Résumé / Abstract Seminaire_IAP
« Structure of photodissociation fronts in star-forming regions revealed by observations of high-J CO emission lines with Herschel. A new view of the morphology of PDRs, their chemistry and of the impact of radiative feedback »

Franck Le Petit
Laboratoire d'Etudes du Rayonnement et de la Matière en Astrophysique et Atmosphères, Observatoire de Paris (Paris, France)

Photodissociation regions (PDRs) are key regions in the study of the interstellar medium. They are the interfaces between molecular gas, where stars form, and the surrounding galactic medium (Hollenbach et al. 1999). Understanding the structure and physical and chemical processes in PDRs is necessary to constrain stellar feedback on molecular clouds. The Herschel Space Observatory has opened the possibility to observe, more systematically the warm molecular gas in galactic and extragalactic sources by covering all CO excitation lines from Jup = 4 to Jup = 50. This allows us to build full CO spectral line energy distributions (SLEDs) including high-J levels. Such CO SLEDs have been particularly studied in star-forming regions in order to provide information on the energetic processes acting in these objects.
Nevertheless, few clear explanations have been proposed to explain these CO SLEDs and up to recently, it was unclear if, in star forming regions, the CO high-J excitation probes direct heating by UV photons or if additional energy sources are required as shocks or X-rays. To answer to this question, we have studied three typical Galactic PDRs: Orion Bar, NGC 7023 and the Carina nebula. In this talk, I will show that the CO high-J excitation in these objects can be explained by UV heating. I will also explain how this result changes the typical view we have of the morphology of PDRs and of the dynamical effects generated by young stars on their parent cloud. Indeed, we found that the edge of PDRs are characterised by a high pressure zone created by the photo-evaporation of the gas where a hot chemistry takes place. Finally, I will present why it is important to understand the detailed chemical processes leading to the formation of H2 and CH+ in PDRs to interpret such observations.
vendredi 23 février 2018 - 11:00
Amphithéâtre Henri Mineur, Institut d'Astrophysique de Paris
Page web du séminaire / Seminar's webpage