Nitrogen production in intermediate-mass stars by hot-bottom burning

The PHOENIX spectrograph has been used on Gemini South in order to determine C, N and O abundances for intermediate mass AGB stars in the Magellanic Clouds. Although seven stars were observed, the spectra of three stars were unusable because they were too cool, causing the molecular features to be too strong for modelling. In another two stars, the spectra were contaminated by emission from shock waves. Pulsation modelling showed that spectra should only be taken near minimum visual light to avoid this problem. The two stars analysed for abundances showed strong enhancements of N and reductions of C, consistent with the expectation of hot-bottom burning. The observed N enhancements and C reductions suggest that envelope convection goes deeper than predicted by evolutionary models using the mixing-length theory of convection. These results provide the first observational in-situ confirmation of primary nitrogen production by hot-bottom burning in intermediate mass AGB stars.