First Measurements of <SUP>15</SUP>N Fractionation in N<SUB>2</SUB>H<SUP>+</SUP> toward High-mass Star-forming Cores

Abstract

We report on the first measurements of the isotopic ratio <SUP>14</SUP>N/<SUP>15</SUP>N in N<SUB>2</SUB>H<SUP>+</SUP> 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 <SUP>14</SUP>N/<SUP>15</SUP>N ratio in CN have been made. The <SUP>14</SUP>N/<SUP>15</SUP>N ratios derived from N<SUB>2</SUB>H<SUP>+</SUP> 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 N<SUB>2</SUB>H<SUP>+</SUP> isotopologues are more than those detected in the CN ones. The <SUP>14</SUP>N/<SUP>15</SUP>N 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 <SUP>14</SUP>N/<SUP>15</SUP>N (as derived from N<SUB>2</SUB>H<SUP>+</SUP>) and the H/D isotopic ratios. This suggests that <SUP>15</SUP>N 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 <SUP>14</SUP>N/<SUP>15</SUP>N, pointing out that some important routes of nitrogen fractionation could be still missing in the models. <P />Based on observations carried out with the IRAM-30 m Telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).