The detection of extragalactic N-15: Consequences for nitrogen nucleosynthesis and chemical evolution

Detections of extragalactic N-15 are reported from observations of the rare hydrogen cyanide isotope (HCN)-N-15 reward the Large Magellanic Cloud (LMC ) and the core of the (post-)starburst galaxy NGC 4945. Accounting for opti cal depth effects, the LMC data from the massive star-forming region N113 i nfer an N-14/N-15 ratio of 111 +/- 17, which is about twice the C-12/C-13 v alue. For the LMC star-forming region N159HW and for the central region of NGC 4945, N-14/N-15 ratios are also approximate to 100. The N-14/N-15 ratio s are smaller than all interstellar nitrogen isotope ratios measured in the disk and center of the Milky Way, strongly supporting the idea that N-15 i s synthesized by massive stars. Although this appears to be in contradictio n with standard stellar evolution and nucleosynthesis calculations, it supp orts recent findings of abundant N-15 production due to rotationally induce d mixing of protons into the helium-burning shells of massive stars.