Nitrogen and hydrogen fractionation in high-mass star-forming cores from observations of HCN and HNC

Abstract

The ratio between the two stable isotopes of nitrogen, <SUP>14</SUP>N and <SUP>15</SUP>N, is well measured in the terrestrial atmosphere ( 272), and for the pre-solar nebula ( 441, deduced from the solar wind). Interestingly, some pristine solar system materials show enrichments in <SUP>15</SUP>N with respect to the pre-solar nebula value. However, it is not yet clear if and how these enrichments are linked to the past chemical history because we have only a limited number of measurements in dense star-forming regions. In this respect, dense cores, which are believed to be the precursors of clusters and also contain intermediate- and high-mass stars, are important targets because the solar system was probably born within a rich stellar cluster, and such clusters are formed in high-mass star-forming regions. The number of observations in such high-mass dense cores has remained limited so far. In this work, we show the results of IRAM-30 m observations of the J = 1-0 rotational transition of the molecules HCN and HNC and their <SUP>15</SUP>N-bearing counterparts towards 27 intermediate- and high-mass dense cores that are divided almost equally into three evolutionary categories: high-mass starless cores, high-mass protostellar objects, and ultra-compact HII regions. We have also observed the DNC(2-1) rotational transition in order to search for a relation between the isotopic ratios D/H and <SUP>14</SUP>N/<SUP>15</SUP>N. We derive average <SUP>14</SUP>N/<SUP>15</SUP>N ratios of 359 ± 16 in HCN and of 438 ± 21 in HNC, with a dispersion of about 150-200. We find no trend of the <SUP>14</SUP>N/<SUP>15</SUP>N ratio with evolutionary stage. This result agrees with what has been found for N<SUB>2</SUB>H<SUP>+</SUP> and its isotopologues in the same sources, although the <SUP>14</SUP>N/<SUP>15</SUP>N ratios from N<SUB>2</SUB>H<SUP>+</SUP> show a higher dispersion than in HCN/HNC, and on average, their uncertainties are larger as well. Moreover, we have found no correlation between D/H and <SUP>14</SUP>N/<SUP>15</SUP>N in HNC. These findings indicate that (1) the chemical evolution does not seem to play a role in the fractionation of nitrogen, and that (2) the fractionation of hydrogen and nitrogen in these objects is not related. <P />IRAM data used in the paper (FITS) is only available at the CDS via anonymous ftp to <A href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A> (<A href="http://130.79.128.5">http://130.79.128.5</A>) or via <A href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/609/A129">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/609/A129