SPITZER OBSERVATIONS OF THE λ ORIONIS CLUSTER. II. DISKS AROUND SOLAR-TYPE AND LOW-MASS STARS

We present IRAC/MIPS Spitzer Space Telescope observations of the solar-type and the low-mass stellar population of the young (similar to 5Myr) lambda Orionis cluster. Combining optical and Two Micron All Sky Survey photometry, we identify 436 stars as probable members of the cluster. Given the distance (450 pc) and the age of the cluster, our sample ranges in mass from 2 M-circle dot to objects below the substellar limit. With the addition of the Spitzer mid-infrared data, we have identified 49 stars bearing disks in the stellar cluster. Using spectral energy distribution slopes, we place objects in several classes: non-excess stars (diskless), stars with optically thick disks, stars with "evolved disks" (with smaller excesses than optically thick disk systems), and "transitional disk" candidates (in which the inner disk is partially or fully cleared). The disk fraction depends on the stellar mass, ranging from similar to 6% for K-type stars (R-C - J < 2) to similar to 27% for stars with spectral-type M5 or later (R-C - J > 4). We confirm the dependence of disk fraction on stellar mass in this age range found in other studies. Regarding clustering levels, the overall fraction of disks in the. Orionis cluster is similar to those reported in other stellar groups with ages normally quoted as similar to 5 Myr.