Nitrogen and hydrogen fractionation in high-mass star-forming cores from observations of HCN and HNC

Abstract

The ratio between the two stable isotopes of nitrogen, 14 N and 15 N, is well measured in the terrestrial atmosphere (∼ 272), and for the pre-Solar nebula (∼ 441, deduced from the Solar wind). Interestingly, some pristine Solar System materials show enrichments in 15 N with respect to the pre-Solar nebula value. However, it is not yet clear if, and how, these enrichments are linked to the past chemical history, due to the limited number of measurements in dense star-forming regions. In this respect, dense cores believed to be precursors of clusters containing also intermediate-and high-mass stars are important targets, as the Solar System was probably born within a rich stellar cluster. The number of observations in such high-mass dense cores has remained limited so far. In this work, we show the results of IRAM-30m observations of the J=1-0 rotational transition of the molecules HCN and HNC, and their 15 N-bearing counterparts, towards 27 intermediate/high-mass dense cores divided almost equally in three evolutionary categories: high-mass starless cores, high-mass protostellar objects, and ultra-compact Hii regions. We have also observed the DNC(2-1) rotational transition, in order to search for a relation between the isotopic ratios D/H and 14 N/ 15 N. We derive average 14 N/ 15 N ratios of 359±16 in HCN and of 438±21 in HNC, with a dispersion of about 150-200. We find no trend of the 14 N/ 15 N ratio with the evolutionary stage. This result agrees with what found from N 2 H + and its isotopologues in the same sources, although the 14 N/ 15 N ratios from N 2 H + show a dispersion larger than that in HCN/HNC. Moreover, we have found no correlation between D/H and 14 N/ 15 N in HNC. These findings indicate that: (1) the chemical evolution does not seem to play a role in the fractionation of nitrogen; (2) the fractionation of hydrogen and nitrogen in these objects are not related.

Nobeyama 45 m mapping observations toward Orion A. III. Multi-line observations toward an outflow-shocked region, Orion Molecular Cloud 2 FIR 4

“…On the other hand, in cold dense gas, Tennekes et al (2006) derived a somewhat large ratio of 3 − 4 toward Cha-MM1. Colzi et al (2018) showed that for high-mass starless cores, the Schilke et al 1992;Talbi et al 1996). Colzi et al (2018) ratios for stronger FUV.…”

“…Colzi et al (2018) showed that for high-mass starless cores, the Schilke et al 1992;Talbi et al 1996). Colzi et al (2018) ratios for stronger FUV. However, the parameter space is not fully explored for the effects of the FUV radiation.…”

“…As we mentioned above, there are at least two effects discussed for reducing the [HNC]/ [HCN] ratio (Hirota et al 1998;Colzi et al 2018;Aguado et al 2017 (2018) estimated the dimensionless FUV strength G 0 from the Herschel 70 µm data and they found that the G 0 parameter of the FIR 4 region exceeds 100 and peaked at around FIR 3 (∼ 7000) and…”

First sample of N2H+ nitrogen isotopic ratio measurements in low-mass protostars

“…Usually, nitriles (HCN, HNC, and CN) appear enriched in 15 N with respect to the protosolar value: Wampfler et al (2014) found A&A 644, A29 (2020) 14 N/ 15 N = 160-460 in a sample of low-mass protostars; Hily-Blant et al (2013a,b) found values of 140-360 in HCN/HNC and 470-510 in CN in prestellar cores. In high-mass star forming regions, Colzi et al (2018) found 14 N/ 15 N = 250-650. It is important to highlight, however, that all these measurements were derived using the double-isotope methods, i.e.…”

Nitrogen isotopic ratio across the Galaxy through observations of high-mass star-forming cores

“…We have derived the 14 N/ 15 N ratio in a sample of 87 high-mass star-forming cores, from observations of HCN(1-0) and HNC(1-0), observed with the IRAM-30m telescope (Colzi et al 2018a andColzi et al 2018b) and we have found that the ratio spans the range 100-1000, with most of the sources (25%) having values in the range 310 ¡ 14 N/ 15 N ¡ 350 (see Fig. 2 in Colzi et al 2018b), namely below the PSN value and just above the terrestrial atmosphere (TA) value (272).…”