Astron. Astrophys. 363, 555-567 (2000)

The heavy-element abundances of AGB stars and the angular momentum conservation model of wind accretion for barium stars

Y.C. Liang ^1,3, G. Zhao ^1,3 and B. Zhang <sup>2,4

1</sup> Beijing Astronomical Observatory, Chinese Academy of Sciences, Beijing 100012, P.R. China
² Department of Physics, Hebei Normal University, Shijiazhuang 050016, P.R. China
³ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, P.R. China
⁴ Chinese Academy of Sciences-Peking University Joint Beijing Astrophysical Center, Beijing 100871, P.R. China

Received 10 July 1998 / Accepted 22 September 2000

Abstract

Adopting new s-process nucleosynthesis scenario and branch s-process path, we calculate the heavy-element abundances of solar metallicity 3 thermal pulse asymptotic giant branch (hereafter TP-AGB) stars, and then discuss the correlation between heavy-element abundances and C/O ratio. ¹³CO reaction is the major neutron source, which is released in radiative condition during the interpulse period, hence gives rise to an efficient s-processing that depends on the ¹³C profile in the ¹³C pocket. A second small neutron burst from ²²Ne source marginally operates during convective pulses over previously s-processed material diluted with fresh Fe seed and H-burning ashes. The calculated heavy-element abundances and C/O ratio on the surfaces of AGB stars are compared with the observations of MS, S and C (N-type) stars. The observations are characterized by a spread in neutron exposures: 0.5-2.5 times of the corresponding exposures reached in the three zones of the ¹³C pocket showed by Fig. 1 of Gallino et al. (1998). The evolutionary sequence from M to S to C stars is explained naturally by the calculated heavy-element abundances and C/O ratio.

Then the heavy-element abundances on the surfaces of TP-AGB stars are used to calculate the heavy-element overabundances of barium stars, which are generally believed to belong to binary systems and their heavy-element overabundances are produced by the accreting material from the companions (the former TP-AGB stars and the present white dwarfs). To achieve this, firstly, the change equations of binary orbital elements are recalculated by taking the angular momentum conservation in place of the tangential momentum conservation, and the change of term is considered; then the heavy-element overabundances of barium stars are calculated, in a self-consistent manner, through wind accretion during successive pulsed mass ejection, followed by mixing. The calculated relationships of heavy-element abundances to orbital periods P of barium stars can fit the observations within the error ranges. Moreover, the calculated abundances of nuclei of different atomic charge Z, corresponding to different neutron exposures of TP-AGB stars, can fit the observational heavy-element abundances of 14 barium stars in the error ranges. Our results suggest that the barium stars with longer orbital period P1600 d may form through accreting part of the ejecta from the intrinsic AGB stars through stellar wind, and the mass accretion rate is in the range of 0.1-0.5 times of Bondi-Hoyle's accretion rate. Those with shorter orbital period P600 d may be formed through other scenarios: dynamically stable late case C mass transfer or common envelope ejection.

Key words: nuclear reactions, nucleosynthesis, abundances stars: abundances stars: AGB and post-AGB stars: carbon stars: mass-loss stars: binaries: close

Send offprint requests to: Y.C. Liang (lyc@yac.bao.ac.cn)

© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000

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