The CN/C¹⁵N isotopic ratio towards dark clouds

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 ± 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 15 N and HCN can simultaneously be enriched in heavy nitrogen. The non-fractionation of N 2 H + , however, remains an open issue and we propose some chemical way of alleviating the discrepancy between model predictions and the observed ratios.

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

“…The high isotopic ratio found in L1544 implies an equivalently high ratio for molecular nitrogen, which strongly contradicts our findings. These results are markedly different from those derived by Hily-Blant et al (2013b), who find a moderate 15 N enrichment, because these authors introduced the 15 N + N 2 H + fractionation reaction, which does not occur 1 .…”

“…The future availability of collisional excitation rates of NH 2 D by H 2 (Daniel et al 2014) may give rise to additional changes. NH does not follow the same trend as NH 2 and NH 3 and is only slightly enriched in 15 N. As discussed by Hily-Blant et al (2013b), NH is mainly Table 4; the black heavy dotted line represents the elemental N/ 15 N. Lower panel: time dependence of the fractional abundances relative to H 2 of N, N 2 , and N 2 H + for models (a) and (b). The value reported for N 2 H + towards TMC1 (Ohishi et al 1992) is displayed as a horizontal green dashed line.…”

The origin of gas-phase HCO and CH3O radicals in prestellar cores

“…If the actual T ex is above 4 K, this will not change the column density values very much. We cannot, however, completely exclude T ex values between 3 and 4 K because such values have already been observed in other species (Padovani et al 2011;Hily-Blant et al 2013). Should the excitation temperatures actually be below 4 K, the column densities would be higher than those quoted in Table 3.…”

THE C 14 N/C 15 N RATIO IN DIFFUSE MOLECULAR CLOUDS

“…Hily-Blant et al (2013a, 2013b found significant differences in the nitrogen isotopic ratios derived from observations of CN and HCN isotopologues in dark molecular cloud cores. An overall ratio of 500 ± 75 was obtained for CN, while HCN exhibited ratios between 140 and 360.…”