SIMBAD references

We construct a gas-grain chemical network model which includes carbon isotopes (¹²C and ¹³C) with an emphasis on isotopomer-exchange reactions. Temporal variations of molecular abundances, the carbon isotope ratios (¹²CX/¹³CX), and the isotopomer ratios (¹²C¹³CX/¹³C¹²CX) of CCH and CCS in cold dense cloud cores are investigated by numerical calculations. We confirm that the isotope ratios of molecules, both in the gas phase and grain surfaces, are significantly different depending on whether the molecule is formed from the carbon atom (ion) or the CO molecule. Molecules formed from carbon atoms have CX/¹³CX ratios greater than the elemental abundance ratio of [¹²C/¹³C]. On the other hand, molecules formed from CO molecules have CX/¹³CX ratios smaller than the [¹²C/¹³C] ratio. We reproduce the observed C¹³CH/¹³CCH ratio in TMC-1, if the isotopomer-exchange reaction, ¹³CCH + H \rightleftharpoons C¹³CH + H + 8.1 K, proceeds with the forward rate coefficient k_f> 10^–11 cm³ s^–1. However, the C¹³CS/¹³CCS ratio is lower than that observed in TMC-1. We then assume the isotopomer-exchange reaction catalyzed by the H atom, ¹³CCS + H \rightleftharpoons C¹³CS + H + 17.4 K. In the model with this reaction, we reproduce the observed C¹³CS/¹³CCS, CCS/C¹³CS, and CCS/¹³CCS ratios simultaneously.