Star and Protoplanetary Disk Properties in Orion's Suburbs

This contribution aims to advance our knowledge of the physical mechanisms driving accretion onto pre-main sequence stars and the dissipation of their circumstellar disks, which are currently not well understood. We present results of a survey of the L1630N and L1641 star-forming clouds in Orion. For a sample of ~400 YSOs we determine their fundamental stellar parameters, disk geometry and accretion rates from a combined data set of optical and infrared photometry and optical spectroscopy. We show that stars residing in clusters or aggregates disperse their disks faster than those formed in isolation, even in very small clusters with low number densities, and in the absence of OB stars. We exclude primordial binarity as a mechanism causing a "transition disk" appearance. We argue that our observations provide observational support for a scenario put forward by Hartmann et al. (2006), in which disks around low-mass stars and brown dwarfs are fully viscous, yielding a steep dependency of the accretion rate on stellar mass of αM_accMαM_* 5/2, and stars in the ~solar to intermediate mass regime have substantial dead zones in the disk interior resulting in a shallower relation of approximately αM_accαM_*.