The radiative feedback of massive stars on molecular clouds creates pillars , globules and other features at the interface between the H II 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 HOBYS ( Herschel imaging survey of OB Young Stellar objects ) 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 ( \sim 500 M _ { \odot } ) and large ( equivalent radius r \sim 0.6 pc ) structures , corresponding to what is defined as ‘ clumps ’ for molecular clouds .
EGGs and proplyd-like objects are smaller ( r \sim 0.1 and 0.2 pc ) and less massive ( \sim 10 and \sim 30 M _ { \odot } ) .
Cloud condensations are small ( \sim 0.1 pc ) , have an average mass of 35 M _ { \odot } , are dense ( \sim 6 \times 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 photoevaporation 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 .