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

Abstract

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 $\tfrac{{{\rm{C}}}^{14}{\rm{N}}}{{{\rm{C}}}^{15}{\rm{N}}}\,=\,(-4.85\pm 1.89)\,{\mathrm{kpc}}^{-2}\times {R}_{\mathrm{GC}}^{2}+(82.11\pm 31.93)\,{\mathrm{kpc}}^{-1}\times {R}_{\mathrm{GC}}\,-(28.12\pm 126.62)$ . Toward the outer galaxy, the isotope ratio tends to decrease, supporting an earlier finding by 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.

Keywords

• Interstellar medium
• Molecular gas
• Galaxy abundances
• Galaxy evolution