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

TL;DR

This study uses U-band photometry combined with literature data to measure accretion rates in the sigma Orionis star-forming region, revealing how accretion depends on stellar mass and disk properties.

Contribution

It provides new measurements of mass accretion rates across a range of stellar masses using U-band excess emission, and analyzes the relation between accretion and disk evolution.

Findings

Accretion rates range from <10^{-11} to a few 10^{-9} Msun/yr.

Accretion dependence on stellar mass is stronger for low-mass stars.

Inner disks transition from optically thin to thick at Macc ~10^{-10} Msun/yr.

Abstract

This paper presents the results of an U band survey with FORS1/VLT of a large area in the sigma Orionis 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. (2007) as likely members of the cluster. The sample stars range in mass from ~0.06 to ~1.2 Msun; 72 of them show evidence of disks and we measure mass accretion rates Macc between <10^{-11} and few 10^{-9} Msun/yr, using the colors of the diskless stars as photospheric templates. Our results confirm the dependence 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 Msun; the spread 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 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} Msun/yr the inner disks change from optically thin to optically thick.