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Astron. Astrophys. 353, 1101-1114 (2000)

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6. Conclusion

A molecular survey of the exceptionally bright comet Hale-Bopp near its perihelion has led to the identification of six new molecular cometary species. In total, 16 molecules were observed (not including isotopes and ions) through their rotational transitions in the radio range and their abundances relative to water were derived. Upper limits obtained for a number of species not yet detected in comets will be discussed separately.

One of the major surprises of these observations was the detection of SO and SO2, as very stringent upper limits on their abundances in comets were obtained in the past from UV observations. Detailed excitation models are needed to understand this discrepancy, as well as to better estimate the SO and SO2 abundances in comet Hale-Bopp. Our present study shows that the SO and SO2 production rates agree within a factor of two, suggesting that a large part of the SO molecules observed in the coma are produced by the photodissociation of SO2. This agrees with interferometric observations which show that SO has not a parent molecule distribution.

Several species detected in this molecular survey (HNCO, NH2CHO, HCOOH, HCOOCH3) have not been observed before in other Solar System objects (except the Earth), while they are observed in star forming regions. Interestingly, these species are predominantly found in molecular hot cores in the ISM, which are dense warm gas clumps located close to massive young stars whose gas phase composition is believed to reflect at least partly the sublimation of icy grains. As there are other striking similarities between the volatile composition of comets and hot cores (such as the dominance of hydrogenated molecules), we performed a quantitative comparison of the molecular abundances inferred in these objects. We also compared cometary abundances to molecular abundances measured in bipolar flows, which probe the composition of the envelopes of low-mass stars. We assumed that the volatile composition measured in the coma of comet Hale-Bopp is representative of the composition of cometary ices, although detailed chemical models have to be developed to check whether some of the species observed with minor abundances could not be produced in the coma. This comparative study shows a good correlation for N-bearing and CHO-bearing species. An interesting result is the similarity of the [HCOOCH3]/[CH3OH] mixing ratio in comets and hot cores. Indeed, methyl formate is rather considered as a second-generation species in hot cores, although this has been recently questioned. There is less agreement for the S-bearing species, an expected result since the abundances of sulphur species in hot cores and bipolar flows are not believed to reflect those in the grains due to a rapid gas-phase chemistry.

These correlations between molecular abundances ranging over several orders of magnitude reinforce the similarity between interstellar and cometary ices. The high ethane abundance in comets, which cannot be explained by thermochemical processes in the hot Solar Nebula, is another evidence for a possible direct link between cometary and interstellar ices. Formation of C2H6 by ion-molecule reactions is inefficient at the low temperature of dark clouds, but can proceed from hydrogenation of C2H2 on grain surfaces or UV photolysis of ice mixtures (Mumma et al. 1996). All this suggests that cometary ices were formed to a large extent by the same low temperature non-equilibrium processes which produce interstellar ices: grain-surface reactions, condensation of products of ion-molecule reactions, UV processing. But is there a direct link between interstellar and cometary ices? In the inner part of the protoplanetary disk, the high temperature led to a complete volatilization (up to atomisation) of the grains and initiated a specific Solar Nebula thermochemistry. Recent non-equilibrium chemical models (Willacy et al. 1998; Finocchi et al. 1997) show that molecular abundances in this region are very different from those in comets. In the outermost regions of the disk, the icy mantles coating the infalling grains may have been preserved or only partially or temporarily evaporated during the accretion phase, allowing for the interstellar signature to be kept in the pre-cometary grains. Recent modelling of the outer cold regions of protoplanetary disks suggests that a gas-phase ion-molecule chemistry followed by condensation (Aikawa et al. 1999), in many respects similar to interstellar chemistry, could have also produced there icy mantles. Assessing to which extent nebular chemistry and additional processes, such as accretion shocks and radial mixing in the nebula, have also contributed to cometary ice composition, will need further studies, but the similarities with interstellar ices shown by the present data clearly set a limit to their relative importance.

Note added in proof: Further analysis of the IRAM data on comet Hale-Bopp revealed rotational signatures of acetaldehyde (CH3CHO), an ubiquitous compound of moleacular hot cores.

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© European Southern Observatory (ESO) 2000

Online publication: January 18, 2000