2001A&A...376.1064V

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 43µm and 197µm, and high resolution (∼10⁴) Fabry-Perot spectra of the [CII] 158µm and [OI] 63 and 145µm lines. The SEST observations consist of C¹⁶O J=1->0 and C¹⁸O J=2->1 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 n=10⁴cm^–3 and a temperature T∼130 K. The PDR is created by a FUV field G₀=3x10⁵ 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 G₀/n 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.