CO2 is not predicted to have appreciable abundance in the gas phase in the interstellar medium (e.g., Herbst & Leung 1986) so that its condensation on interstellar grains can be neglected. Thus its presence may be due to reaction of gas-phase species on grain cores or to energetic processing such as UV and particle irradiation. Indeed laboratory experiments have shown that CO2 is produced after UV and ion irradiation of astrophysically relevant ices (Gerakines et al. 1996; Palumbo et al. 1998). The comparison reported in Fig. 3 shows that along the line of sight of NGC7538 IRS1 ice mantles could have suffered energetic processing and warmup. This latter is probably due to the temperature gradient along the circumstellar envelope caused by the forming star which is heating up its surroundings. The former, simulated in laboratory by ion irradiation, could be caused by particle irradiation due to stellar flares and/or low energy cosmic rays (Teixeira et al. 1998 and references therein). Another possibility could be UV irradiation. However in this case the UV photons emitted by the embedded object are not able to penetrate the thick envelope and are absorbed by the material close to the star, then the UV field inside the cloud would be responsible for the processing of icy grain mantles. If this is the case the same processing should occur in quiescent interstellar clouds. In this view it would be very interesting to compare the profile of the CO2 bands along the line of sight of embedded objects with those to be observed towards field stars in order to evaluate the relative importance of energetic processing (UV vs ions) on ice mantles.
Furthermore also fits of the solid CO band profile observed towards several obscured infrared objects have pointed to ice mantles that could have suffered energetic processing and warmup (Palumbo & Strazzulla 1993; Teixeira et al. 1998). In addition the observed CO bands have been fitted with the same laboratory mixtures used above. A further support to these results will be given by the simultaneous comparison of the high resolution profile of the stretching and bending modes of CO2 and CO towards NGC7538 IRS1 and all other sources with the same laboratory spectra. This will be done when ISO high resolution spectra are available to the scientific community.
Finally, it has often been pointed out that composition of ice mantles in the interstellar medium might be similar to that of comets (e.g., Mumma et al. 1993), suggesting that comets are a direct aggregate of interstellar dust after a modest processing in the solar nebula. After the estimation of the abundance of CO2 in icy mantles (15-20% with respect to water) which is significantly higher than that observed in comets (a few percent with respect to water) this model on the origin of comets seems to have an additional problem.
© European Southern Observatory (ESO) 1998
Online publication: June 2, 1998