Dr Harvey Liszt (NRAO)
Cosmic rays induce a low level of ionization of helium in dense dark gas where the main sink of He+ is He+ + CO -> C+ + O + He, at least until CO depletes onto grains. So C+ is introduced into the gas relatively quickly, but the carbon exchange reaction of C+ and CO selectively returns 13C+ to CO (because it is more tightly bound than 12CO, by 35K), depleting the C+ pool in 13C+. This leads to substantial carbon isotope fractionation in molecules that from or exchange carbon with C+. However there is also a pool of neutral atomic carbon in the gas, perhaps a few percent of CO, whose origin is less clear but probably arises from photodissociation of CO and is not fractionated. So the pools of free neutral and ionized carbon may have different isotope ratios, leading to a variety of complex chemical effects that provide particular signatures of the chemistry but greatly complicate the interpretation of molecular isotopomeric abundance ratios. I will demonstrate various of these effects and discuss why CO itself should not be strongly fractionated.