Accretion in brown dwarfs: An infrared view

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Abstract
This paper presents a study of the accretion properties of 19 very low mass objects (~ ) in the regions Chamaeleon I and ρ Oph. For 8 objects we obtained high resolution Hα profiles and determined mass accretion rate 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. ([CITE]) for T Tauri stars in Taurus, extends to objects with mass ~0.03 ; 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 . 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 . 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 for lower , 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 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.
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