Molecular line study of evolution in protostellar cloud cores

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. The molecules selected for this study are supposed to show significant abundance variations in the course of the chemical evolution. We find that the cores have very different chemical compositions. Cha-MMS1 exhibits characteristics of so-called ‘early-type’ chemistry with high abundances of carbon-chain molecules such as HC3N, CH3CCH and c-C3H2. However, 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. On the other hand, CrA C has a higher SO abundance than Cha-MMS1, which according to chemistry models 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-MMS1 than in CrA C. This result is somewhat surprising since starless cores are usually thought to be chemically younger than star-forming cores. Because of the high N2H abundance, we suggest that ChaMMS1 represents the ‘late-time cyanopolyyne peak’ that is predicted to occur when heavy molecules start to freeze onto grain surfaces (Ruffle et al. 1997). 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. The abundances observed in CrA C can be explained either by pure gas-phase models at late stages of evolution, or by the ‘SO peak’ which follows the second cyanopolyyne peak (Ruffle et al. 1999). Thus, the dynamical evolution in CrA C seems to have been very slow compared with that of ChaMMS1, and we discuss possible reasons for this. We detected two SO emission maxima around Cha-MMS1, which lie symmetrically on both sides of the core, approximately on the line connecting the centre of Cha-MMS1 and the position of Herbig-Haro object HH49/50. These SO peaks may signify the lobes of a bipolar outflow, and the observation supports Send offprint requests to: S. Kontinen the suggestion by Reipurth et al. 1996 that Cha-MMS1 is the central source of HH49/50.