The detection of extragalactic ${}^{15}$N: Consequences for nitrogen nucleosynthesis and chemical evolution

Abstract

Detections of extragalactic ${}^{15}$N are reported from observations of the rare hydrogen cyanide isotope HC${}^{15}$N toward the Large Magellanic Cloud (LMC) and the core of the (post-) starburst galaxy NGC 4945. Accounting for optical depth effects, the LMC data from the massive star-forming region N113 infer a ${}^{14}N{/}^{15}$N ratio of 111 $\pm$ 17, about twice the ${}^{12}C{/}^{13}$C value. For the LMC star-forming region N159HW and for the central region of NGC 4945, ${}^{14}N{/}^{15}$N ratios are also $\approx$ 100. The ${}^{14}N{/}^{15}$N ratios are smaller than all interstellar nitrogen isotope ratios measured in the disk and center of the Milky Way, strongly supporting the idea that ${}^{15}$N is predominantly of `primary' nature, with massive stars being its dominant source. Although this appears to be in contradiction with standard stellar evolution and nucleosynthesis calculations, it supports recent findings of abundant ${}^{15}$N production due to rotationally induced mixing of protons into the helium-burning shells of massive stars.

15 pages including one postscript figure, accepted for publication by ApJ Letter, further comments: please contact Yi-nan Chin

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• Nucleosynthesis in Classical Novae: ONe vs. CO White Dwarfs
• Abundances in Spiral Galaxies: Evidence for Primary Nitrogen Production

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