The Detection of Extragalactic [TSUP]15[/TSUP]N: Consequences for NitrogenNucleosynthesis and Chemical Evolution

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Abstract
Detections of extragalactic 15N are reported from observations of the rare hydrogen cyanide isotope HC15N 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 an 14N/15N ratio of 111±17, which is about twice the 12C/13C value. For the LMC star-forming region N159HW and for the central region of NGC 4945, 14N/15N ratios are also ≈100. The 14N/15N ratios are smaller than all interstellar nitrogen isotope ratios measured in the disk and center of the Milky Way, strongly supporting the idea that 15N is synthesized by massive stars. Although this appears to be in contradiction with standard stellar evolution and nucleosynthesis calculations, it supports recent findings of abundant 15N production due to rotationally induced mixing of protons into the helium-burning shells of massive stars.
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