2015ApJ...808L..46F

We report on the first measurements of the isotopic ratio ¹⁴N/¹⁵N in N₂H⁺ toward a statistically significant sample of high-mass star-forming cores. The sources belong to the three main evolutionary categories of the high-mass star formation process: high-mass starless cores, high-mass protostellar objects, and ultracompact H ii regions. Simultaneous measurements of the ¹⁴N/¹⁵N ratio in CN have been made. The ¹⁴N/¹⁵N ratios derived from N₂H⁺ show a large spread (from ∼180 up to ∼1300), while those derived from CN are in between the value measured in the terrestrial atmosphere (∼270) and that of the proto-solar nebula (∼440) for the large majority of the sources within the errors. However, this different spread might be due to the fact that the sources detected in the N₂H⁺ isotopologues are more than those detected in the CN ones. The ¹⁴N/¹⁵N ratio does not change significantly with the source evolutionary stage, which indicates that time seems to be irrelevant for the fractionation of nitrogen. We also find a possible anticorrelation between the ¹⁴N/¹⁵N (as derived from N₂H⁺) and the H/D isotopic ratios. This suggests that ¹⁵N enrichment could not be linked to the parameters that cause D enrichment, in agreement with the prediction by recent chemical models. These models, however, are not able to reproduce the observed large spread in ¹⁴N/¹⁵N, pointing out that some important routes of nitrogen fractionation could be still missing in the models.