Globules and Pillars in Cygnus X I. Herschel Far-infrared imaging of the
  Cyg OB2 environment
  The radiative feedback of massive stars on molecular clouds creates pillars,
globules and other features at the interface between the HII region and
molecular cloud. We present here Herschel observations between 70 and 500
micron of the immediate environment of the Cygnus OB2 association, performed
within the HOBYS program. All structures were detected based on their
appearance at 70 micron, and have been classified as pillars, globules,
evaporating gasous globules (EGGs), proplyd-like objects, and condensations.
From the 70 and 160 micron flux maps, we derive the local FUV field on the PDR
surfaces. In parallel, we use a census of the O-stars to estimate the overall
FUV-field, that is 10^3-10^4 G_0 close to the central OB cluster (within 10 pc)
and decreases down to a few tens G_0, in a distance of 50 pc. From a SED fit to
the four longest Herschel wavelengths, we determine column density and
temperature maps and derive masses, volume densities and surface densities for
these structures. We find that the morphological classification corresponds to
distinct physical properties. Pillars and globules have the longest estimated
photoevaporation lifetimes, a few 10^6 yr, while all other features should
survive less than that. These lifetimes are consistent with that found in
simulations of turbulent, UV-illuminated clouds. We propose a tentative
evolutionary scheme in which pillars can evolve into globules, which in turn
then evolve into EGGs, condensations and proplyd-like objects.
