${}^{14}$N${/}^{15}$N abundance ratio toward massive star-forming regions with different Galactic distances

Abstract

The abundance ratio of ${}^{14}$N${/}^{15}$N is, in principle, a powerful tool for tracing stellar nucleosynthesis. This work aims to measure and analyze (${}^{14}$N/${}^{15}$N)$\times$(${}^{13}$C/${}^{12}$C) and ${}^{14}$N${/}^{15}$N abundance ratios in massive star-forming regions across a range of galactocentric distances to provide constraints on galactic chemical evolution (GCE) models. We present H${}^{13}$CN and HC${}^{15}$N J=2-1 results toward 51massive star-forming regions obtained with the Institut de Radioastronomie Millimétrique (IRAM) 30 meter telescope.\ We used these results to derive (${}^{14}$N/${}^{15}$N)$\times$(${}^{13}$C/${}^{12}$C) abundance ratios as well as ${}^{14}$N${/}^{15}$N ratios using the double isotope method. We find an overall decreasing trend in the (${}^{14}$N/${}^{15}$N)$\times$(${}^{13}$C/${}^{12}$C) abundance ratio and an increasing trend in the ${}^{14}$N${/}^{15}$N ratio with increasing galactocentric distance ($D_{\mathrm{GC}}$), which provides a good constraint for the GCE model based on high signal to noise ratio measurements. While the predicted (${}^{14}$N/${}^{15}$N)$\times$(${}^{13}$C/${}^{12}$C) ratios between 6 and 12 kpc determined using current GCE models are consistent with our observational results, the ratios from models for $D_{\mathrm{GC}}$ less than 6 kpc are significantly higher than the observational results, which indicates GCE models for ${}^{14}$N/${}^{15}$N and/or ${}^{13}$C/${}^{12}$C ratios need to be updated for at least this range.

9 pages, 5 figures, 2 tables, accepted for publication in the A&A Letters

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