Interstellar Nitrogen Isotope Ratios: Measurements on Tracers of C¹⁴N and C¹⁵N

The nitrogen isotope ratio 14 N/ 15 N is a powerful tool to trace Galactic stellar nucleosynthesis and constrain Galactic chemical evolution. Previous observations have found lower 14 N/ 15 N 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 12 C/ 13 C, which introduces additional uncertainties. Here we present observations of C 14 N and its rare isotopologue, C 15 N, 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 C 15 N. 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 14 N/ 15 N values. Our measured 14 N/ 15 N isotope ratios from C 14 N and C 15 N measurements are compatible with those from our earlier measurements of NH 3 and 15 NH 3 (Paper I), i.e., increasing ratios to a Galactocentric distance of ∼9 kpc. The unweighted second-order polynomial fit yields C 14 N C 15 N = ( − 4.85 ± 1.89 ) kpc − 2 × R GC 2 + ( 82.11 ± 31.93 ) kpc − 1 × R GC − ( 28.12 ± 126.62 ) . Toward the outer galaxy, the isotope ratio tends to decrease, supporting an earlier finding by H 13 CN/HC 15 N. Galactic chemical evolution models are consistent with our measurements of the 14 N/ 15 N 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.