The sulphur saga in TMC-1: Discovery of HCSCN and HCSCCH
  We report the detection, for the first time in space, of cyano
thioformaldehyde (HCSCN) and propynethial (HCSCCH) towards the starless core
TMC-1. Cyano thioformaldehyde presents a series of prominent a- and b-type
lines, which are the strongest previously unassigned features in our Q-band
line survey of TMC-1. Remarkably, HCSCN is four times more abundant than cyano
formaldehyde (HCOCN). On the other hand, HCSCCH is five times less abundant
than propynal (HCOCCH). Surprisingly, we find an abundance ratio HCSCCH/HCSCN
of 0.25, in contrast with most other ethynyl-cyanide pairs of molecules for
which the CCH-bearing species is more abundant than the CN-bearing one. We
discuss the formation of these molecules in terms of neutral-neutral reactions
of S atoms with CH2CCH and CH2CN radicals as well as of CCH and CN radicals
with H2CS. The calculated abundances for the sulphur-bearing species are,
however, significantly below the observed values, which points to an
underestimation of the abundance of atomic sulphur in the model or to missing
formation reactions, such as ion-neutral reactions.
