Hydrodynamical influence on the formation of HCN and DCN in protostellar clouds

Abstract

HCN emission or absorption lines observed in the collapsing cloud around low and high mass protostars indicate abundances relative to H which range from 10^-9 to 10^-7. These large variations are caused by the physical conditions of the molecular cloud. In addition to these properties, the cosmic ray ionisation rate influence the formation efficiency of the HCN molecules. In order to study the impact of a temporally changing gas density, velocity, and temperature a reduced chemical network derived from the UMIST data base is combined with a self similar hydrodynamical model of a collapsing gas sphere. Using a very general initial element distribution of a young molecular cloud the question is discussed whether a gas phase chemistry coupled to the ice phase is able to reproduce the HCN abundance and the DCN/HCN ratio observed in the vicinity of protostars. The differences between the calculated abundances of a hydrodynamical and a hydrostatic model are discussed as well.