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Astron. Astrophys. 339, L17-L20 (1998)

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4. Discussion

Having established that the 15.2 µm CO2 bending mode exibits a substructure associated with the formation of molecular complexes, it is important to confirm that these laboratory results are compatible with the interstellar abundances of the major ice species involved. Current estimates toward several protostellar sources indicate an abundance ratio of H2O-CH3OH-CO2 [FORMULA] 10:1:2 (see Fig. 2, upper panel and Ehrenfreund et al. 1997a). The strong 9.6 µm CO stretch of CH3OH, could not be measured in the ISO spectrum of RAFGL7009S, because it is embedded in the heavily saturated silicate band. However, recent UKIRT observations of both, the fundamental and combination modes of CH3OH in the 3-4 µm region indicate a very high abundance of CH3OH of [FORMULA] 30% relative to H2O toward this source (Dartois et al. 1998a), which suggests a ratio of H2O-CH3OH-CO2 [FORMULA]10:3:2 toward RAFGL7009S. Therefore most of the [FORMULA] 20% CO2 measured toward RAFGL7009S could be mixed with CH3OH and the remaining CH3OH may be present in pure form or mixed with H2O ice. It is evident from Fig. 2 (upper panel) that CO2 embedded in a water-rich (polar) mixture cannot be responsible for the bending mode structure, neither at low nor high temperature, showing no substructure and an extended red wing. It can however not be excluded that some polar CO2 could be present and hidden in the large bending mode feature (Gerakines et al. 1999, Boogert et al. 1999).

Fig. 3 displays a schematic drawing of the evolution of interstellar ices composed of a silicate core and an ice mantle in the environment of massive protostars (see also van Dishoeck & Blake 1998). Only the major ice species have been considered in this scheme.

[FIGURE] Fig. 3. A schematic drawing of the line-of-sight conditions toward massive protostars. Details are explained in the text.

Many recent laboratory results may be used to test the scenario shown in Fig. 3. Polar ices are dominated by H2O, and contain also some CO, CH3OH, CO2, CH4, NH3 and other minor species. From band profile studies we can determine that most of the NH3 and CH4 but only minor parts of CH3OH and CO2 are embedded in water-rich (polar) ice mixtures (Ehrenfreund et al. 1997a, Schutte 1998). Far from the protostar in colder (below 20 K) and denser regions or in "clumps", where CO is abundant in the interstellar gas, apolar ice mantles, dominated by CO, N2 and some O2 may accrete as an additional grain mantle layer. The narrow band width of many apolar CO features indicates only negligible admixtures of other species, such as CO2 (Ehrenfreund et al. 1997b). The temperature rise in the vicinity of protostars is responsible for the evolution of interstellar ice mantles. All pure and trapped ices sublimate at specific temperatures. Above [FORMULA] 50 K the major ice species H2O, CH3OH and CO2 dominate the interstellar ice spectrum and show in comparison with laboratory data that ice layers rearrange, and that complexes between CO2 and CH3OH become spectroscopically visible. The multipeak structure of the CO2 bending mode is only observed in ice mixtures containing CH3OH (or C2H5OH) and heated to temperatures equivalent to [FORMULA] 60 K in interstellar space. Above 80 K, CH3OH seperates from H2O, which is consistent with the observations of CH3OH-dominated ice layers (Skinner et al. 1992, Dartois et al. 1998a). Above [FORMULA] 90-100 K all major ice species sublimate.

From the presented results we can conclude that an initial ice layer of CO2, CH3OH and H2O in roughly equal abundances must be formed on the grain surface and thereafter be exposed to thermal processing. The efficient production of CO2 by UV photolysis is well demonstrated in the laboratory (e.g. Ehrenfreund et al. 1997b). Impacts of cosmic rays can also form CO2 from simple ices and can provide a reasonable fit to the CO2 bending mode (Strazzulla et al. 1998). The challenging question remains concerning the relative roles of UV and cosmic ray energetic processing or grain surface reactions in the formation of abundant ice species such as CO2 and CH3OH.

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© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998