SEARCHES FOR THE PROTONATED INTERSTELLAR SPECIES HC3NH+, CH3CNH+, AND HOCS+ - IMPLICATIONS FOR ION-MOLECULE CHEMISTRY

Based on recent laboratory determinations of the molecular constants, we have made sensitive searches for the interstellar polyatomic ions HC3NH+, CH3CNH+, and HOCS+, which are the protonated versions of the abundant interstellar species HC3N, CH3CN, and OCS. The abundance ratios HC3NH+/HC3N, CH3CNH+/CH3CN, and HOCS+/OCS are good tests of ion-molecule formation models because they can be accurately determined observationally (since the ion and neutral are coextensive in such models), and because these ratios depend strongly only on the fast protonation rates of the neutrals and on fast electron recombination rates of the ions, both of which are now becoming known. We find that observed limits for all three ratios are consistent with current ion-molecule models, although the low abundance of HC2NH+ precludes several reaction pathways for forming higher order cyanopolyynes. We also find that HC3NH+/HC3N is at least 10 times smaller than HCNH+/HCN, a result that is not explainable solely by ion-molecule processes and which may suggest a non-gas-phase depletion mechanism for HCN, such as adsorption onto grains. There is no evidence for high-temperature HOCS+/OCS chemistry.