强钡星与弱钡星的Monte-carlo模拟

【作者】 全亚民；

【导师】 张波；

【作者基本信息】 河北师范大学 ， 理论物理， 2005， 硕士

【摘要】 钡星对于重元素核合成及星系化学演化的研究都有非常重要的意义。钡星的重元素是过丰的,但其光度太低,认为还未演化到AGB阶段,而且并没有观测到⁹⁹Tc核素。研究表明,Ba星应该属于双星系统,其重元素超丰来自双星吸积,它的伴星已经经历了s-过程核合成,而且由在以前发生的同其伴星（以前是AGB星,现已演化为白矮星）的质量传输过程中,从伴星那里获得了富含重元素的物质,这样造成了Ba星的重元素超丰。 本文采用星风质量吸积的角动量守恒模型,并考虑到初始金属丰度对AGB星s-过程核合成的影响,用Monte-Carlo方法研究了弱钡星系统和强钡星系统的轨道根数的变化规律。计算结果表明,随着星风吸积过程的进行,在星风质量损失阶段系统轨道半长轴将增大,导致轨道周期增大,而偏心率变化不大,由此可以模拟并解释与钡星系统轨道根数及丰度分布相关的系列观测事实。通过本文关于钡星轨道根数及重元素丰度的计算结果和讨论还可得到如下主要结论: （a） 能演化成弱钡星系统的红巨星双星系统的周期的对数分布得比较均匀,在大周期（大于10000天）范围仍有分布,而能演化成强钡星系统的红巨星双星系统的分布在短周期区密集。其原因是双星系统的轨道周期越短,星风吸积过程中物质的吸积量就越大,则钡星的重元素丰度就越高,从而越容易形成强钡星。 （b） 对于低质量AGB星,由于He壳层的钡丰度在一定范围内随金属丰度的降低而增加,因此在其它条件相同的情况下,双星系统的金属丰度越低,则钡星的重元素丰度就越高。 （C） 弱钡星的丰度分布随主星初始质量的变化不大,但其数目分布却随主星初始质量的增大而减小,强钡星的Ba丰度的分布及数目分布均在2—3M范围内达到峰值,弱钡星的钡丰度随伴星初始质量的增大变化不大,而对于强钡星,钡丰度随伴星初始质量的增大而降低,不论弱钡星还是强钡星,其数目分布都随其初始质量的增大而减少。更多还原

【Abstract】 It is very significant to Ba stars for heavy-element nucleosynthesis and Galactic chemical evolution. The obvious overabundance of heavy-element are observed in Ba stars, however, owing to the luminosity of Ba stars is too faint, so we contemplate it has not evolved into the stage of AGB, and the nuclear ⁹⁹Tc have not observed in Ba stars. In general, Ba stars belong to binary system, and their heavy-element overabundance are due to the accreting material from the companions （the former TP-AGB stars and the present white dwarfs）.Carrying out a series of Monte-Carlo simulations, the works of orbital elements of binaries are recalculated considering the influence of initial metallicity on S-process nucleosynthesis of intrinsic AGB stars,and here we adopt wind accretion scenario with the total angular momentum conservation. The calculation demonstrate that in the course of wind accretion, in the mass-losing stage of the wind the orbital semi-major axis increases,and result in the orbital period increases, but the eccentricity dose not vary significantly. This can simulate and interpret the observational facts that are as to the orbital elements and abundance distribution . On the basis of the calculation and discuss with respect to the orbit elements and heavy—element abundance, our results show the following conclusions:（a）the period of logarithm distribution of G-K red giant binaries that evolved into mild barium stars is even. The are still orbital distribution for periods longer than 10000 days and the G-K red giant binaries that evolved into strong barium stars are dense in the region of short periods. The reason lie in that, the shorter the orbital period of the binary system ,the larger the mass accretion, and the higher the heave-element abundance. So it is easier for red giant binaries evolve into strong barium stars with shorter orbital period.（b）for lower-mass AGB stars, due to a smaller metallicity in the He stratum probably result in larger heavy-element overabundances , therefore the barium abundance of binary system increase with the decline of initial metallicity on condition that other factors are equal.（c）there is little relationship between the variation of the first star initial mass and the distribution of Ba—stars abundance, but the distribution of its number decline with increasing initial mass of first stars .the distribution of Ba—abundance in strong barium stars更多还原