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

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1. Introduction

Models of chemistry in interstellar clouds indicate that molecules are formed efficiently in the gas phase through ion-molecule reactions (e.g. CO; Millar & Nejad 1985), on grain surfaces through heterogeneous catalysis (e.g. H2O; Tielens & Hagen 1982), or by ultraviolet (UV) processing of ice mantles (e.g. `XCN'; Lacy et al. 1984). Observations of both the solid and gas phase are essential to test and further constrain these models. With the Short Wavelength Spectrometer on board of the Infrared Space Observatory (ISO-SWS; Kessler et al. 1996; de Graauw et al. 1996) it is possible to observe the ro-vibrational bands of several important molecules, that cannot, or with extreme difficulty, be observed through the earth atmosphere.

One of the simplest organic species, methane (CH4), was recently studied in the solid state (Boogert et al. 1996). ISO-SWS observations towards the massive protostellar objects W 33A and C 7538 : IRS9 show an absorption feature at 7.67 µm. The peak position and width of this absorption band reveal that interstellar solid methane is embedded in an ice mantle of primarily polar molecules (H2O, [FORMULA]). The methane ice column density is low (1-2% of water ice; [FORMULA] compared to atomic H). It was concluded that methane is probably formed on grain surfaces, at a low gas phase atomic C abundance.

An important test to this hypothesis is the observation and analysis of gas phase CH4. Severely hindered by the earth atmosphere, Lacy et al. (1991) detected the R(0) and R(2) lines with ground based observations toward C 7538 : IRS9 and tentatively toward W 33A. We observed these two sources with ISO-SWS. Dartois et al. (1998) report observations of gaseous and solid phase CH4 toward the embedded protostar GL 7009S. The derived temperature and abundance of gaseous CH4, as well as the CH4 gas/solid state abundance ratio, are used to further constrain the origin of interstellar CH4.

In Sect. 2 we discuss the reduction and quality of the ISO-SWS observations. The observed solid state and gas phase absorption features in the 7.35-7.85 µm spectral region are identified in Sect. 3. Models of the [FORMULA] dyad of gaseous CH4 are described in Sect. 4, and compared to the interstellar spectra. Rotational diagrams are constructed to derive the temperature and column density of interstellar gaseous CH4. In Sect. 5 we discuss the observational constraints that can be put on existing chemical models for the formation of interstellar CH4. The conclusions are given in Sect. 6.

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© European Southern Observatory (ESO) 1998

Online publication: July 7, 1998