Interstellar diazenylium recombination and nitrogen isotopic fractionation

In an earlier paper (Charnley & Rodgers), we demonstrated that gas-phase chemistry in selectively depleted interstellar cores and/or protostellar discs can lead to significant N-15 enhancements. Recent laboratory measurements of the electron dissociative recombination of N-2 H+ show that this reaction is able to break the N-N bond and so will release nitrogen atoms and NH radicals into the gas at a much faster rate than previous models have assumed. We have assessed the importance of this new result for the N-15 chemistry and find that the reaction acts to reduce the degree of N-15 enhancement. We discuss two modifications to our original model that each allow the high N-15/N-14 ratios to be recovered: N-2 H+ recombination on polycyclic aromatic hydrocarbon (PAH) molecules and a small barrier in the N + NH reaction. We find that higher N-15/N-14 ratios can be produced in this revised model. Some observational consequences of these are outlined.