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

Abstract

The fractionation of nitrogen (N) in star-forming regions is a poorly\nunderstood process. To put more stringent observational constraints on the\nN-fractionation, we have observed with the IRAM-30m telescope a large sample of\n66 cores in massive star-forming regions. We targeted the (1-0) rotational\ntransition of HN$^{13}$C, HC$^{15}$N, H$^{13}$CN and HC$^{15}$N, and derived\nthe $^{14}$N/$^{15}$N ratio for both HCN and HNC. We have completed this sample\nwith that already observed by Colzi et al. (2018), and thus analysed a total\nsample of 87 sources. The $^{14}$N/$^{15}$N ratios are distributed around the\nProto-Solar Nebula value with a lower limit near the terrestrial atmosphere\nvalue ($\\sim$272). We have also derived the $^{14}$N/$^{15}$N ratio as a\nfunction of the Galactocentric distance and deduced a linear trend based on\nunprecedented statistics. The Galactocentric dependences that we have found are\nconsistent, in the slope, with past works but we have found a new local\n$^{14}$N/$^{15}$N value of $\\sim$400, i.e. closer to the Prosolar Nebula value.\nA second analysis was done, and a parabolic Galactocentric trend was found.\nComparison with Galactic chemical evolution models shows that the slope until 8\nkpc is consistent with the linear analysis, while the flattening trend above 8\nkpc is well reproduced by the parabolic analysis.\n