The interstellar heritage of comets

Comets have similar compositions to the interstellar medium leading to the suggestion that at least some of their molecules maybe inherited from an earlier stage of evolution. To investigate the degree to which this might have occurred we compare the composition of individual comets to that of the well-studied protostellar region IRAS 16293-2422B. We show that the observed molecular abundance ratios in several comets correlate well with those observed in the protostellar source. However, this does not necessarily mean that the cometary abundances are identical to the protostellar values. We find that the abundance ratios of many molecules present in comets are enhanced compared to their protostellar counterparts. For COH- molecules, the data suggest that higher abundances relative to methanol of more complex species, such as HCOOH, CH$_3$CHO, and HCOOCH$_3$, are found in comets. For N-bearing molecules, the ratio of nitriles relative to CH$_3$CN -- HC$_3$N/CH$_3$CN and HCN/CH$_3$CN -- tend to be enhanced. The abundances of both SO and SO$_2$ relative to H$_2$S are enhanced in comets, whereas OCS/H$_2$S is reduced. Using a subset of our comets with a common set of observed molecules we suggest a possible means of determining the relative degree to which they retain interstellar ices. This analysis suggests that over 90\% of the COH-bearing molecules can be explained by the protostellar composition. The possible fraction inherited from the protostellar region is lower for N-molecules at only 25--70\%, and lower still for S-molecules at 28--33\%. While this is still speculative, especially since few comets have large numbers of observed molecules, it provides a possible route for determining the relative degree to which comets contain disk-processed material. In the future, this method may, in conjunction with models, lead to a means of identifying the location and/or time at which comets formed.