Abundances in intermediate-mass AGB stars undergoing third dredge-up and hot-bottom burning |
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| Authors: | J A McSaveney P RWood M Scholz J C Lattanzio K H Hinkle |
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| Institution: | Mount Stromlo Observatory, Research School of Astronomy and Astrophysics, Australian National University, Cotter Road, Weston Creek, ACT 2611, Australia;Centre for Stellar and Planetary Astrophysics, Department of Mathematical Sciences, Monash University, Melbourne, Australia;Institut f. Theoretische Astrophysik d. Univ. Heidelberg, Albert-Ueberle-Strasse 2, 69120 Heidelberg, Germany;School of Physics, University of Sydney, NSW 2600, Australia;National Optical Astronomy Observatory, PO Box 26732, Tucson AZ 85726-6732, USA |
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| Abstract: | High-dispersion near-infrared spectra have been taken of seven highly evolved, variable, intermediate-mass (4–6 M⊙) asymptotic giant branch (AGB) stars in the Large Magellanic Cloud and Small Magellanic Cloud in order to look for C, N and O variations that are expected to arise from third dredge-up and hot-bottom burning. The pulsation of the objects has been modelled, yielding stellar masses, and spectral synthesis calculations have been performed in order to derive abundances from the observed spectra. For two stars, abundances of C, N, O, Na, Al, Ti, Sc and Fe were derived and compared with the abundances predicted by detailed AGB models. Both stars show very large N enhancements and C deficiencies. These results provide the first observational confirmation of the long-predicted production of primary nitrogen by the combination of third dredge-up and hot-bottom burning in intermediate-mass AGB stars. It was not possible to derive abundances for the remaining five stars: three were too cool to model, while another two had strong shocks in their atmospheres which caused strong emission to fill the line cores and made abundance determination impossible. The latter occurrence allows us to predict the pulsation phase interval during which observations should be made if successful abundance analysis is to be possible. |
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| Keywords: | stars: abundances stars: AGB and post-AGB stars: oscillations |
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