  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 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. In this paper, we
analyze the spatial variation of the disk fraction in Cygnus OB2 and we study
its correlation with the local values of Far and Extreme ultraviolet radiation
fields and the local stellar surface density. We present definitive 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. We find that disk dissipation due to close
stellar encounters is negligible in Cygnus OB2, and likely to have affected 1%
or fewer of the stellar population. Disk dissipation is instead dominated by
photoevaporation. We also compare our results to what has been found in other
young clusters with different massive populations, concluding 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.
