The physical conditions in the PDR of W49N^*

¹ CESR CNRS-UPS, BP 4346, 31028 Toulouse Cedex 04, France
² Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
³ Observatoire de Bordeaux, 2 rue de l'Observatoire, BP 89, 33270 Floirac, France
⁴ Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 09, France

Corresponding author: C. Vastel, vastel@cesr.fr

Received: 24 April 2001
Accepted: 12 July 2001

Abstract

We present ISO-LWS and ground-based SEST observations which probe the Photo Dissociation Region (PDR) of W49N. The ISO-LWS observations consist of low resolution (~200) grating spectra between 43 μm and 197 μm, and high resolution (~10⁴) Fabry-Pérot spectra of the [CII] 158 μm and [OI] 63 and 145 μm lines. The SEST observations consist of C¹⁶O and C¹⁸O maps of the region. Using all these observations and with the help of a detailed PDR model, we have determined the physical and chemical structure of the PDR of W49N. We find that only a tiny fraction, 2.5% , of the cloud gas is in the atomic form. This gas has an average density cm^-3 and a temperature K. The PDR is created by a FUV field times brighter than the average FUV interstellar field, which is emitted by the stars associated with the several HII regions inside the cloud. The photoelectric heating efficiency is very low, ~10^-4, relative to other known PDRs, probably because of the relatively high ratio. Overall, W49N seems to be one of the youngest giant molecular clouds actively forming stars in the Galaxy, since most of the cloud material is still molecular and dense and not yet dispersed by the several embedded newly formed massive protostars.