More sulphur in TMC-1: Discovery of the NC_3S and HC_3S radicals with the QUIJOTE line survey

Abstract

We present the detection of the free radicals NC$_3$S and HC$_3$S towards TMC-1 with the QUIJOTE line survey. The derived column densities are (1.4pm 0.2)times 1011 for NC$_3$S and (1.5pm 0.2)times 1011 for HC$_3$S. We searched for NCCS, but only three transitions are within the domain of our QUIJOTE line survey and the observed lines are marginally detected at the 3sigma level, providing an upper limit to its column density of leq 6times 1010 cm$^ $. We also unsuccessfully searched for longer species of the NC$_n$S (nge 4) and HC$_n$S (nge 5) families in our TMC-1 data. A chemical model based on a reduced set of reactions involving HC$_3$S and NC$_3$S predicts abundances that are 10-100 times below the observed values. These calculations indicate that the most efficient reactions of formation of HC$_3$S and NC$_3$S in the model are S + C$_3$H$_2$ and N + HC$_3$S, respectively, while both radicals are very efficiently destroyed through reactions with neutral atoms.