Carbon isotope fractionation and depletion in TMC1

¹²C/¹³C isotopologue abundance anomalies have long been predicted for gas-phase chemistry in molecules other than CO and have recently been observed in the Taurus molecular cloud (TMC) in several species hosting more than one carbon atom, i.e., CCH, CCS, CCCS, and HC₃N. Here we work to ascertain whether these isotopologic anomalies actually result from the predicted depletion of the ¹³C⁺ ion in an oxygen-rich optically shielded dense gas, or from some other more particular mechanism or mechanisms. We observed λ3mm emission from carbon-, sulfur-, and nitrogen-bearing isotopologues of HNC, CS, and H₂CS at three positions in Taurus (TMC1, L1527, and the NH₃ peak) using the ARO 12 m telescope. We saw no evidence of ¹²C/¹³C anomalies in our observations. Although the pool of C⁺ is likely to be depleted in ¹³C, ¹³C is not depleted in the general pool of carbon outside CO, which probably exists mostly in the form of C⁰. The observed isotopologic abundance anomalies are peculiar to those species in which they are found.