More sulphur in TMC-1: Discovery of the NC₃S and HC₃S radicals with the QUIJOTE line survey

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 (1.4 ± 0.2)×10 11 for NC 3 S and (1.5 ± 0.2)×10 11 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 3 σ level, providing an upper limit to its column density of ≤6 × 10 10 cm −2 . We also unsuccessfully searched for longer species of the NC n S ( n ≥ 4) and HC n S ( n ≥ 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.