The CN/C¹⁵N isotopic ratio towards dark clouds ^⋆,⋆⋆

¹ Université Joseph Fourier and CNRS, Institut de Planétologie et d’Astrophysique, 38041 Grenoble, France
e-mail: Pierre.Hily-Blant@obs.ujf-grenoble.fr
² Université Paris-Orsay and CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
³ LRA/CNRS, Observatoire de Paris & École normale supérieure, 75005 Paris, France

Received: 26 February 2013
Accepted: 26 June 2013

Abstract

Understanding the origin of the composition of solar system cosmomaterials is a central question, not only in the cosmochemistry and astrochemistry fields, and requires various approaches to be combined. Measurements of isotopic ratios in cometary materials provide strong constraints on the content of the protosolar nebula. Their relation with the composition of the parental dark clouds is, however, still very elusive. In this paper, we bring new constraints based on the isotopic composition of nitrogen in dark clouds, with the aim of understanding the chemical processes that are responsible for the observed isotopic ratios. We have observed and detected the fundamental rotational transition of C¹⁵N towards two starless dark clouds, L1544 and L1498. We were able to derive the column density ratio of C¹⁵N over ¹³CN towards the same clouds and obtain the CN/C¹⁵N isotopic ratios, which were found to be 500 ± 75 for both L1544 and L1498. These values are therefore marginally consistent with the protosolar value of 441. Moreover, this ratio is larger than the isotopic ratio of nitrogen measured in HCN. In addition, we present model calculations of the chemical fractionation of nitrogen in dark clouds, which make it possible to understand how CN can be deprived of ¹⁵N and HCN can simultaneously be enriched in heavy nitrogen. The non-fractionation of N₂H⁺, however, remains an open issue, and we propose some chemical way of alleviating the discrepancy between model predictions and the observed ratios.