ASpitzer Space TelescopeStudy of Disks in the Young σ Orionis Cluster

We report new Spitzer Space Telescope observations from the IRAC and MIPSinstruments of the young (~ 3 Myr) sigma Orionis cluster. We identify 336 starsas members of the cluster using optical and near-infrared color magnitudediagrams. Using the spectral energy distribution (SED) slopes in the IRACspectral range, we place objects in several classes: non-excess stars, starswith optically thick disks(like classical T Tauri stars), class I(protostellar) candidates, and stars with ``evolved disks''; the last exhibitsmaller IRAC excesses than optically thick disk systems. In general, thisclassification agrees with the location expected in IRAC-MIPS color-colordiagrams for these objects. We find that the evolved disk systems are mostly acombination of objects with optically thick but non-flared disks, suggestinggrain growth and/or settling, and transition disks, systems in which the innerdisk is partially or fully cleared of small dust. In all, we identify 7transition disk candidates and 3 possible debris disk systems. As in otheryoung stellar populations, the fraction of disks depends on the stellar mass,ranging from ~10% for stars in the Herbig Ae/Be mass range (>2 msun) to ~35% inthe T Tauri mass range (1-0.1 msun). We find that the disk fraction does notdecrease significantly toward the brown dwarf candidates (<0.1 msun). The IRACinfrared excesses found in stellar clusters and associations with and withoutcentral high mass stars are similar, suggesting that external photoevaporationis not very important in many clusters. Finally, we find no correlation betweenthe X-ray luminosity and the disk infrared excess, suggesting that the X-raysare not strongly affected by disk accretion.Comment: 44pages, 17 figures. Sent to Ap