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

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 Macc 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 ̇ of Macc for any value of the stellar mass is ∼2 orders of magnitude. We discuss the implications of these results in the context of ̇ Macc 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 ̇ disk evolution models. Finally, we analyze the relation between Macc and the excess emission in the Spitzer bands, and find that at ̇ Macc ∼ 10−10 M⊙/y the inner disks change from optically thin to optically thick.