Accretion in brown dwarfs: An infrared view

This paper presents a study of the accretion properties of 19 very low mass objects ($M_\star$~ $0.01{-}0.1$ $M_\odot$) in the regions Chamaeleon I and ρ  Oph. For 8 objects we obtained high resolution H α  profiles and determined mass accretion rate $\dot M_{\rm ac}$  and accretion luminosity L ac . Pa β  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 H α and a Pa β  detection, we show that the correlation between the Pa β  luminosity and luminosity L ac , found by Muzerolle et al. ([CITE]) for T Tauri stars in Taurus, extends to objects with mass ~0.03 $M_\odot$; L (Pa β ) can be used to measure L ac  also in the substellar regime. The results were less conclusive for Br γ , which was detected only in 2 objects, neither of which had an H α  estimate of $\dot M_{\rm ac}$. Using the relation between L (Pa β ) and L ac  we determined the accretion rate for all the objects in our sample (including those with no H α spectrum), more than doubling the number of substellar objects with known $\dot M_{\rm ac}$. 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 $\dot M_{\rm ac}$ 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 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 H α  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 $\dot M_{\rm ac}$ 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.