Formation of interstellar CCS and CCCS: a case for radical/neutral chemistry?

Pathways to the abundant sulphur-containing carbon chain molecules, CCS and CCCS, are examined. The previously proposed ion/molecule mechanisms for the formation of these species appear satisfactory in the case of CCS, but arguments for the formation of CCCS via ion/molecule reactions rely on several assumptions which do not appear justified in the absence of sufficient experimental evidence. Alternative, radical/neutral pathways to CCS and CCCS (namely the reactions of C2H with S, and with CS respectively) are proposed which can account for the observed high abundances of these species within TMC-1 and other objects. Thermochemical aspects of the various CxS production mechanisms are assessed in the light of recent ab initio calculations on the chemical species concerned. Experimental study of the reaction of C2H with CS is strongly urged.