Astron. Astrophys. 351, 1066-1074 (1999)

1. Introduction

The CO₂ bending mode, as observed with the ISO satellite, presents a pattern that significantly varies from one line of sight to another and can exhibit two to three distinct components at 15.1, 15.25 and 15.4 µm (see de Graauw et al. 1996 and Fig. 1). These features were at first associated with possible absorption arising from other molecules present in the ice mantles, such as formic acid, whose mode falls not far from the carbon dioxide bending mode. This particular hypothesis can be rejected due to the absence, in the full spectrum, of the other formic acid modes, at least in strength compatible with the 15.2 µm mode, such as a CO stretch around 1700 cm^-1. Another hypothesis one may consider is the grain size and shape effects that could influence light absorption and scattering. For example, it is well known that scattering is responsible for the long wavelength wing of the water ice stretching mode at 3 µm (Rouan & Leger 1984). Very specific grain shapes (e.g. needles) would cause an asymmetry in some bands, and in extreme cases give rise to apparent new features (Bohren & Huffman 1983). It is however difficult to imagine a strong effect arising at 15.2 µm, a wavelength well above typical interstellar grain radii, when the CO₂ stretching mode as well as other solid state molecular absorptions in the infrared seem little affected (e.g. CH₄, Dartois et al. 1998).

Fig. 1. Peculiar substructure observed in the solid CO2 bending mode absorption band toward RAFGL7009S. The sharp feature at 14.98 µm is the gas phase CO2 counterpart (Dartois et al. 1998).

Recently, fairly good fits to the astronomical CO₂ bending mode data has been obtained with the spectra of CO₂-CH₃OH ice mixtures (Ehrenfreund et al. 1998), showing that interactions between the molecules in the ice represent an alternative way to justify the peculiar line shapes observed. Irradiated ice mixtures have also been investigated in that way (Palumbo et al. 1998).

In a different region of the spectrum, the "6.85 µm" band is a prominent feature in the line of sight to various protostellar sources, becoming almost as strong as the 6 µm water bending mode in some of them (Willner et al. 1982). Its identification remains however unclear. The "6.85 µm" band has been attributed to various molecules including CH₃OH and NH based on its spectral position and experiments (Tielens et al. 1984, Grim & Greenberg 1987, Schutte & Greenberg 1997, Demyk et al. 1998). In addition, CH₃OH may represent a significant fraction of ice mantles, as derived from observations of modes at shorter wavelength (e.g. 3.53 µm, Allamandola et al. 1992) or through comparisons between the shape of the CH₃ deformation modes in the mid infrared region ( 1400 cm^-1, Schutte et al. 1996) and laboratory results.

We show hereafter that a physical link exists between carbon dioxide and methanol: a complex involving these two molecules can form and the intermolecular interactions influence the carbon dioxide bending mode line shape. Furthermore, ground based observations using UKIRT have shown that methanol is particularly abundant towards RAFGL7009S and W 33A (Dartois et al. 1999). From our experiments we demonstrate that this last molecule forms an electron donor-acceptor complex with carbon dioxide.

In this paper, we assess in the first part experimentally the process responsible for these changes in the CO₂ spectra and in the second part we develop the astrophysical implications.

© European Southern Observatory (ESO) 1999

Online publication: November 16, 1999
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