Nitrogen fractionation in high-mass star-forming cores across the Galaxy

Abstract

The fractionation of nitrogen (N) in star-forming regions is a poorly understood process. To put more stringent observational constraints on the N-fractionation, we have observed with the IRAM-30m telescope a large sample of 66 cores in massive star-forming regions. We targeted the (1-0) rotational transition of HN${}^{13}$C, HC${}^{15}$N, H${}^{13}$CN and HC${}^{15}$N, and derived the ${}^{14}$N/${}^{15}$N ratio for both HCN and HNC. We have completed this sample with that already observed by Colzi et al. (2018), and thus analysed a total sample of 87 sources. The ${}^{14}$N/${}^{15}$N ratios are distributed around the Proto-Solar Nebula value with a lower limit near the terrestrial atmosphere value ($\sim$272). We have also derived the ${}^{14}$N/${}^{15}$N ratio as a function of the Galactocentric distance and deduced a linear trend based on unprecedented statistics. The Galactocentric dependences that we have found are consistent, in the slope, with past works but we have found a new local ${}^{14}$N/${}^{15}$N value of $\sim$400, i.e. closer to the Prosolar Nebula value. A second analysis was done, and a parabolic Galactocentric trend was found. Comparison with Galactic chemical evolution models shows that the slope until 8 kpc is consistent with the linear analysis, while the flattening trend above 8 kpc is well reproduced by the parabolic analysis.

arXiv comment32 pages, 6 figures, 3 tables, 2 appendices

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