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Astron. Astrophys. 317, 929-941 (1997)

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Infrared observations and laboratory simulations of interstellar CH4 and SO2

A.C.A. Boogert 1, W.A. Schutte 2, F.P. Helmich 2, A.G.G.M. Tielens 3 and D.H. Wooden 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 MS 245-6, NASA Ames Research Center, Moffet Field, CA 94035, USA

Received 11 March 1996 / Accepted 22 May 1996


Interstellar [FORMULA] may consume a fair amount of the carbon budget in dense molecular clouds, but probably less than CO, [FORMULA], and [FORMULA]. However, it can only be observed at wavelength regions in the infrared that are heavily affected by the earth atmosphere. With new space and airborne missions (e.g. ISO, SOFIA) in mind we have studied the near infrared absorption spectra of solid and gaseous [FORMULA]. We obtained laboratory spectra of the [FORMULA] deformation mode (1302 [FORMULA], 7.68 µm) of solid [FORMULA] in astrophysically relevant mixtures. We found that the peak position and width of this absorption band vary strongly as a function of molecular environment, compared to temperature and particle shape effects. Hence, observations of this feature will provide a powerful probe of the molecular composition of interstellar ices. Also the gas phase [FORMULA] ro-vibrational spectrum of the same band has been calculated. Using observed physical conditions around the protostar W 33A, we show that unresolved gaseous [FORMULA] lines are detectable (at the 2-5% level) at a resolution [FORMULA] 1000, when the column density N [FORMULA].

An astrophysically relevant molecule with a very strong transition in the same wavelength regime, is [FORMULA]. We studied the [FORMULA] asymmetric stretching mode (1319 [FORMULA], 7.58 µm) of solid [FORMULA] in several mixtures, revealing that the peak position, width and detailed profile of this band are very sensitive to the molecular environment. Besides probing the composition of ice mantles, observations of solid [FORMULA] will provide important information on the sulfur budget locked up in grain mantles, which is currently poorly known.

We compare the laboratory and calculated spectra of [FORMULA] and [FORMULA] with previously published ground based spectra and new airborne observations of young stellar objects in the 7-8 µm region. W 33A, NGC 7538 : IRS1 and IRS9 show a feature near [FORMULA] that is consistent with absorption by solid [FORMULA] or the Q-branch of gaseous [FORMULA]. The column density of solid [FORMULA] would be 0.3-4% of solid [FORMULA], indicating that solid [FORMULA] consumes [FORMULA] of the cosmic carbon abundance. A gaseous origin would imply a column density of at least this amount, being highly dependent on the assumed temperature of the absorbing gas. A second absorption feature is detected toward W 33A and NGC 7538 : IRS1 at [FORMULA]. The peak position and width of this feature are consistent with the [FORMULA] mode of solid [FORMULA] in a matrix of solid [FORMULA] or pure [FORMULA]. The derived column density is 0.1-1% of solid [FORMULA], indicating that solid [FORMULA] locks up 0.6-6% of the cosmic sulfur abundance.

This study shows that 7-8 µm spectroscopy of dense molecular clouds, using new airborne and space-based platforms, will provide valuable information on the composition of icy grain mantles and molecular cloud chemistry.

Key words: infrared: ISM: lines and bands – ISM: molecules – ISM: abundances – ISM: dust, extinction – molecular data – stars: individual: W 33A

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

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998