Astronomy and Astrophysics, volume 363, 555-567 (2000/11-2)
The heavy-element abundances of AGB stars and the angular momentum conservation model of wind accretion for barium stars.
LIANG Y.C., ZHAO G. and ZHANG B.
Abstract (from CDS):
Adopting new s-process nucleosynthesis scenario and branch s-process path, we calculate the heavy-element abundances of solar metallicity 3M☉ thermal pulse asymptotic giant branch (hereafter TP-AGB) stars, and then discuss the correlation between heavy-element abundances and C/O ratio. 13C(α,n)16O 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. (1998ApJ...497..388G). 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 δr/r 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 P>1600 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 P<600 d may be formed through other scenarios: dynamically stable late case C mass transfer or common envelope ejection.