H alpha emission produced in magnetospheric accretion flows, the most widely applicable primary calibrator now available, we determine the first estimates of mass accretion rates for objects at such extremely low masses. For the six objects with masses less than or similar to 0.03M(circle dot), we find accretion rates of similar to 5 x 10(-12) M(circle dot)yr(-1), among the smallest yet measured. These new results continue the trend of decreasing mass accretion rate with decreasing (sub) stellar mass that we have noted previously for samples of more massive objects; the overall correlation is. M proportional to M-2.1, and it now extends over a mass range of over 2 orders of magnitude. Finally, the absence of a discontinuity in the distribution of accretion rates with mass tends to suggest that stars and brown dwarfs share similar formation histories.