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Third-dredge-up oxygen in planetary nebulae

Pequignot, D. and Walsh, J.R. and Zijlstra, A.A. and Dudziak, G. (2000) Third-dredge-up oxygen in planetary nebulae. Astronomy and Astrophysics, 361 (1). L1-L4. ISSN 0004-6361

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The planetary nebulae He 2-436 and Wray 16-423 in the Sagittarius dwarf galaxy appear to result from nearly twin stars, except that third-dredge-up carbon is more abundant in He 2-436. A thorough photoionization-model analysis implies that ratios Ne/O, S/O and Ar/O are significantly smaller in He 2-436, indicative of third-dredge-up oxygen enrichment. The enrichment of oxygen with respect to carbon is (7±4)%. Excess nitrogen inWray 16-423 suggests third dredge-up of late-CN-cycle products even in these low-mass, intermediate-metallicity stars.