FIRST MEASUREMENTS OF N-15 FRACTIONATION IN N2H+ TOWARD HIGH-MASS STAR-FORMING CORES

We report on the first measurements of the isotopic ratio N-14/N-15 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 N-14/N-15 ratio in CN have been made. The N-14/N-15 ratios derived from N2H+ show a large spread (from similar to 180 up to similar to 1300), while those derived from CN are in between the value measured in the terrestrial atmosphere (similar to 270) and that of the proto-solar nebula (similar to 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 N-14/N-15 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 N-14/N-15 (as derived from N2H+) and the H/D isotopic ratios. This suggests that N-15 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 N-14/N-15, pointing out that some important routes of nitrogen fractionation could be still missing in the models.