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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mstyle displaystyle="false"> <mml:mfrac> <mml:mrow> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">C</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>14</mml:mn> </mml:mrow> </mml:msup> <mml:mi mathvariant="normal">N</mml:mi> </mml:mrow> <mml:mrow> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">C</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>15</mml:mn> </mml:mrow> </mml:msup> <mml:mi mathvariant="normal">N</mml:mi> </mml:mrow> </mml:mfrac> </mml:mstyle> <mml:mspace width="0.25em"/> <mml:mo>=</mml:mo> <mml:mspace width="0.25em"/> <mml:mo stretchy="false">(</mml:mo> <mml:mo>−</mml:mo> <mml:mn>4.85</mml:mn> <mml:mo>±</mml:mo> <mml:mn>1.89</mml:mn> <mml:mo stretchy="false">)</mml:mo> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mi>kpc</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> <mml:mo>×</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>GC</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>+</mml:mo> <mml:mo stretchy="false">(</mml:mo> <mml:mn>82.11</mml:mn> <mml:mo>±</mml:mo> <mml:mn>31.93</mml:mn> <mml:mo stretchy="false">)</mml:mo> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mi>kpc</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:mo>×</mml:mo> <mml:msub> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>GC</mml:mi> </mml:mrow> </mml:msub> <mml:mspace width="0.25em"/> <mml:mo>−</mml:mo> <mml:mo stretchy="false">(</mml:mo> <mml:mn>28.12</mml:mn> <mml:mo>±</mml:mo> <mml:mn>126.62</mml:mn> <mml:mo stretchy="false">)</mml:mo> </mml:math> . 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.