Molecular line study of evolution in protostellar cloud cores

Two dense dark cloud cores representing different stages of dynamical evolu tion were observed in a number of molecular spectral lines. One of the core s, 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 mole cules selected for this study are supposed to show significant abundance va riations 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 abun dances of carbon-chain molecules such as HC3N, CH3CCH and c-C3H2 However, i t also has a large N2H+ abundance, which is expected only to build up at la ter stages. In contrast, none of the carbon-chain molecules were detected i n 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' t han 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 h igher in Cha-MMS1 than in CrA C. This result is somewhat surprising since s tarless cores are usually thought to be chemically younger than star-formin g cores. Because of the high N2H+ abundance, we suggest that Cha-MMS1 represents the 'late-time cyanopolyyne peak' that is predicted to occur when heavy molecu les start to freeze onto grain surfaces (Ruffle et al. 1997). This would al so be a more natural explanation for the carbon-chain molecules than the 'e arly-time' picture in view of the fact that the core is presently collapsin g 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' w hich follows the second cyanopolyyne peak (Ruffle et al. 1999). Thus, the d ynamical evolution in CrA C seems to have been very slow compared with that of Cha-MMS1, 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 t he suggestion by Reipurth et al. 1996 that Cha-MMS 1 is the central source of HH49/50,