Astronomy & Astrophysics

Issue		A&A Volume 459, Number 3, December I 2006
Page(s)		837 - 842
Section		Interstellar and circumstellar matter
DOI		10.1051/0004-6361:20065575

A&A 459, 837-842 (2006)
DOI: 10.1051/0004-6361:20065575

Accretion rates in Herbig Ae stars

R. Garcia Lopez^{1, 2}, A. Natta¹, L. Testi¹, and E. Habart^{1, 3}

¹ INAF - Osservatorio Astrofisico di Arcetri, Largo Fermi 5, 50125 Firenze, Italy
e-mail: natta@arcetri.astro.it
² Università degli Studi di Roma "Tor Vergata" - Dipartimento di Fisica, via della Ricerca Scientifica 1, 00133 Roma, Italy
³ Université Paris-Sud, 91405 Orsay, France

(Received 9 May 2006 / Accepted 29 August 2006 )

Abstract
Aims.Accretion rates from disks around pre-main sequence stars are of importance for our understanding of planetary formation and disk evolution. We provide in this paper estimates of the mass accretion rates in the disks around a large sample of Herbig Ae stars.
Methods.We obtained medium resolution 2 $\mu$ m spectra and used the results to compute values of $\dot M_{\rm acc}$ from the measured luminosity of the Br $_\gamma$ emission line, using a well established correlation between L(Br $_\gamma$ ) and the accretion luminosity $L_{\rm acc}$ .
Results.We find that 80% of the stars, all of which have evidence of an associated circumstellar disk, are accreting matter, with rates $3\times 10^\mathbin{\lower 3pt\hbox {$\rlap{\raise 5pt\hbox{$\char'074$ }}... ...ower 3pt\hbox {$\rlap{\raise 5pt\hbox{$\char'074$ }}\mathchar''7218$ }}10^$ $M_\odot$ /yr; for 7 objects, 6 of which are located on the ZAMS in the HR diagram, we do not detect any line emission. Few HAe stars (25%) have $\dot M_{\rm acc}$ >10^-7 $M_\odot$ /yr.
Conclusions.In most HAe stars the accretion rate is sufficiently low that the gas in the inner disk, inside the dust evaporation radius, is optically thin and does not prevent the formation of a puffed-up rim, where dust is directly exposed to the stellar radiation. When compared to the $\dot M_{\rm acc}$ values found for lower-mass stars in the star forming regions Taurus and Ophiuchus, HAe stars have on average higher accretion rates than solar-mass stars; however, there is a lack of very strong accretors among them, probably due to the fact that they are on average older.

Key words: stars: formation -- stars: circumstellar matter -- stars: pre-main sequence -- accretion, accretion disks

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