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 15 N 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 15 N chemistry and find that the reaction acts to reduce the degree of 15 N enhancement. We discuss two modifications to our original model that each allow the high 15 N/ 14 N 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 15 N/ 14 N ratios can be produced in this revised model. Some observational consequences of these are outlined.