Accretion in brown dwarfs:: An infrared view

This paper presents a study of the accretion properties of 19 very low mass objects (M-star similar to0.01-0.1 M-circle dot) in the regions Chamaeleon I and rho Oph. For 8 objects we obtained high resolution Halpha profiles and determined mass accretion rate (M)over dot(ac) and accretion luminosity L-ac. Pabeta is detected in emission in 7 of the 10 rho Oph objects, but only in one in Cha I. Using objects for which we have both a determination of L-ac from Halpha and a Pabeta detection, we show that the correlation between the Pabeta luminosity and luminosity L-ac, found by Muzerolle et al. (1998) for T Tauri stars in Taurus, extends to objects with mass similar to0.03 M-circle dot; L(Pabeta) can be used to measure L-ac also in the substellar regime. The results were less conclusive for Brgamma, which was detected only in 2 objects, neither of which had an Halpha estimate of (M)over dot(ac). Using the relation between L(Pabeta) and L-ac we determined the accretion rate for all the objects in our sample (including those with no Halpha spectrum), more than doubling the number of substellar objects with known When plotted as a function of the mass of the central object together with data from the literature, our results confirm the trend of lower (M)over dot(ac) for lower M-star, although with a large spread. Some of the spread is probably due to an age effect; our very young objects in rho Oph have on average an accretion rate at least one order of magnitude higher than objects of similar mass in older regions. As a side product, we found that the width of Ha measured at 10% peak intensity is not only a qualitative indicator of the accreting nature of very low mass objects, but can be used to obtain a quantitative, although not very accurate, estimate of (M)over dot(ac) over a large mass range, from T Tauri stars to brown dwarfs. Finally, we found that some of our objects show evidence of mass-loss in their optical spectra.