¹⁵N fractionation in infrared-dark cloud cores

Abstract

Context. Nitrogen is one of the most abundant elements in the Universe and its 14 N/ 15 N isotopic ratio has the potential to provide information about the initial environment in which our Sun formed. Recent findings suggest that the Solar System may have formed in a massive cluster since the presence of short-lived radioisotopes in meteorites can only be explained by the influence of a supernova. Aims. To determine the 14 N/ 15 N ratio towards a sample of cold, massive dense cores at the initial stages in their evolution. Methods. We have observed the J=1→0 transitions of HCN, H 13 CN, HC 15 N, HN 13 C and H 15 NC toward a sample of 22 cores in 4 Infrared-Dark Clouds (IRDCs). IRDCs are believed to be the precursors of high-mass stars and star clusters. Assuming LTE and a temperature of 15 K, the column densities of HCN, H 13 CN, HC 15 N, HN 13 C and H 15 NC are calculated and their 14 N/ 15 N ratio is determined for each core. Results. The 14 N/ 15 N ratio measured in our sample of IRDC cores range between ∼70 and ≥763 in HCN and between ∼161 and ∼541 in HNC. They are consistent with the terrestrial atmosphere (TA) and protosolar nebula (PSN) values, and with the ratios measured in low-mass pre-stellar cores. However, the 14 N/ 15 N ratios measured in cores C1, C3, F1, F2 and G2 do not agree with the results from similar studies toward the same massive cores using nitrogen bearing molecules with nitrile functional group (-CN) and nitrogen hydrides (-NH) although the ratio spread covers a similar range. Conclusions. Amongst the 4 IRDCs we measured relatively low 14 N/ 15 N ratios towards IRDC G which are comparable to those measured in small cosmomaterials and protoplanetary disks. The low average gas density of this cloud suggests that the gas density, rather than the gas temperature, may be the dominant parameter influencing the initial nitrogen isotopic composition in young PSN.

Gas phase Elemental abundances in Molecular cloudS (GEMS)

“…In order to avoid opacity and self-absorption effects, we prefer to trace the behavior of this molecule using its isotopolog, H 13 CN. We are aware that fractionation might be important for HCN and that the HCN/H 13 CN ratio can vary by a factor of about two (Roueff et al 2015;Zeng et al 2017;Loison et al 2019a;Colzi et al 2020;Loison et al 2020). Thus, H 13 CN is a good proxy for HCN with an uncertainty of a factor of two.…”

Nitrogen isotope fractionation in protoplanetary disks

“…Nitrogen isotope fractionation is also observed outside the solar system, from diffuse clouds to prestellar cores and circumstellar disks (Lucas & Liszt, 1998;Wampfler et al, 2014;Zeng et al, 2017;Guzmán et al, 2017;Hily-Blant et al, 2017). The lowest and highest 14 N/ 15 N ratio are a factor of 40 apart, clearly indicating various degrees of fractionation towards different sources.…”

The nitrogen isotopic ratio of HC3N towards the L1544 prestellar core

“…More specifically, the main repository of nitrogen in the protosolar nebula (PSN) is still unclear, although there is some consensus that it may be atomic, N, or molecular, N 2 (Schwarz & Bergin 2014). Furthermore, the large variations of the isotopic ratio of nitrogen ( 14 N/ 15 N), as measured in various carriers within different types of solar system objects, remain unexplained (Aléon 2010;Füri & Marty 2015;Hily-Blant et al 2013a, 2017. One striking problem is the 14 N/ 15 N isotopic ratio of nitrogen in comets.…”