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

This paper presents the results of an U band survey with FORS1/VLT of a large area in the sigma 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 similar to 0.06 to similar to 1.2 M-circle dot; 72 of them show evidence of disks and we measure mass accretion rates (M) over dot(acc) between <10(-11) and few 10(-9) M-circle dot/y, using the colors of the diskless stars as photospheric templates. Our results confirm the dependence of (M) over dot(acc) on the mass of the central object, which is stronger for low-mass stars and flattens out for masses larger than similar to 0.3 M-circle dot; the spread of (M) over dot(acc) for any value of the stellar mass is similar to 2 orders of magnitude. We discuss the implications of these results in the context of disk evolution models. Finally, we analyze the relation between. (M) over dot(acc) and the excess emission in the Spitzer bands, and find that at (M) over dot(acc) similar to 10(-10) M-circle dot/y the inner disks change from optically thin to optically thick.