A sensitive λ 3 mm line survey ofL483: A broadview of the chemical composition of a core around a Class 0object

摘要

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 3 mm line survey of L483, a dense core around a Class 0 protostar, which was observed with the IRAM 30m telescope in the 80-116 GHz 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 ~0.5 km s⁻¹) and low rotational temperatures (≲10 K). 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 aresystematically lower than in TMC-1 (especially for carbon chains), tend to behigher than in L1544 and B1-b, and are similar to those in L1527. Apart from theoverabundance of carbon chains in TMC-1, we find that L483 does not have amarked chemical differentiation with respect to starless/prestellar cores likeTMC-1 and L1544, although it does chemically differentiate from Class 0 hotcorino sources like IRAS 16293–2422. This fact suggests that the chemicalcomposition of the ambient cloud of some Class 0 sources could be largelyinherited from the dark cloud starless/prestellar phase. We explore the use ofpotential chemical evolutionary indicators, such as the HNCO/C3S,SO2/C2S, and CH3SH/C2S ratios,to trace the prestellar/protostellar transition. We also derived isotopic ratiosfor a variety of molecules, many of which show isotopic ratios close to thevalues for the local interstellar medium (remarkably all those involving³⁴S and ³³S), while there are also several isotopicanomalies like an extreme depletion in ¹³C for one of the twoisotopologs of c-C3H2, a drasticenrichment in ¹⁸O for SO and HNCO (SO being also largely enriched in¹⁷O), and different abundances for the two ¹³Csubstituted species of C2H and the two ¹⁵N substitutedspecies of N₂H⁺. We report the first detection in space ofsome minor isotopologs like c-C₃D. The exhaustivechemical characterization of L483 presented here, together with similar studiesof other prestellar and protostellar sources, should allow us to identify themain factors that regulate the chemical composition of cores along the processof formation of low-mass protostars.