The CN/C15 N isotopic ratio towards dark clouds

TL;DR

This study measures the nitrogen isotopic ratios in dark clouds to understand chemical processes affecting isotopic compositions, providing new observational data and chemical models that shed light on nitrogen fractionation in starless clouds.

Contribution

It presents the first measurements of CN isotopic ratios in dark clouds and offers chemical models explaining nitrogen fractionation processes.

Findings

CN/C$^{15}$N ratio is about 500 in two dark clouds.

The ratios are marginally consistent with the protosolar value of 441.

Models suggest CN can be depleted in $^{15}$N while HCN is enriched.

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 C$^{15}$N towards two starless dark clouds, L1544 and L1498. We were able to derive the column density ratio of C$^{15}$N over $^{13}$CN towards the same clouds, and obtain the CN/C$^{15}$N isotopic ratios, which were found to be $500\pm75$ 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 $^{15}$N 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.