Transition disks in Lambda Orionis low mass members

D. Barrado. J.R. Stauffer, M. Morales-Calderon, A. Bayo

We present multi-wavelength optical and infrared photometry of 170 previously known low mass 
stars and brown dwarfs of the 5 Myr Collinder 69 cluster (Lambda Orionis). The near
infrared photometry allows us to identify stars with IR excesses, and we find that the Class 
II population is very large, around 25% for stars and 40% for brown dwarfs, 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 inhomogenous way, lying preferently in a 
filament running toward the south-east.  The IR excesses for the Collinder 69 members range 
from pure Class II (flat or nearly flat spectra longward of 1 micron), 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 micron. Collinder 69 thus appears to be at an age 
where it provides a natural laboratory for the study of primordial disks and their 
dissipation.