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

¹ IRAP, Université de Toulouse, CNRS, UPS, CNES, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
e-mail: christine.joblin@irap.omp.eu
² Instituto de Física Fundamental (CSIC), Calle Serrano 121-123, 28006 Madrid, Spain
³ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités UPMC Univ., Paris 06, 92190 Meudon, France
⁴ Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
⁵ Université Paris-Diderot, Paris, France
⁶ Observatorio Astronómico Nacional, Apdo. 112, 28803 Alcalá de Henares, Madrid, Spain
⁷ Institut d’Astrophysique Spatiale (IAS), Université Paris Sud & CNRS, 91405 Orsay, France
⁸ European Space Astronomy Centre, ESA, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁹ I. Physikalisches Institut der Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany
¹⁰ Institut Utinam, CNRS UMR 6213, OSU THETA, Université de Franche-Comté, 41 bis avenue de l’Observatoire, 25000 Besançon, France
¹¹ Department of Astronomy, University of Michigan, 311 West Hall 1085 S. University Avenue, Ann Arbor, MI 48109, USA

Received: 9 January 2018
Accepted: 10 April 2018

Abstract

Context. In bright photodissociation regions (PDR) associated with 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 to present a comprehensive view of the modelling 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, and 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 were analysed 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 with typical thicknesses of a few 10⁻³ pc and at high thermal pressures (P_th ~ 10⁸ K cm⁻³).

Conclusions. Compiling data from the literature, we find 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 to rationalise the analysis of high-J CO emission in massive star formation and provides an observational constraint for models which study stellar feedback on molecular clouds.