Isotopic fractionation of carbon, deuterium, and nitrogen: a full chemical study^⋆

¹ LERMA, Observatoire de Paris, PSL Research University, CNRS, UMR8112, Place Janssen, 92190 Meudon Cedex, France
e-mail: evelyne.roueff@obspm.fr
² Sorbonne Universités, UPMC Univ. Paris 6, 4 Place Jussieu, 75005 Paris, France
³ ISM, Université de Bordeaux – CNRS, UMR 5255, 351 cours de la Libération, 33405 Talence Cedex, France
e-mail: jean-christophe.loison@u-bordeaux.fr

Received: 6 October 2014
Accepted: 5 January 2015

Abstract

Context. The increased sensitivity and high spectral resolution of millimeter telescopes allow the detection of an increasing number of isotopically substituted molecules in the interstellar medium. The ¹⁴N/¹⁵N ratio is difficult to measure directly for molecules containing carbon.

Aims. Using a time-dependent gas-phase chemical model, we check the underlying hypothesis that the ¹³C/¹²C ratio of nitriles and isonitriles is equal to the elemental value.

Methods. We built a chemical network that contains D, ¹³C, and ¹⁵N molecular species after a careful check of the possible fractionation reactions at work in the gas phase.

Results. Model results obtained for two different physical conditions that correspond to a moderately dense cloud in an early evolutionary stage and a dense, depleted prestellar core tend to show that ammonia and its singly deuterated form are somewhat enriched in ¹⁵N, which agrees with observations. The ¹⁴N/¹⁵N ratio in N₂H⁺ is found to be close to the elemental value, in contrast to previous models that obtain a significant enrichment, because we found that the fractionation reaction between ¹⁵N and N₂H⁺ has a barrier in the entrance channel. The high values of the N₂H⁺/¹⁵NNH⁺ and N₂H⁺/N¹⁵NH⁺ ratios derived in L1544 cannot be reproduced in our model. Finally, we find that nitriles and isonitriles are in fact significantly depleted in ¹³C, thereby challenging previous interpretations of observed C¹⁵N, HC¹⁵N, and H¹⁵NC abundances from ¹³C containing isotopologues.