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

¹ Università degli studi di Firenze, Dipartimento di fisica e Astronomia, via Sansone, 1 –50019 Sesto Fiorentino, Italy
² INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
e-mail: colzi@arcetri.astro.it
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching bei München, Germany
⁴ CNRS, IPAG, 38000 Grenoble, France
⁵ Univ. Grenoble Alpes, IPAG, 38000 Grenoble, France

Received: 8 February 2017
Accepted: 22 October 2017

Abstract

The ratio between the two stable isotopes of nitrogen, ¹⁴N and ¹⁵N, 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 ¹⁵N 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 ¹⁵N-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 ¹⁴N/¹⁵N. We derive average ¹⁴N/¹⁵N 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 ¹⁴N/¹⁵N ratio with evolutionary stage. This result agrees with what has been found for N₂H⁺ and its isotopologues in the same sources, although the ¹⁴N/¹⁵N ratios from N₂H⁺ 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 ¹⁴N/¹⁵N 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.