2004A&A...424..603N

This paper presents a study of the accretion properties of 19 very low mass objects (M_*∼0.01-0.1M_☉) in the regions Chamaeleon I and ρ Oph. For 8 objects we obtained high resolution Hα profiles and determined mass accretion rate {dot}(M)_ac and accretion luminosity L_ac. Paβ is detected in emission in 7 of the 10 ρ Oph objects, but only in one in Cha I. Using objects for which we have both a determination of L_ac from Hα and a Paβ detection, we show that the correlation between the Paβ luminosity and luminosity L_ac, found by Muzerolle et al. (1998AJ....116.2965M) for T Tauri stars in Taurus, extends to objects with mass ∼0.03M_☉; L(Paβ) can be used to measure L_ac also in the substellar regime. The results were less conclusive for Brγ, which was detected only in 2 objects, neither of which had an Hα estimate of {dot}(M)_ac. Using the relation between L(Paβ) and L_ac we determined the accretion rate for all the objects in our sample (including those with no Hα spectrum), more than doubling the number of substellar objects with known {dot}(M)_ac. 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 {dot}(M)_ac for lower M_*, although with a large spread. Some of the spread is probably due to an age effect; our very young objects in ρ 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 Hα 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 {dot}(M)_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.