Structure of photodissociation fronts in star-forming regions revealed by observations of high-J CO emission lines with Herschel.

Context: In bright photodissociation regions (PDRs) associated to massive star formation, the presence of dense "clumps" that are immersed in a less dense interclump medium is often proposed to explain the difficulty of models to account for the observed gas emission in high-excitation lines.

Aims: We aim at presenting a comprehensive view of the modeling of the CO rotational ladder in PDRs, including the high-J lines that trace warm molecular gas at PDR interfaces.

Methods: We observed the 12 CO and 13 CO ladders in two prototypical PDRs, the Orion Bar and NGC 7023 NW using the instruments onboard Herschel . We also considered line emission from key species in the gas cooling of PDRs (C+ , O, H2 ) and other tracers of PDR edges such as OH and CH+ . All the intensities are collected from Herschel observations, the literature and the Spitzer archive and are analyzed using the Meudon PDR code.

Results: A grid of models was run to explore the parameter space of only two parameters: thermal gas pressure and a global scaling factor that corrects for approximations in the assumed geometry. We conclude that the emission in the high-J CO lines, which were observed up to J up =23 in the Orion Bar (J up =19 in NGC 7023), can only originate from small structures of typical thickness of a few 10-3 pc and at high thermal pressures ( P th ~ 108 K cm-3 ).

Conclusions: Compiling data from the literature, we found that the gas thermal pressure increases with the intensity of the UV radiation field given by G 0 , following a trend in line with recent simulations of the photoevaporation of illuminated edges of molecular clouds. This relation can help rationalising the analysis of high-J CO emission in massive star formation and provides an observational constraint for models that study stellar feedback on molecular clouds.