Globules and pillars in Cygnus X - I. Herschel far-infrared imaging of the Cygnus 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. Optical and near-infrared observations from the ground as well as with the Hubble or Spitzer satellites have revealed numerous examples of such cloud structures. We present here Herschel far-infrared observations between 70 mu m and 500 mu m of the immediate environment of the rich Cygnus OB2 association, performed within the Herschel imaging survey of OB Young Stellar objects (HOBYS) program. All of the observed irradiated structures were detected based on their appearance at 70 mu m, and have been classified as pillars, globules, evaporating gasous globules (EGGs), proplyd-like objects, and condensations. From the 70 mu m and 160 mu m flux maps, we derive the local far-ultraviolet (FUV) field on the photon dominated 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) (Habing field) 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 spectral energy distribution (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 are massive (similar to 500 M-circle dot) and large (equivalent radius r similar to 0.6 pc) structures, corresponding to what is defined as "clumps" for molecular clouds. EGGs and proplyd-like objects are smaller (r similar to 0.1 and 0.2 pc) and less massive (similar to 10 and similar to 30 M-circle dot). Cloud condensations are small (similar to 0.1 pc), have an average mass of 35 M-circle dot, are dense (similar to 6 x 10(4) cm(-3)), and can thus be described as molecular cloud "cores". All pillars and globules are oriented toward the Cyg OB2 association center and have the longest estimated photo-evaporation lifetimes, a few million years, while all other features should survive less than a million years. 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.