Photoevaporation and close encounters: How the environment around Cygnus OB2 affects the evolution of protoplanetary disks | Mario Guarcello

In our Galaxy, star formation occurs in a variety of environments, with a large fraction of stars formed in clusters hosting massive stars. OB stars have an important feedback on the evolution of protoplanetary disks orbiting around nearby young stars and likely on the process of planet formation occurring in them. The nearby massive association Cygnus OB2 is an outstanding laboratory to study this feedback. It is the closest massive association to our Sun, and hosts hundreds of massive stars and thousands of low mass members, both with and without disks. We have analyzed the spatial variation of the disk fraction (i.e. the fraction of cluster members bearing a disk) in Cygnus OB2 and and studied its correlation with the local values of Far and Extreme ultraviolet radiation fields and the local stellar surface density. We found evidence that disks are more rapidly dissipated in the regions of the association characterized by intense local UV field and large stellar density. In particular, the FUV radiation dominates disks dissipation timescales in the proximity (i.e. within 0.5 pc) of the O stars. In the rest of the association, EUV photons potentially induce a significant mass loss from the irradiated disks across the entire association, but the efficiency of this process is reduced at increasing distances from the massive stars due to absorption by the intervening intracluster material. Comparing our results to what has been found in other young clusters with different massive populations, it is possible to conclude that massive associations like Cygnus OB2 are potentially hostile to protoplanetary disks, but that the environments where disks can safely evolve in planetary systems are likely quite common in our Galaxy.