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Astron. Astrophys. 336, 352-358 (1998)


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The gas/solid methane abundance ratio toward deeply embedded protostars *

A.C.A. Boogert 1, 5, F.P. Helmich 2, E.F. van Dishoeck 2, W.A. Schutte 2, A.G.G.M. Tielens 1, 3, 5 and D.C.B. Whittet 4

1 Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands
2 Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands
3 MS 245-3, NASA Ames Research Center, Moffet Field, CA 94035, USA
4 Department of Physics, Applied Physics & Astronomy, Rensselaer Polytechnic Institute, Troy, NY 12180, USA
5 SRON, P.O. Box 800, 9700 AV Groningen, The Netherlands

Received 1 October 1997 / Accepted 24 April 1998

Abstract

We present the detection of ro-vibrational absorption lines of the deformation mode of gaseous CH4 toward the massive protostars W 33A, and C 7538 : IRS9, using the SWS spectrometer on board of the Infrared Space Observatory. The observed lines indicate that the CH4 gas is warm ([FORMULA] K), and has a low abundance ([FORMULA]). The CH4 ice in these lines of sight is embedded in a polar matrix (Boogert et al. 1996), and the gas/solid state abundance ratio is low ([FORMULA]). We discuss models for the formation of interstellar CH4. The observations impose strong limitations on time dependent gas phase models, e.g. a low initial CO/C ratio would be required, the CH4 must have been formed and subsequently condensed on the grains within a narrow time window, and an additional mechanism would be necessary to form polar ice mantles. More likely, interstellar CH4 is formed through grain surface reactions at a high CO/C ratio, which explains the low observed CH4 abundance ([FORMULA]), the presence of CH4 in a polar ice, the low gas/solid ratio, and the absence of a strong cold CH4 gas component. The observed warm CH4 gas probably has sublimated from the grains in the `hot core' region surrounding the protostar.

Key words: ISM: dust, extinction – molecules – abundances – infrared: ISM: lines and bands – stars: individual: NGC 7538: IRS9; W 33A

* 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) and with the participation of ISAS and NASA.

Send offprint requests to: A.C.A. Boogert, (boog@astro.rug.nl)

© European Southern Observatory (ESO) 1998

Online publication: July 7, 1998

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