Sources of Carbon and Heavy‐Element Abundances in Asymptotic Giant Branch Stars and Ba Stars

Abstract

Abundance trends of CNO elements are crucial to nucleosynthesis of elements and Galactic chemical evolution (GCE) studies. They play important roles in stellar interiors as opacity sources and energy production through the CNO cycle. Thus, they can influence stellar lifetimes, positions in the H-R diagram, and heavy-element yields. In general, oxygen is produced mainly by massive stars through Type II supernova explosions, and nitrogen is produced mainly by intermediate-mass stars. However, the main source of carbon is not clear. Therefore, in this dissertation, we carefully compare and analyze the stellar nucleosynthesis yields of different mass stars (e.g., S. E. Woosley & T. A. Weaver 1995, ApJS, 101, 181; A. Maeder 1992, AA K. Nomoto et al. 1997, Nucl. Phys. A, 616, 79; L. Portinari et al. 1998, AA L. B. van den Hoek & M. A. T. Groenewegen 1997, AA A. Renzini & M. Voli 1981, AA P. Marigo 2001, AA P. Marigo et al. 1996, AA P. Marigo et al. 1996, AA K. Nomoto et al. 1997, Nucl. Phys. A, 621, 467 for Type Ia supernova explosions). Then, according to these different stellar yields, we set up eight GCE models (infall) to explore the sources of carbon. The results show that in the early stages of the Galaxy, massive stars are the main carbon producers and that as the Galaxy evolves to later stages, the longer lived intermediateand low-mass stars play an increasingly important role. At the same time, metal-rich Wolf-Rayet stars eject a significant amount of carbon into the interstellar medium by radiative-driven stellar winds. However, from the published nucleosynthesis yields, we cannot distinguish, for the present Galactic time, whether the main source of carbon is just the massive stars ( ) alone or just the intermediateM 1 8 M, mass stars, low-mass stars, and massive ( ) stars that M ≤ 40 M, do not go through the Wolf-Rayet stage. Simultaneously, our results show that the nitrogen contribution is dominated by intermediateand low-mass stars. A secondary source of massive stars cannot explain the observed [N/Fe] in metal-poor stars. Most of the oxygen is produced by massive stars. Besides producing carbon and nitrogen, low-mass stars are the main nucleosynthesis site of slow neutron capture process (s-process) elements during their asymptotic giant branch (AGB) stage. Thus, adopting a new s-process nucleosynthesis scenario (R. Gallino et al. 1998, ApJ, 497, 388; M. Busso et al. 1999, ARA&A, 37, 239) and branch s-process path, we calculate the heavy-element nucleosynthesis of solar metallicity 3 thermal-pulse AGB stars. The C O reaction is 16 M (a, n) , the major neutron source, released in a radiative condition during the interpulse period. A second small neutron burst from the Ne source marginally operates during convective pulses. The calculated heavy-element abundances and C/O ratio on the surfaces of AGB stars are well fitted by the observations of MS, S, SC, and C (N-type) stars. Thus, the evolutionary sequence from M to S to SC to C stars of AGB stars is explained naturally. Another important point is that AGB stars can cause heavyelement overabundances of Ba stars through binary accretion. Therefore, using our AGB star nucleosynthesis results, combined with our angular momentum conservation model of wind accretion, the heavy-element overabundances of Ba stars are calculated. The results are well matched by the observed orbital periods and heavy-element abundances of 14 barium stars taken from L. Zacs (1994, A&A, 283, 937) and M. Busso et al. (1995, ApJ, 446, 775). These results suggest that the Ba stars with longer orbital periods, days, may form through the P 1 1600 accreting part of the ejecta as a result of stellar winds from the intrinsic AGB stars. Those with shorter orbital periods, days, may be formed through other scenarios such as P ! 600 dynamically stable late-case C mass transfer or common envelope ejection.