Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008acasn..49..133z&link_type=abstract
Acta Astronomica Sinica, vol. 49, no. 2, p. 133-143
Astronomy and Astrophysics
Astronomy
Stars: Agb, Stars: Neutron, Methods: Nnumerical
Scientific paper
An investigation on the characteristics of the distribution of neutron exposures(hereafter DNE) in low-mass asymptotic giant branch (hereafter AGB) stars is present, according to the s-process nucleosynthesis model with the 13C(α,n) 16O reaction occurring under radiative conditions in the interpulse phases. The exposure distribution in the He intershell is calculated and the effects of model parameters on it are discussed. The results show that, the DNE is indeed much more complex than those of the previous convective s-process scenarios. Nevertheless, with regard to the effect of the major 13C neutron source, the final exposure distribution can still be approximated by an exponential DNE. However, it is must be pointed out that the formula for the calculation of the mean neutron exposure T0 is no longer the well-known T0 = -ΔT / ln r(ΔT is the neutron exposure per pulse and r denotes the fractional overlap of two successive convective thermal pulses). With the assumption that the 13C pocket has uniform composition and undergoes a fixed neutron exposure per interpulse ΔT, the analytic formula that approximates the T0 for a wide range of r and the mass fraction of ^{13}C pocket q in the He intershell is T0 =-ΔT / ln{q [1.0020+0.6602(r-q)+4.6125(r-q)2-10.8962 (r-q)3 +13.9138(r-q) 4]}. When q=r, the new formula is degenerated into the old one. This well matches the stellar models for the s-process nucleosynthesis mechanism. The new relation definitely reflects some characteristics in which the radiative 13C pocket s-process nucleosynthesis model differs from all the previous convective s-process scenarios. For example, the former needs a much bigger ΔT than the latter in order to reproduce a same element abundance distribution (corresponding to a same value of T0). Based on the s-synthesis calculating performed in a single AGB star to reproduce the main component, the DNE for the solar system is also calculated according to the new formula. The result is compared with those by some parameterized models.A good agreement is found, and this confirms the reliability of the results of this paper. The new formula provides strong supports for the consistency of the radiative s-process stellar model with the classical model from the viewpoint of DNE. Since the radiative 13C pocket s-process nucleosynthesis model is very typical, the new formula may be quite useful to the study of the s-process nucleosynthesis.
Zhang Baigang
Zhang Feng-Hua
Zhou Gui-De
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