Abstract
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. 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. 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. 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). 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.
Highlights
Photodissociation regions (PDR) are key regions in the study of the interstellar medium
Our work shows that the emission of the warm molecular gas at the photodissociation regions (PDR) edge can be attributed to a slab at a high thermal pressure of 108 and 3 × 108 K cm−3 in NGC 7023 NW and Orion Bar, respectively
The excitation temperature deduced for Jup ≥ 15 from the rotational diagrams are 112 and 147 K, respectively, showing the presence of warm CO gas at the irradiated PDR edge
Summary
Photodissociation regions (PDR) are key regions in the study of the interstellar medium. PDRs are the interfaces between molecular gas (where stars form), and the surrounding galactic medium (see review by Hollenbach & Tielens 1999). Dissociating UV photons produced by young stars are absorbed in PDRs by dust and gas allowing a transition from the atomic phase to the molecular phase. Intense emission from fine-structure lines of C+, O, and C, as well as H2 rotational and rovibrational transitions and CO rotational transitions can be observed in PDRs. It is admitted that emission in these atomic and molecular lines is mainly induced by the heating of the gas by UV photons from nearby massive stars involving the photo-electric effect on grains (Bakes & Tielens 1994; Weingartner & Draine 2001) as well as the collisional deexcitation of H2 excited by UV pumping
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