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 12CO and 13CO 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 (Pth ~ 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 G0, 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.

Top-down formation of fullerenes in the interstellar medium

Fullerenes have been recently detected in various circumstellar and interstellar environments, raising the question of their formation pathway. It has been proposed that they can form at the low densities found in the interstellar medium by the photo-chemical processing of large polycyclic aromatic hydrocarbons (PAHs). Following our previous work on the evolution of PAHs in the NGC 7023 reflection nebula, we evaluate, using photochemical modeling, the possibility that the PAH C66H20 (i.e. circumovalene) can lead to the formation of C60 upon irradiation by ultraviolet photons. The chemical pathway involves full dehydrogenation of C66H20, folding into a floppy closed cage and shrinking of the cage by loss of C2 units until it reaches the symmetric C60 molecule. At 10" from the illuminating star and with realistic molecular parameters, the model predicts that 100% of C66H20 is converted into C60 in ~ 105 years, a timescale comparable to the age of the nebula. Shrinking appears to be the kinetically limiting step of the whole process. Hence, PAHs larger than C66H20 are unlikely to contribute significantly to the formation of C60, while PAHs containing between 60 and 66 C atoms should contribute to the formation of C60 with shorter timescales, and PAHs containing less than 60 C atoms will be destroyed. Assuming a classical size distribution for the PAH precursors, our model predicts absolute abundances of C60 are up to several 10-4 of the elemental carbon, i.e. less than a percent of the typical interstellar PAH abundance, which is consistent with observational studies. According to our model, once formed, C60 can survive much longer (> 107 years for radiation fields below G0 = 104) than other fullerenes because of the remarkable stability of the C60 molecule at high internal energies. Hence, a natural consequence is that C60 is more abundant than other fullerenes in highly irradiated environments.