2007ApJ...662.1067H

We report new Spitzer Space Telescope observations, using the IRAC and MIPS instruments, of the young (∼3 Myr) σ Orionis cluster. We identify 336 stars as members of the cluster, using optical and near-infrared color-magnitude diagrams. Using the spectral energy distribution slopes in the IRAC spectral range, we place objects into several classes: non-excess stars, stars with optically thick disks (such as classical T Tauri stars), class I (protostellar) candidates, and stars with ``evolved disks''; the last exhibit smaller IRAC excesses than optically thick disk systems. In general, this classification agrees with the location expected in IRAC-MIPS color-color diagrams for these objects. We find that the evolved disk systems are mostly a combination of objects with optically thick but nonflared disks, suggesting grain growth and/or settling, and transition disks, systems in which the inner disk is partially or fully cleared of small dust. In all, we identify seven transition disk candidates and three possible debris disk systems. As in other young stellar populations, the fraction of disks depends on the stellar mass, ranging from ∼10% for stars in the Herbig Ae/Be mass range (>2 M_☉) to ∼35% for those in the T Tauri mass range (1-0.1 M_☉). The IRAC infrared excesses found in stellar clusters and associations with and without central high-mass stars are similar, suggesting that external photoevaporation is not very important in many clusters. Finally, we find no correlation between the X-ray luminosity and the disk infrared excess, suggesting that the X-rays are not strongly affected by disk accretion.