Abundance of HCN and its C and N isotopologues in L1498 (1803.11140v1)

Abstract: The isotopic ratio of nitrogen in nearby protoplanetary disks, recently measured in CN and HCN, indicates that a fractionated reservoir of nitrogen is available at the stage of comet formation. This reservoir presents a 3:1 enrichment in 15N relative to the elemental ratio of 330, identical to that between comets and the sun, suggesting that similar processes are responsible for the fractionation in the protosolar nebula (PSN) and in PSN analogs. However, how the fractionation of nitrogen takes place is an open question. Previously obtained HCN/HC15N abundance ratios suggest that HCN may 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). We use state-of-the-art radiative transfer calculations using ALICO, a 1D radiative transfer code capable of treating hyperfine (hf) overlaps. From an analysis of dust emission maps of L1498, we derive a new physical structure. We also use new, high-accuracy HCN-H2 hf collisional rates, which enable us to quantitatively reproduce all the features of the HCN(1-0) and HCN(3-2) spectra, especially the anomalous hf 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 fractionated in HCN, but nitrogen is not. For the HCN/HC15N abundance ratio, our study validates to some extent analyses based on the single excitation temperature assumption. Comparisons with other measurements suggest significant core-to-core variability. Furthermore, the 13C enrichment we found in HCN could explain the superfractionation of nitrogen measured in solar system chondrites.