AGB星辐射s-过程核合成模型中子辐照量分布的计算方法和特点

Calculating Method and Characteristics of the Distribution of Neutron Exposures in Radiative S-process Nucleosynthesis Model for AGB Stars

考虑到模型参量随脉冲数的变化,推导出^(13)C辐射燃烧的低质量AGB星s-过程核合成模型中子辐照量分布的计算方法,该方法具有普适性和简洁性.利用该方法,计算了3M_⊙、太阳金属丰度恒星模型的中子辐照量分布.结果表明,若合理假设^(13)C薄层内中子数密度均匀分布,则辐射核合成模型最终的中子辐照量分布趋近于指数分布.对于初始质量一定、金属丰度一定的恒星模型,平均中子辐照量τ_0与每个脉冲的中子辐照量△7存在确定的关系:τ_0=0.434λ(q_1,q_2…q_(m_(max))+1,r_1,r_2…r_(m_(max))+1)△7,式中m_(max)是带挖掘的脉冲总数,比例系数λ(q_1,q_2…q_(m_(max))+1,r_1,r_2…r_(m_(max))+1)可通过对最终中子辐照量分布的指数拟合得到.该式从中子辐照量分布角度定量地将经典模型和辐射s-过程核合成模型统一起来,使经典模型能为恒星模型核合成数值计算提供指导和约束.

An investigation on the distribution of neutron exposures in the low-mass asymptotic giant branch （AGB） stars is presented, according to the s-process uucleosyn-thesis model with the 12C（α,n）16O reaction occurring under radiative conditions in the interpulse phases. The model parameters, such as the fractional overlap of two successive convective thermal pulses r, the mass fraction of 13C pocket in the He intershell q, and the mass of the effective 13C in the 13C pocket, vary with pulse number. Considering these factors, the calculating method for the distribution of neutron exposures in the He intershell is presented. This method has the characteristics of simplicity and universality. Using this method, the exposure distributions of the stellar model for a star with a mass of 3 M⊙ and solar metalficity are calculated. The results suggest that, with the reasonable assumption that the 13C pocket has a uniform composition, the final exposure distribution can still be approximated by an exponential law. For a stellar model with a fixed initial mass and metallicity, there is a definite relation between the mean neutron exposure To, and the neutron exposure △τ for per interpulse. That is τ0=0.434λ（q1,q2…qmmax＋1,r1,r2…rmmax＋1）△τ, where mmax is the total number of thermal pulses with the third dredge-up episode, and the proportional coefficient λ（q1,q2…qmmax＋1,r1,r2…rmmax＋1） can be determined through an exponential curve fitting to the final exposure distribution. This new formula quantitatively unifies the classical model with the stellar model in terms of the distribution of neutron exposures, and makes the classical model continue to offer guidance and constraints to the s-process numerical calculations in stellar models.