低质量AGB星重元素的核合成

THE HEAVY ELEMENTS NUCLEOSYNTHESIS IN THE LOW MASS AGB STARS

本文以（１３）Ｃ（α，ｎ）（１６）Ｏ作为中子源，考虑到恒星核心质量随热脉冲数的变化及星风、超星风质量损失的影响，采用从（５６）Ｆｅ到（２１０）Ｂｉ的无分支ｓ过程反应通道，拟合了ＭＳ、Ｓ的重元素超丰．本文特别将恒星质量与ＡＧＢ内Ｓ过程核合成模型结合起来讨论．结果表明质量较大的恒星因对流较强而稀释因子较大，ＭＳ、Ｓ星在轻、重Ｓ元素丰度关系图中分别落入四个不同区域，由此可以粗略估计这些恒星的质量．２．５Ｍ 的ＡＧＢ星形成具有环状空腔的星体，最后演化成Ｔｃ—ｎｏ单星，可以解释双星系统伴星为主序星的ＡＧＢ星无Ｔｃ现象．

A comparison is presented between the predication of nucleosythesis calculations for population I low-mass stars during the He-shell flashes of the asymptotic giant branch (AGB) and the distribation of heavy elements observed in MS and S stars. The neutron irradiation is assumed to be derived by the reacation (13)C(a, n)(16)O The core mass varies with the thermal pulse number. steller winds and superwinds. We adopt an unbranded sprocess path from (56)Fe to (210)Bi and standard models connected with star masses. By varying the main parameters within the range allowed by present models, a set of results are constructed. First, more heavier elements are produced with unvaried overlap facter; Second, the star with a larger mass has a larger dilution factor because of stonger convection. The MS, S stars are located in 4 different areas in the diagram of ([Nd/Y]+[Nd /Zr])vs.([Y/Fel+[Zr/Fel]), by which we can estimate MS,S star masses. We show that the third dredge-up of 2.5M stars stops after some pulses because of hollow shell structure, and they will not show Tc,so the problem that binary stars have a Tc-no AGB and a companion in the main sequence can be resolved.