Molecular line study of evolution in protostellar cloud cores

Abstract

Two dense dark cloud cores representing different stages of dynamical evolution were observed in a number of molecular spectral lines. One of the cores, Cha- MMS1 in the Chamaeleon cloud I contains a Class 0 protostar, whereas the other, CrA C in the R Coronae Australis cloud, is pre-stellar. We find that the cores have very different chemical compositions. Cha-MMS1 exhibits characteristics of so-called `early-type' chemistry with high abundan- ces of carbon-chain molecules such as HC3N, but it also has a large N2H+ abundance, which is expected only to build up at later stages. In contrast, none of the carbon-chain molecules were detected in CrA C. CrA C has a higher SO abundance than Cha-MMS1, which implies that it is chemically `older' than Cha- MMS1. The most striking difference between the two cores is seen in the HC3N/SO abundance ratio, which is at least three orders of magnitude higher in Cha-MMS than in CrA C. This result is surprising since starless cores are usually thought to be chemically younger than star-forming cores. Because of the high N2H+ abundance, we suggest that Cha-MMS1 represents the `late-time cyanopolyyne peak'. This would also be a more natural explanation for the carbon-chain molecules than the `early-time' picture in view of the fact that the core is presently collapsing to form a star. Thus, the dynamical evolution in CrA C seems to have been very slow compared with that of Cha-MMS1, and we discuss possible reasons for this.