Hydrodynamical influence on the formation of HCN molecules

Abstract

HCN emission or absorption lines observed in the environment of low and high mass protostars indicate abundances relative to H2 which range from 10−9 to 10−6. These large variations are caused by the physical conditions in the surrounding envelope where the molecules are excited. In addition to the gaseous properties, the amount of UV radiation and the cosmic ray ionisation rate influence the formation efficiency of the HCN molecules. In order to study the impact of a temporally changing density and temperature a reduced chemical network derived from the UMIST data base (Le Teuff et al., 2000) is combined with different hydrodynamical models of a collapsing gas sphere. Using a very general initial element distribution (Markwick et al., 2000) the question is discussed whether a gas phase chemistry is able to reproduce the measured variability of the HCN abundances. Besides, we compare our conclusions concerning the influence of the infall time with the results published by Lintott et al. (2005).