15N fractionation in infrared-dark cloud cores

Context. Nitrogen is one of the most abundant elements in the Universe and its N-14/N-15 isotopic ratio has the potential to provide information about the initial environment in which our Sun formed. Recent findings suggest that the solar system may have formed in a massive cluster since the presence of short-lived radioisotopes in meteorites can only be explained by the influence of a supernova. Aims. We seek to determine the N-14/N-15 ratio towards a sample of cold and dense cores at the initial stages in their evolution. Methods. We observed the J = 1 -> 0 transitions of HCN, (HCN)-C-13, (HCN)-N-15, (HNC)-C-13, and (HNC)-N-15 towards a sample of 22 cores in four infrared-dark clouds (IRDCs) which are believed to be the precursors of high-mass stars and star clusters. Assuming LTE and a temperature of 15 K, the column densities of HCN, (HCN)-C-13, (HCN)-N-15, (HNC)-C-13, and (HNC)-N-15 are calculated and their N-14/N-15 ratio is determined for each core. Results. The N-14/N-15 ratios measured in our sample of IRDC cores range between similar to 70 and >= 763 in HCN and between similar to 161 and similar to 541 in HNC. These ratios are consistent with the terrestrial atmosphere (TA) and protosolar nebula (PSN) values, and with the ratios measured in low-mass prestellar cores. However, the N-14/N-15 ratios measured in cores C1, C3, F1, F2, and G2 do not agree with the results from similar studies towards the same cores using nitrogen bearing molecules with nitrile functional group (-CN) and nitrogen hydrides (-NH) although the ratio spread covers a similar range. Conclusions. Relatively low N-14/N-15 ratios amongst the four-IRDCs were measured in IRDC G which are comparable to those measured in small cosmomaterials and protoplanetary disks. The low average gas density of this cloud suggests that the gas density, rather than the gas temperature, may be the dominant parameter influencing the initial nitrogen isotopic composition in young PSN.