星系中子辐照量分布函数

【作者】 崔文元；

【导师】 张波；

【作者基本信息】 河北师范大学 ， 理论物理， 2004， 硕士

【摘要】 本文首先对三成份模型中与本工作密切相关的几个参量及其观测约束进行了比较，可以看出三成份模型是一个描述星系化学演化的比较理想的模型，它可以作为我们研究星系中子辐照量分布函数ρ_gal（t,τ）的基础。 本文首先引入了一个新概念——星系中子辐照量分布函数ρ_gal（t,τ），来描述星系慢中子俘获过程核合成的元素丰度分布，并由星系化学演化方程导出了ρ_gal（t,τ）的演化方程。经过数值计算我们得到了星系演化各个时期薄盘的中子辐照量分布函数ρ_gal,D（t,τ），由计算结果可见，对某一给定的时刻，星系中薄盘的中子辐照量分布函数ρ_gal,D（t,τ）)都可近似看成两个指数分布的叠加，这与太阳系的中子辐照量分布类似。星系中子辐照量分布的变化可以分为以下两个阶段： ● 第一阶段（t＜4Gyr）：τ₀较t=4Gyr时小，原因是在这一阶段寿命较短的中等质量AGB星（3—8M_⊙）在s^-核素核合成过程中起重要作用，它们在τ值较低处对s^-过程核合成的结果贡献较大(τ∈(0-0.5mb^-1))，因此平均中子辐照量τ₀值也小，曲线也较t=4Gyr时的曲线陡； ● 第二阶段（t＜4Gyr）：τ₀值较大，在t=4Gyr左右达到最大值，原因是在这一阶段贫金属的小质量星开始大量死亡，其在慢中子俘获核合成过程中的贡献已大于此时死亡的中等质量AGB星的贡献；随后，星族Ⅰ的AGB星即富金属AGB星的贡献逐渐增加，在小中子辐照量区间(τ＜0.5mb^-1)分布函数的斜率逐渐增加。 可以看出，计算结果与太阳系中子辐照量分布函数的变化规律基本相同，但符合得并不是太好，尤其是在中子辐照量τ值较大处，其原因可能是本文所选取得AGB星核合成区域的中子辐照量分布函数形式过于单一所致。尽管存在不足，但本文初步从理论上定量说明了太阳系的中子辐照量分布函数ρ_sun（τ）的指数衰减形式，进而阐明了AGB星的中子辐照量分布函数ρ_AGB（τ）与星系中子辐照量分布函数ρ_gal（t,τ）的关系以及星系中子辐照量河泌布店又学硕少学戎论戈分布函数彻(‘，弓与太阳系的中子辐照量分布函数编(叻的内在联系。更多还原

【Abstract】 In the present paper, we firstly compare some important parameters with the observational constraints and illustrate that the Galaxy chemical evolution model of three zone (such as halo, thick disk and thin disk) and multi-phase (diffuse gas, molecular clouds, stars of both low and high mass, the remnants) is a good model for the Galaxy chemical evolution, so we can use it to study the neutron exposure distribution function of the Galaxy pgal (r, ).In this paper, we mainly introduce a new conception which is the neutron exposure distribution function of the Galaxy pgal (t, ) and obtain the evolution function of the pgal(t,) through the chemical evolution function of the Galaxy. Through the numerical calculation, we get the pga,D(t, of the thin disk at every epoch of the Galaxy chemical evolution. As a result, we know the pgal,D (t, ) isclose to two exponential distribution which is similar to the neutron exposure distribution of the solar system psun ( ) . With the time increasing, the Pgal,D (t,) is changing, too. The changing process can be distinguished to two phases: The first phase (t<4Gyr): 0 is smaller than the value when it is at t=4Gyr, because the AGB stars with medium mass(3-8 M) play an important role in the s-process nucleosynthesis and they make big contribution when is smaller( (0-0.5mb-1)), so the mean neutron exposure is smaller and the curve is also steeper.The second phase: 0 is greater and reach the maximum value near 4Gyr, because large numbers of the poor metal stars with low mass died, they make bigger contribution than the AGB stars with medium mass. The contribution of the AGB stars of pop I become larger. The slope of the neutron exposure distribution function pgal,D(t,) increase gradually in the small neutronexposure range.In spite of the result of our work isn’t accord completely with the variation law of the distribution function of neutron exposure of the solar system psun for the single form of the neutron exposure distribution function of AGB stars pAGB we adopted. We has explained the exponential attenuation form of the psun and show the relation of the PAGB and pgal(t, ) .We also interpret the intrinsic connection between pgal,D(t,) and sun(r).更多还原