低质量AGB星He壳层内重元素核合成

NUCLEOSYNTHESIS IN THE HE-SHELL OF THE ASYMPTOTIC GIANT BRANCH STARS

以13C（α，n）16O及22Ne（α，n）25Mg作为双脉冲中子源，对于星族I低质量AGB星，采用无分又s一过程反应通道，结合最新恒星演化的计算结果，在各物理参量合理取值范围内，计算了He壳层内重元素核合成。结果表明，达到渐近分布时所需的脉冲数N0的范围是6～16个。渐近分布达到后，He壳层内重元素的丰度仅与平均中子辐照量τ0有关。与低质量AGB星相应的平均中子辐照量范围是τ0＝0．15～1．0mb－1。

In this paper , we present the nucleosynthesis calculations in the convective He -shell for Population I low - mass 8tars during the He - shell flashes of the asymptotic giant branch (AGB). The main neutron source at the origin of s - process nucleosynthesis is assumed to be the reaction13C(a,n)16O; a minor contribution derives also from the marginal activation of the reaction22Ne(a,n)25Mg at the end of each flash. We adopt a unbranched s - process network, and use standard models for the recurrent thermal pulses: core mass increasing with pulse number, overlap factor and stellar winds varying with core mass. It is found that asymptotic conditions were reached after a number of pulses that depends on the adopted value of effective neutron exposure To, ranging from 16-18 for ro = O. 15 mb-1 to about 6-8 for To = 1. 0 mb-1. After asymptotic abundances have been achieved, the distributions of s - nuclei can be simply characterized by τo, The various classes of low -mass AGB stars are characterized by neutron exposures covering a large interval, reaching values often in excess of 0. l5 mb-1 and up to 1. 0 mb-1.