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Astron. Astrophys. 315, L377-L380 (1996)

Solid methane toward deeply embedded protostars*

A.C.A. Boogert1, W.A. Schutte2, A.G.G.M. Tielens3, D.C.B. Whittet4, F.P. Helmich2, P. Ehrenfreund2, P.R. Wesselius1,5, Th. de Graauw1,5, and T. Prusti6

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
6 ISO Science Operations Centre, Astrophysics Division, ESA, Villafranca del Castillo, P.O. Box 50727, E-28080 Madrid, Spain

Received 16 July 1996 / Accepted 22 August 1996

Abstract. We report on the detection of an absorption feature near 7.67 img1.gif toward the deeply embedded protostellar objects W 33A and NGC 7538 : IRS9. Comparison with laboratory spectra shows that this feature can be identified as the img2.gif (`deformation') mode of solid state img3.gif embedded in polar molecules (i.e., img4.gif ) in icy grain mantles. The solid img3.gif column density relative to solid img5.gif is 0.4-1.9%. Unlike solid CO, solid img3.gif does not seem to be subjected to out-gassing toward the warm source W 33A. The low gas-to-solid ratio for img3.gif argues that img3.gif is formed on grains either by hydrogenation of accreted C or by ultraviolet processing of img6.gif -rich ices.

Key words: ISM: dust, extinction - ISM: molecules - ISM: abundances - infrared: ISM: lines and bands - stars: individual: NGC 7538: IRS9 - stars: individual: 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.

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