2019A&A...625A.147A

An exhaustive chemical characterization of dense cores is mandatory to our understanding of chemical composition changes from a starless to a protostellar stage. However, only a few sources have had their molecular composition characterized in detail. Here we present a λ 3mm line survey of L483, a dense core around a Class 0 protostar, which was observed with the IRAM 30m telescope in the 80-116GHz frequency range. We detected 71 molecules (140 including different isotopologs), most of which are present in the cold and quiescent ambient cloud according to their narrow lines (FWHM∼0.5km/s) and low rotational temperatures (≤10K). Of particular interest among the detected molecules are the cis isomer of HCOOH, the complex organic molecules HCOOCH₃, CH₃OCH₃, and C₂H₅OH, a wide variety of carbon chains, nitrogen oxides like N₂O, and saturated molecules like CH₃SH, in addition to eight new interstellar molecules (HCCO, HCS, HSC, NCCNH⁺, CNCN, NCO, H₂NCO⁺, and NS⁺) whose detection has already been reported. In general, fractional molecular abundances in L483 are systematically lower than in TMC-1 (especially for carbon chains), tend to be higher than in L1544 and B1-b, and are similar to those in L1527. Apart from the overabundance of carbon chains in TMC-1, we find that L483 does not have a marked chemical differentiation with respect to starless/prestellar cores like TMC-1 and L1544, although it does chemically differentiate from Class 0 hot corino sources like IRAS 16293-2422. This fact suggests that the chemical composition of the ambient cloud of some Class 0 sources could be largely inherited from the dark cloud starless/prestellar phase. We explore the use of potential chemical evolutionary indicators, such as the HNCO/C₃S, SO₂/C₂S, and CH₃SH/C₂S ratios, to trace the prestellar/protostellar transition. We also derived isotopic ratios for a variety of molecules, many of which show isotopic ratios close to the values for the local interstellar medium (remarkably all those involving ³⁴S and ³³S), while there are also several isotopic anomalies like an extreme depletion in ¹³C for one of the two isotopologs of c-C₃H₂, a drastic enrichment in ¹⁸O for SO and HNCO (SO being also largely enriched in ¹⁷O), and different abundances for the two ¹³C substituted species of C₂H and the two ¹⁵N substituted species of N₂H⁺. We report the first detection in space of some minor isotopologs like c-C₃D. The exhaustive chemical characterization of L483 presented here, together with similar studies of other prestellar and protostellar sources, should allow us to identify the main factors that regulate the chemical composition of cores along the process of formation of low-mass protostars.