Contribution Invited Talk
UNSOLVED PROBLEMS IN ASTROCHEMISTRY
Although astrochemistry has enjoyed many significant successes throughout the years, there are also a number of unsolved problems or partially solved problems which remain to be solved. In my view, the top three such problems are:
- Understanding how the larger molecules in interstellar clouds are synthesized, and their connection to astrobiology,
- Understanding how the different distributions of molecular intensities come about; in particular, can we explain why molecular distributions from the same general source tend to have a high overlap, a moderate overlap, or a negligible overlap depending upon the complexity of the source.
- Answering the question of what role, significant or secondary, will chemistry play in the ALMA world of interferometry?
The problem of understanding the synthesis of large or very large interstellar molecules has been growing as it has become evident that a number of mechanisms must be invoked for most sources. Once upon a time, we could say that the chemistry of large molecules in cold cores was dominated by ion-neutral reactions given that most of the large molecules are very unsaturated. But now, we realize that other possibilities must be considered including gas phase neutral-neutral reactions, even those with barriers that can be tunneled under, as well as grain surface mechanisms typically involving the diffusive motion of atoms or small molecules.. Once upon a time, we could assume that most of the so-called gaseous complex molecules in hot cores and corinos were formed by surface chemistry during the warm up stage, but it is becoming clear that gas-phase mechanisms are also important including both neutral-neutral and ion-neutral reactions. Once upon a time, we had no evidence for very large molecules in translucent or diffuse clouds, other than possibly the DIBs, now we know that C60+ is present, and we have to explain how and when it was synthesized, or whether it is simply a product of grain destruction or sputtering. And finally, what about other large molecules detected in in cm-wave absorption in translucent clouds?
The advent of interferometry is beginning to show us that we can no longer blithely regard any source as homogeneous; that at high resolution material can be divided into slivers of smaller portions, often filamentary in nature. Does such knowledge mean that large-scale gas-grain simulations can no longer be used? Can they be made more complicated? Can they distinguish between excitation and chemistry?
Should we, instead, focus on small-scale problems, such as the problem of different vs similar molecular distributions in space? For example, in hot cores, why do methyl formate and dimethyl ether seem to occupy the same local spaces in star-forming regions, while acetaldehyde occupies a much greater space? Is the problem physical, chemical, or both? In general, does anti-correlation imply that one molecule is the precursor of the other? Does correlation imply that both molecules come from the same third source? Answers to these questions are non-trivial, and depend upon relative abundances.