Accretion in brown dwarfs: An infrared view ^*,**

¹ Osservatorio Astrofisico di Arcetri, INAF, Largo E. Fermi 5, 50125 Firenze, Italy e-mail: natta@arcetri.astro.it
² Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
³ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
⁴ Istituto Astrofisica Spaziale e Fisica Cosmica, CNR, via del Fosso del Cavaliere, 00133 Roma, Italy

Received: 27 February 2004
Accepted: 26 May 2004

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.