计算平均中子辐照量的新公式

The New Formula for The Mean Neutron Exposure

根据^(13)C辐射燃烧的低质量AGB(Asymptotic Giant Branch)星s-过程核合成模型,考察了氦壳层核合成区域中子辐照量分布(以下简称DNE)的特点.结果表明,该模型的DNE同以前的对流核合成模型一样仍然非常接近指数分布,但是每个脉冲的中子辐照量Δτ和平均中子辐照量τ_0之间的关系却不再是τ_0=-Δτ/lnr,而近似为τ_0= -Δτ/ln{q[1.0020+0.6602(r-q)+4.6125(r-q)~2-10.8962(r-q)~3+13.9138(r-q)~4]}( r为重叠因子,q为^(13)C壳层占氦壳层的质量比例).该式从DNE角度将辐射s-过程恒星模型和经典模型统一起来.

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）160 reaction occurring under radiative conditions in the interpulse phases. The exposure distribution in the He intershell is calculated and t＇he 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 TO is no longer the well-known τo = --△τ/lnτ （△τ 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 △τ, the analytic formula that approximates the To for a wide range of r and the mass fraction of ^13C pocket q in the He intershell is τo = --△τ/In{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 AT than the latter in order to reproduce a same element abundance distribu-tion （corresponding to a same value of TO ）. 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.