1999ApJ...512L.143C

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.