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

By Keith Cowing
Status Report
June 20, 2024
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Interstellar Nitrogen Isotope Ratios: Measurements on tracers of C14N and C15N
Visual comparison of the 14N/15N ratios obtained from CN (blue circles), NH3 (green squares), HCN (yellow stars) and HNC (red diamonds) in different star forming regions. The purple line shows the nitrogen ratio of the terrestrial atmosphere (TA), derived from N2, ∼ 272 (Marty et al. 2009). The cyan line presents the ratio of 441 ± 6 measured for the Proto-Solar Nebula (PSN) from the solar wind (Marty et al. 2010). — astro-ph.GA

The nitrogen isotope ratio 14N/15N is a powerful tool to trace Galactic stellar nucleosynthesis and constraining 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 therefore 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 Galacticentric distance of ~9 kpc. The unweighted second order polynomial fit yields C14NC15N=(−4.85±1.89)kpc−2×R2GC+(82.11±31.93)kpc−1×RGC−(28.12±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 RGC toward the outer Galaxy.

J. L. Chen, J. S. Zhang, C. Henkel, Y. T. Yan, H. Z. Yu, Y. X. Wang, Y. P. Zou, J. Y. Zhao, X. Y. Wang

Comments: 34 pages, 9 figures, 6 tables
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2406.09683 [astro-ph.GA] (or arXiv:2406.09683v1 [astro-ph.GA] for this version)
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Journal reference: The Astrophysical Journal (2004)
Submission history
From: Jialiang Chen
[v1] Fri, 14 Jun 2024 03:10:01 UTC (1,995 KB)
Astrobiology, astrochemistry,

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻