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Astron. Astrophys. 357, 994-1000 (2000)

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Large [O]/[CO] ratios in cold molecular clouds towards W 49N *

C. Vastel 1, E. Caux 1, C. Ceccarelli 2, A. Castets 3, C. Gry 4,5 and J.P. Baluteau 4

1 CESR CNRS-UPS, B.P. 4346, 31028 Toulouse cedex 04, France
2 Laboratoire d'Astrophysique, Observatoire de Grenoble, B.P. 53, 38041 Grenoble cedex 09, France
3 Observatoire de Bordeaux, 2 Rue de l'Observatoire, B.P. 89, 33270 Floirac, France
4 Laboratoire d'Astronomie Spatiale, Traverse du siphon, 13004 Marseille, France
5 ISO Data Center, ESA Astrophysics Division, Villafranca del Castillo, P.O. Box, 59727 Madrid, Spain

Received 23 July 1999 / Accepted 27 March 2000


We present ISO-LWS high spectral resolution (R [FORMULA] 104) observations of the [OI] 63 µm and 145 µm lines towards the high mass star formation region W49N. At 145 µm the observed spectrum shows an emission component only, while at 63 µm it shows both emission and absorption components. We performed an analysis of the observed spectra using measured profiles of the LWS Fabry-Pérot spectral responses.

We also present 12CO and 13CO J = 1 [FORMULA] 0 and J = 2 [FORMULA] 1 observations of the same line of sight and detected seven molecular clouds.

We interpret the observed [OI] 63 µm line absorption as due to the oxygen associated with both atomic hydrogen clouds and molecular clouds present on the line of sight. We found that HI clouds are responsible only for a fraction of this absorption and that at least 15 times more O than CO has to be present in the molecular clouds to reproduce the observed 63 µm absorption. This implies that the gaseous oxygen is almost totally in atomic form and that carbon is deficient by more than a factor six in these molecular clouds.

Key words: ISM: abundances – ISM: atoms, ions – ISM: clouds – ISM: individual objects: W49N – ISM: molecules

* Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA.

Send offprint requests to: C. Vastel (Charlotte.Vastel@cesr.fr)

© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000