Abundance of HCN and its C and N isotopologues in L1498

Abstract

The isotopic ratio of nitrogen in nearby protoplanetary disks, recently measured in CN and HCN, indicates that a fractionated reservoir of volatile nitrogen is available at the earliest stage of comet formation. This reservoir also presents a 3:1 enrichment in 15 N relative to the elemental ratio of 330, identical to that between the solar system comets and the protosun, suggesting that similar processes are responsible for the fractionation in the protosolar nebula (PSN) and in these PSN analogs. However, where, when, and how the fractionation of nitrogen takes place is an open question. Previously obtained HCN/HC 15 N abundance ratios suggest that HCN may already be enriched in 15 N in prestellar cores, although doubts remain on these measurements, which rely on the double-isotopologue method. Here we present direct measurements of the HCN/H 13 CN and HCN/HC 15 N abundance ratios in the L1498 prestellar core based on spatially resolved spectra of HCN(1-0), (3-2), H 13 CN(1-0), and HC 15 N(1-0) rotational lines. We use state-of-the-art radiative transfer calculations using ALICO, a 1D radiative transfer code capable of treating hyperfine overlaps. From a multiwavelength analysis of dust emission maps of L1498, we derive a new physical structure of the L1498 cloud. We also use new, high-accuracy HCN-H 2 hyperfine collisional rates, which enable us to quantitatively reproduce all the features seen in the line profiles of HCN(1-0) and HCN(3-2), especially the anomalous hyperfine line ratios. Special attention is devoted to derive meaningful uncertainties on the abundance ratios. The obtained values, HCN/H 13 CN=45±3 and HCN/HC 15 N=338±28, indicate that carbon is heavily fractionated in HCN, but nitrogen is not. For the H 13 CN/HC 15 N abundance ratio, our detailed study validates to some extent analyses based on the single excitation temperature assumption. Comparisons with other measurements from the literature suggest significant core-to-core variability. Furthermore, the heavy 13 C enrichment we found in HCN could explain the superfractionation of nitrogen measured in solar system chondrites.

A sensitive λ 3 mm line survey of L483

“…In the case of nitrogen, direct 14 N/ 15 N ratios could only be obtained for HCN (more specifically for H 13 CN) and HC 3 N. In both cases the observed ratios are consistent with the local ISM values, and therefore we can safely state that 15 N fractionation is not important for these two molecules in L483. Similar conclusions have been found for these two molecules in other cold dense environments like TMC-1, B1b, L1544, or L1498 (e.g., Daniel et al 2013; Taniguchi & Saito 2017; Magalhães et al 2018; Hily-Blant et al 2018). In the case of N 2 H + , the two 15 N substituted isotopologs have slightly different abundances, with a N 15 NH + / 15 NNH + ratio of 1.6, and it is very likely that this molecule is affected by a dilution in 15 N, as the N 2 H + / 15 NNH + and N 2 H + /N 15 NH + ratios we find are >747 and >450, respectively.…”

A primordial 15N-depleted organic component detected within the carbonaceous chondrite Maribo

“…1). Moreover, their nitrogen isotopic ratios are close to values found in the local interstellar medium [19][20][21][22] and overlap with the ranges observed for several molecular clouds [23][24][25][26][27][28][29][30][31][32][33] . They also point towards the solar value (δ 15 N air ~ -380 ‰) obtained by analyses of solar wind samples of the Genesis mission 34 and the composition of Jupiter´s atmosphere [35][36][37] (Fig.…”

Depletion and fractionation of nitrogen in collapsing cores

“…Further evidence is provided by the interferometric study of Colzi et al (2019), showing spatial variation of the abundance ratio in a single protostar. In the context of low-mass prestellar cores, source-to-source variations -even when located in the same large-scale environment -were also proposed by Magalhães et al (2018) to explain the HCN/HC 15 N ratio. Hily-Blant et al (2017) have noted the large scatter of the directly measured CN/C 15 N ratio along various lines of sight through the diffuse ISM (Ritchey et al 2015), with the values being mutually inconsistent by more than 2σ.…”

“…It has been proposed that the 15 N-rich nitrogen reservoirs observed in the Solar System could be formed by chemical mass fractionation in the parent interstellar cloud (Charnley & Rodgers 2002;Hily-Blant et al 2013a,b;Furuya & Aikawa 2018); but more sophisticated modeling of the observations has called this interpretation of nitrogen fractionation into question (Roueff et al 2015). That chemical fractionation is not efficient, even in cold gas, is also supported by state-of-the-art determinations of the HCN:HC 15 N ratio in the L1498 pre-stellar core by Magalhães et al (2018). These authors obtained HCN:HC 15 N = 338 ± 28, which is in harmony with the bulk nitrogen isotopic ratio R tot = 330 in the ISM (Hily-Blant et al 2017) and indicates that chemical mass fractionation is inefficient, at least for HCN.…”

“…The derived ratio was R = 476 ± 70, where the error was only statistical and did not include the uncertainty intrinsic to the double-isotope method. To obtain a direct measurement, we used the source model of Magalhães et al (2018) and followed the same methodology and assumptions. The minimization, performed using Markov chains, was done simultaneously on the CN, 13 CN, and C 15 N, spectra obtained along cuts across the core (see Figs.…”