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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
Received 1 October 1997 / Accepted 24 April 1998
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 ( K), and has a low abundance (). 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 (). 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 (), 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.
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Online publication: July 7, 1998