Low-mass stars and accretion at the ages of planet formation in the cepheus OB2 region

We present the first identification of low-mass (spectral types K-M) stars in the young clusters Tr 37 and NGC 7160, members of the CepOB2 association. This is part of a program to follow the evolution of protoplanetary accretion disks through the ages thought to be crucial to understanding disk dissipation and planet formation (similar to3-10 Myr). Combining optical photometry and optical spectroscopy, we have identified similar to40 members in Tr 37 and similar to15 in NGC 7160, using several independent tests for determining the membership (optical colors, optical variability, Halpha emission, and Li lambda6707 absorption). We confirm previous age estimates of 1-5 Myr for Tr 37 and 10 Myr for NGC 7160. We find active accretion in some of the stars in Tr 37, with average accretion rates of similar to10(-8) M. yr(-1), derived from their U-band excesses. These results expand the existing samples of accreting stars and are consistent with the models of viscous accretion disk evolution. No signs of active accretion have been detected so far in the older cluster NGC 7160, suggesting that disk accretion ends before the age of 10 Myr. These results are consistent with those from other populations and are a clear sign of disk evolution within the CepOB2 region. We also investigate the spatial asymmetries in Tr 37 and the possible presence of younger populations triggered by Tr 37 itself, and we outline an efficient method to detect and study the rest of the clusters members and their characteristics.