α增丰对极端水平分支星的影响

The Effects of α-Enhancement on the Evolution of EHB Stars

摘要: 金属元素分析表明：在银河系的球状星团和椭圆星系中，普遍存在α增丰现象。极端水平分支星是一种处在氦燃烧阶段的天体，它们具有非常薄的氢包层。我们用Eggleton’s恒星演化程序，研究了α增丰对极端水平分支星的影响。我们计算了不同金属丰度、不同核心质量和不同包层质量的极端水平分支星的演化；金属丰度取0.001、0.004、0.02和0.04，核心质量取0.400、0.450、0.475、0.500和0.550 M☉，包层质量取0.001、0.005、0.010、0.015和0.020 M☉。通过研究发现，α增丰可以使极端水平分支星的有效温度升高，表面重力加速度和光度增大，演化寿命变短。金属丰度越大，α增丰效应越明显；包层越厚，α增丰效应越明显。

Abstract: Analyses of metal elements have shown α-enhanced phenomenon in some globular clusters of the Milky Way and in some elliptical galaxies. Extreme horizontal branch (EHB) stars are helium burning objects with especially thin hydrogen envelopes. We study the effects of α-enhancement on the evolution of EHB stars, using Eggleton’s code. We calculate the evolution of EHB stars with metallicities 0.001, 0.004, 0.02 and 0.04, core masses 0.400, 0.450, 0.475, 0.500 and 0.550 M☉, and envelope masses 0.001, 0.005, 0.010, 0.015 and 0.020 M☉. It is found that α-enhancement can raise effective temperatures, surface gravities and luminosities, and shorten evolutionary lifetime for EHB stars. The higher values their metallicities are, the more obvious the α-enhanced effects are; and the thicker their envelopes are, the more obvious the α-enhanced effects are.