U-band study of the accretion properties in the σ Orionis star-forming region^⋆,⋆⋆

¹ Osservatorio Astrofisico di Arcetri, INAF, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: erigliaco@arcetri.astro.it
² Università di Firenze, Dipartimento di Astronomia, Largo E. Fermi 2, 50125 Firenze, Italy
³ ESO, Karl-Schwarszchild Strasse 2, 85748 Garching bei München, Germany

Received: 29 June 2010
Accepted: 9 October 2010

Abstract

This paper presents the results of an U band survey with FORS1/VLT of a large area in the σ Ori star-forming region. We combine the U-band photometry with literature data to compute accretion luminosity and mass accretion rates from the U-band excess emission for all objects (187) detected by Spitzer in the FORS1 field and classified by Hernandez et al. as likely members of the cluster. The sample stars range in mass from  ~0.06 to  ~1.2 M_⊙; 72 of them show evidence of disks and we measure mass accretion rates Ṁ_acc between  <10^-11 and few 10^-9 M_⊙/y, using the colors of the diskless stars as photospheric templates. Our results confirm the dependence of Ṁ_acc on the mass of the central object, which is stronger for low-mass stars and flattens out for masses larger than  ~0.3 M_⊙; the spread of Ṁ_acc for any value of the stellar mass is  ~2 orders of magnitude. We discuss the implications of these results in the context of disk evolution models. Finally, we analyze the relation between Ṁ_acc and the excess emission in the Spitzer bands, and find that at Ṁ_acc  ~ 10^-10 M_⊙/y the inner disks change from optically thin to optically thick.