Accretion in brown dwarfs: An infrared view

Abstract

This paper presents a study of the accretion properties of 19 very low mass objects (M*∼0.01-0.1 M⊙) in the regions Chamaeleon I and ρ Oph. For 8 objects we obtained high resolution Hα profiles and determined mass accretion rate Ṁac and accretion luminosity Lac. 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 Lac from Hα and a Paβ detection, we show that the correlation between the Paβ luminosity and luminosity L ac, found by Muzerolle et al. (1998) for T Tauri stars in Taurus, extends to objects with mass ∼0.03 M⊙; L(Paβ) can be used to measure Lac also in the substellar regime. The results were less conclusive for Bry, which was detected only in 2 objects, neither of which had an Hα estimate of Ṁac. Using the relation between L(Paβ) and Lac 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 Ṁac. 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 Ṁac 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 Ṁac 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.