Carbon isotope fractionation and depletion in TMC1

TL;DR

This study investigates carbon isotope fractionation in TMC1, testing whether observed anomalies are due to 13C+ depletion or other mechanisms, and finds no evidence of 12C/13C anomalies in the observed molecules.

Contribution

The paper provides observational evidence that 12C/13C anomalies are species-specific and not due to 13C+ depletion in dense gas environments.

Findings

No 12C/13C anomalies observed in HNC, CS, and H2CS isotopologues.

13C+ likely depleted, but general carbon pool remains unaffected outside CO.

Anomalies are specific to certain molecules, not universal across all carbon-bearing species.

Abstract

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 in several species hosting more than one carbon atom, i.e. CCH, CCS, CCCS and HC$_3$N. 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 $\lambda$3mm emission from carbon, sulfur and nitrogen-bearing isotopologues of HNC, CS and \HH CS at three positions in Taurus(TMC1, L1527 and the ammonia peak) using the ARO 12m 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 C^0. The observed isotopologic abundance anomalies are peculiar to those species in which they are found.