The Detection of Extragalactic [TSUP]15[/TSUP]N: Consequences for NitrogenNucleosynthesis 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 an 14 N/ 15 N ratio of 111±17, which is 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 ≈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 synthesized by massive stars. 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.