3.1. Laboratory spectroscopy
Laboratory data show that solid CO2 invokes particular interactions with other polar and apolar molecules, which result in strong spectroscopic diversity (Reed et al. 1986, Sandford & Allamandola 1990, Ehrenfreund et al. 1996, 1997b). Therefore the profiles of major ice species can be used to determine if and how those components are mixed together.
Fig. 1 shows the CO2 bend during a warm-up sequence of a H2O-CH3OH-CO2 mixture, characterized by a broad asymmetric profile at 10 K. During heating this profile converts in a multipeak structure. A quite symmetric double peak is observed at 15.15 µm (660 cm-1) and 15.25 µm (655 cm-1) as well as a shoulder at 15.4 µm (649 cm-1), which disappears when the temperature is increasing. The pronounced double peak at 15.2 µm is observed exclusively for pure and annealed CO2, the vibration being doubly degenerate (Ehrenfreund et al. 1997b).
A survey in the laboratory, using different molecules, allowed us to identify the shoulder at 15.4 µm with specific complexes formed between CO2 and another polar molecule. In order to reveal the nature of the particular complex several molecules of astronomical importance, such as CH3OH, C2H5OH, HCOOH or NH3 have been tested. The band at 15.4 µm is formed due to an acid-base interaction between the C atom of CO2 and the oxygen atom of a specific polar molecule, which in interstellar space is likely CH3OH. The CO2 molecule is acting as a Lewis acid and has the ability to form a very stable complex. A similar complex formation is also observed for CO2-C2H5OH mixtures (see Fig. 2). A detailed analysis of the band position and width let us conclude that the abundance of H2O must have approximately the same proportion as CH3OH and CO2 to fit the astrophysical data. When polar molecules, such as H2O, NH3 and HCOOH are dominant in the ice, their presence inhibits the complex formation with CO2, because they are involved in efficient H-bonding (e.g. Ehrenfreund et al. 1997b, see Fig. 11). When water ice is more abundant than CH3OH and CO2 the shoulder at 15.4 µm is not present (see Fig. 2, upper panel). The nature of this complex is furthermore constrained by the fact that only mixtures close to a 1:1 ratio provide a good fit to astronomical spectra. For a detailed description of the Lewis complex involving CO2 and the spectroscopic properties of CO2/CH3OH mixtures the reader is referred to more extended papers (Dartois et al. 1999, Ehrenfreund et al. 1999).
3.2. Comparison of laboratory and astronomical data
RAFGL7009S is a massive young stellar object (YSO) located apart from the galactic plane. Infrared observations with ISO already showed that it is an extraordinary source to study solids as well as gas phase species, as it is deeply embedded (d'Hendecourt et al. 1996, Dartois et al. 1998b). Among the species detected in the solid phase is CO2, with a line of sight ratio relative to H2O of 20%.
Fig. 2 shows the bending mode of CO2 toward the massive protostar RAFGL7009S, which is characterized by a triple-peak structure. This characteristic multi-peak feature is currently observed with ISO toward 20 protostars (Gerakines et al. 1999, Boogert et al. 1999). This particular mode shows two sharp peaks at 15.15 and 15.25 µm and a broad wing at 15.4 µm. Comparison with laboratory data are made in two steps. The first trace above ISO data in Fig. 2 represents the laboratory spectrum of CO2-C2H5OH=1:1 ice, annealed to 90 K. Ethanol is likely not responsible for the observed multipeak structure of the CO2 bending mode of RAFGL7009S, because of the absence of the main other vibrational modes in its spectrum. It is used to illustrate the nature of the complex between ethanol and CO2. Just above we display the same spectrum with the addition of the CO2 gas phase absorption responsible for the sharp Q branch at 14.97 µm (Dartois et al. 1998b). On the middle panel the same comparison has been performed with a H2O-CH3OH-CO2=1:1:1 mixture. Methanol is a more logical candidate to account for the structure of the CO2 bending mode, since it is observed toward many protostellar targets (e.g. Allamandola et al. 1992, Dartois et al. 1998a). If the amount of water is slowly increased, the bending mode profile changes, shifts and the 15.4 µm feature progressively vanishes. Note that the results also show that intermolecular interactions completely dominate the behavior of the line profiles and particle shape/size effects in heated ices are negligible, at least in this class of objects. The present laboratory results provide strong evidence that in dense clouds around massive protostars thermal processing dominates the evolution of interstellar ices.
© European Southern Observatory (ESO) 1998
Online publication: September 30, 1998