Astronomy & Astrophysics

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A&A 424, 603-612 (2004)
DOI: 10.1051/0004-6361:20040356

Accretion in brown dwarfs: An infrared view

A. Natta¹, L. Testi¹, J. Muzerolle², S. Randich¹, F. Comerón³ and P. Persi⁴

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 ( $M_\star$ ~ 0.01-0.1 $M_\odot$ ) in the regions Chamaeleon I and $\rho$ Oph. For 8 objects we obtained high resolution H $\alpha$ profiles and determined mass accretion rate $\dot M_{\rm ac}$ and accretion luminosity $L_{\rm ac}$ . Pa $\beta$ is detected in emission in 7 of the 10 $\rho$ Oph objects, but only in one in Cha I. Using objects for which we have both a determination of $L_{\rm ac}$ from H $\alpha$ and a Pa $\beta$ detection, we show that the correlation between the Pa $\beta$ luminosity and luminosity $L_{\rm ac}$ , found by Muzerolle et al. (1998) for T Tauri stars in Taurus, extends to objects with mass ~0.03 $M_\odot$ ; L(Pa $\beta$ ) can be used to measure $L_{\rm ac}$ also in the substellar regime. The results were less conclusive for Br $\gamma$ , which was detected only in 2 objects, neither of which had an H $\alpha$ estimate of $\dot M_{\rm ac}$ . Using the relation between L(Pa $\beta$ ) and $L_{\rm ac}$ we determined the accretion rate for all the objects in our sample (including those with no H $\alpha$ 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 $\rho$ 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 $\alpha$ 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.

Key words: stars: low-mass, brown dwarfs -- stars: formation -- stars: activity -- line: profiles -- accretion, accretion disks

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