2018A&A...615A..52M

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 ¹⁵N 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/HC¹⁵N abundance ratios suggest that HCN may already be enriched in ¹⁵N in prestellar cores, although doubts remain on these measurements, which rely on the double-isotopologue method. Here we present direct measurements of the HCN/H¹³CN and HCN/HC¹⁵N abundance ratios in the L1498 prestellar core based on spatially resolved spectra of HCN(1-0), (3-2), H¹³CN(1-0), and HC¹⁵N(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-H₂ 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/H¹³CN=45±3 and HCN/HC¹⁵N=338±28, indicate that carbon is heavily fractionated in HCN, but nitrogen is not. For the H¹³CN/HC¹⁵N 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 ¹³C enrichment we found in HCN could explain the superfractionation of nitrogen measured in solar system chondrites.