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The radiative feedback of massive stars on molecular clouds creates pillars,globules and other features at the interface between the HII region andmolecular cloud. We present here Herschel observations between 70 and 500micron of the immediate environment of the Cygnus OB2 association, performedwithin the HOBYS program. All structures were detected based on theirappearance 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 PDRsurfaces. In parallel, we use a census of the O-stars to estimate the overallFUV-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 tothe four longest Herschel wavelengths, we determine column density andtemperature maps and derive masses, volume densities and surface densities forthese structures. We find that the morphological classification corresponds todistinct physical properties. Pillars and globules have the longest estimatedphotoevaporation lifetimes, a few 10^6 yr, while all other features shouldsurvive less than that. These lifetimes are consistent with that found insimulations of turbulent, UV-illuminated clouds. We propose a tentativeevolutionary scheme in which pillars can evolve into globules, which in turnthen evolve into EGGs, condensations and proplyd-like objects.

Globules and Pillars in Cygnus X I. Herschel Far-infrared imaging of the Cyg OB2 environment