2000A&A...354.1127G

Fractional abundances of HC₃N and a number of other species are presented for dense core models based on a variety of different assumptions. The highest calculated values of the HC₃N fractional abundances arise in static models where many species striking grain surfaces are processed in hydrogenation reactions leading to the rapid injection of saturated species into the gas phase; in these models the HC₃N is most abundant well before chemical equilibrium is reached. The effects of the various parameters considered are strongly coupled and lead to markedly differing ratios of the HC₃N abundance relative to abundances of other species including H₂CO, C₃H and C₄H. Although none of these models is specific to any particular astronomical object, we briefly compare our results with abundances observed in TMC-1. We conclude that the molecular abundances observed in TMC-1 can be accounted for on the basis of models of the type discussed here, and that - although this much-studied object has unusually high molecular abundances - it is not chemically anomalous compared to other dense cores.