2024ApJ...971..164C

The nitrogen isotope ratio ¹⁴N/¹⁵N is a powerful tool to trace Galactic stellar nucleosynthesis and constrain Galactic chemical evolution. Previous observations have found lower ¹⁴N/¹⁵N ratios in the Galactic center and higher values in the Galactic disk. This is consistent with the inside-out formation scenario of our Milky Way. However, previous studies mostly utilized double isotope ratios also including ¹²C/¹³C, which introduces additional uncertainties. Here we present observations of C¹⁴N and its rare isotopologue, C¹⁵N, toward a sample of star-forming regions measured by the IRAM 30 m and/or the ARO 12 m telescope at λ ∼ 3 mm wavelength. For those 35 sources detected in both isotopologues, physical parameters are determined. Furthermore, we have obtained nitrogen isotope ratios using the strongest hyperfine components of CN and C¹⁵N. For those sources showing small deviations from local thermodynamical equilibrium and/or self-absorption, the weakest hyperfine component, likely free of the latter effect, was used to obtain reliable ¹⁴N/¹⁵N values. Our measured ¹⁴N/¹⁵N isotope ratios from C¹⁴N and C¹⁵N measurements are compatible with those from our earlier measurements of NH₃ and ¹⁵NH₃ (Paper I), i.e., increasing ratios to a Galactocentric distance of ∼9 kpc. The unweighted second-order polynomial fit yields $\tfrac{{{\rm{C}}}^{14}{\rm{N}}}{{{\rm{C}}}^{15}{\rm{N}}}\,=\,(-4.85\pm 1.89)\,{\mathrm{kpc}}^{-2}\times {R}_{\mathrm{GC}}^{2}+(82.11\pm 31.93)\,{\mathrm{kpc}}^{-1}\times {R}_{\mathrm{GC}}\,-(28.12\pm 126.62)$. Toward the outer galaxy, the isotope ratio tends to decrease, supporting an earlier finding by H¹³CN/HC¹⁵N. Galactic chemical evolution models are consistent with our measurements of the ¹⁴N/¹⁵N isotope ratio, i.e., a rising trend from the Galactic center region to approximately 9 kpc, followed by a decreasing trend with increasing R_GC toward the outer Galaxy.