Abundance of HCN and its C and N isotopologues in L1498
  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.
