Spitzer:: Accretion in low-mass stars and brown dwarfs in the λ orionis cluster

We present multiwavelength optical and IR photometry of 170 previously known low-mass stars and brown dwarfs of the 5 Myr Collinder 69 cluster (lambda Orionis). The new photometry supports cluster membership for most of them, with less than 15% of the previous candidates identified as probable nonmembers. The near-IR photometry allows us to identify stars with IR excesses, and we find that the Class II population is very large, around 25% for stars (in the spectral range M0-M6.5) and 40% for brown dwarfs, down to 0.04 M (circle dot), despite the fact that the H alpha equivalent width is low for a significant fraction of them. In addition, there are a number of substellar objects, classified as Class III, that have optically thin disks. The Class II members are distributed in an inhomogeneous way, lying preferentially in a filament running toward the southeast. The IR excesses for the Collinder 69 members range from pure Class II (flat or nearly flat spectra longward of 1 alpha m), to transition disks with no near-IR excess but excesses beginning within the IRAC wavelength range, to two stars with excess only detected at 24 mu m. Collinder 69 thus appears to be at an age where it provides a natural laboratory for the study of primordial disks and their dissipation.