公共包层抛射通道生成的热亚矮星

发布时间：2018-05-17 20:30

本文选题：热亚矮星 + 双星演化　；参考：《中国科学院大学(中国科学院云南天文台)》2017年硕士论文

【摘要】：热亚矮星是在星团的颜色-星等图上位于水平分支的最蓝端的恒星,因此也被称为极端水平分支星。它们一般被认为是由燃烧的氦核以及很薄的包层所组成。热亚矮星在以下几个方面很重要:(1)对于热亚矮星的形成与演化的研究可以提高天文学上对于恒星和双星演化的认识。(2)很多热亚矮星表现出多周期脉动现象(同时有p模式震动与g模式震动),因此它们是星震学上的重要研究对象。(3)热亚矮星的形成和演化决定了球状星团水平分支的形状。(4)热亚矮星被认为是早型星系中紫外反转现象的一个重要来源。绝大部分热亚矮星被发现在双星系统中(对于场星大约2/3),因此双星机制是目前热亚矮星形成的主流解释。本文首先对热亚矮星的观测性质和形成模型进行了综述。然后,介绍了我们对公共包层抛射形成的热亚矮星的性质和相应天体的研究情况。主要分为以下几个方面。(1)公共包层抛射形成的热亚矮星的性质。我们对主星初始质量分别为Mi=0.8M,1.0M,1.26M,和1.5M的情况进行了研究。对每一个初始质量,从最小氦核点燃质量到第一巨星支顶端,每间隔0.002M取一个模型作为公共包层开始的时刻。然后以10-3M yr-1的速率使恒星丢掉外包层,以此模拟公共包层抛射,直到恒星开始收缩时停止丢失物质。接着我们继续跟踪剩余恒星的演化,直至它演化成为碳氧白矮星。我们发现在有效温度-表面重力加速度图上,这些热亚矮星被清楚地分为两组:一组集中在极端水平分支的最蓝端,另一组则均匀地覆盖了经典热亚矮星的分布范围,两者之间存在明显的空隙。集中在高温端的那一组恒星在沿着白矮星冷却线下降的过程中点燃氦闪过程,由此氦闪所驱动的对流区将穿透富氢包层,从而将富氢物质混入高温氦燃烧区,最终将这些物质点燃并耗尽。因此,这组恒星集中在水平分支最高温端。而另外一组氦点燃较早的恒星,由氦闪驱动的对流区没有穿透富氢包层。富氢物质得以保留,这种方式产生的热亚矮星根据包层质量的不同可以覆盖整个经典热亚矮星区域。但一些观测到的短周期热亚矮星位于两组间的空隙中,并且它们的数密度远大于我们模拟中的理论值。公共包层抛射和对流的处理方式,可以很大的影响两组的参数空间。公共包层抛射后的回落以及闪耀时的物质损失可以分别使两组恒星落入到空隙中,它们很可能可以解释这个矛盾。(2)公共包层抛射通道生成的热亚矮星的周期范围。此结果和观测对比可以用来约束公共包层抛射系数,αCE。我们模拟了初始质量从0.7到1.3M的七个恒星模型,并取出它们在接近第一巨星支顶端时的七组位置下的包层引力束缚能,Egr,和内能,Eth。我们假设这些恒星在这些位置充满洛希瓣并进入公共包层阶段,根据不同的伴星质量,M2(人为设定),我们可以得到此时双星系统的周期和者轨道间距。然后通过公共包层抛射的能量机制,给定一组αCE和αth,我们就可以得到热亚矮星双星的最终周期P。我们模拟得到的周期范围大致与观测和前人的工作相吻合,但是我们工作中的最大周期比Han et al.(2002)[1]得到的最大周期小了一个数量级。这种差别主要是由于核边界定义不同产生的。(3)球状星团中的蓝钩星。蓝钩星在水平分支上非常高温的位置,同时它们有高于一般极端水平分支星的氦丰度。两种机制被提出以解释蓝钩星的形成,氦自增丰机制和延迟氦闪机制。Lei et al.(2015,2016)[2,3]通过双星中潮汐增强星风研究了蓝钩星的形成。我们发现,公共包层抛射通道形成的热亚矮星由于延迟氦闪同样可以产生蓝钩星。因为延迟氦闪是蓝钩星形成的物理原因,而与之前的物质损失机制无关,部分稳定物质转移通道生成的极端水平分支星也可能是蓝钩星。
[Abstract]:The hot sub dwarfs are the bluest end stars located at the horizontal branch in the color star map of the cluster and are also known as extreme horizontal branching stars. They are generally considered to be composed of burning helium nuclei and very thin cladding. The hot dwarfs are important in the following aspects: (1) the study of the formation and evolution of the hot dwarf stars can be raised. The understanding of the evolution of stars and binary stars in high astronomy. (2) many hot sub dwarfs exhibit multi periodic pulsation (with P mode vibration and G mode vibration), so they are important research objects in the study of astrology. (3) the formation and evolution of the hot sub dwarfs determine the shape of the horizontal branch of the globular cluster. (4) the hot sub dwarf is considered to be early An important source of ultraviolet inversion in type galaxies. Most of the hot sub dwarfs are found in the binary system (about 2/3 of the field stars), so the double star mechanism is the mainstream interpretation of the current hot sub dwarf formation. This paper first reviewed the observation properties and the formation model of the hot sub dwarfs. The properties of the hot sub dwarfs and the research on the corresponding celestial bodies are mainly divided into the following aspects. (1) the properties of the hot sub dwarfs formed by the common cladding ejection. We have studied the initial mass of the main star Mi=0.8M, 1.0M, 1.26M, and 1.5M. At the top of the giant star branch, each 0.002M takes a model as the beginning of the common cladding. Then the star loses the outer layer at the rate of 10-3M yr-1 to simulate the common cladding, until the star begins to shrink and stops losing the material. Then we continue to track the evolution of the remaining stars until it evolves into the carbon oxygen white dwarf. It is found that the hot sub dwarfs are clearly divided into two groups in the effective temperature surface gravity acceleration map: one is concentrated at the bluest end of the extreme horizontal branch, and the other is evenly covered by the distribution range of the classic hot sub dwarfs. There is a clear gap between the two groups. The group of stars in Gao Wenduan is cooled down the white dwarf. In the process of line descent, the helium flash process is ignited, thus the convection zone driven by the helium flash will penetrate the hydrogen rich cladding, thus mixing the rich hydrogen into the high temperature helium combustion area, and eventually igniting and exhausting these substances. Therefore, this group of stars is concentrated at the highest temperature end of the horizontal branch. The hydrogen rich material is not penetrated. The hydrogen rich material is retained. The hot sub dwarfs produced in this way can cover the whole classic hot sub dwarf region based on the mass of the cladding. However, some observed short periodic thermo dwarfs are located in the gap between the two groups, and their number density is far greater than the theoretical value in our simulation. The treatment of radiation and convection can greatly affect the parameter space of the two groups. The fall of the common cladding and the material loss of the blazed can make two groups of stars fall into the gap, respectively. They are likely to explain the contradiction. (2) the periodic range of the hot sub dwarf produced by the common cladding channel. The contrast can be used to constrain the common cladding ejection coefficient. Alpha CE. we simulated seven stellar models with initial mass from 0.7 to 1.3M, and removed the cladding gravitational binding energy, Egr, and internal energy at the seven sets of positions near the top of the first giant star branch, and Eth. we assumed that these stars were filled with Loch flaps and entered public packets at these positions. In the layer phase, according to the mass of the companion stars, M2 (artificially set), we can get the cycle and the distance between the two stars at this time. Then, by the energy mechanism of the common cladding, we give a set of alpha CE and alpha th, we can get the final cycle of the P. of the hot sub dwarf star. Human work is consistent, but the maximum period in our work is one order of magnitude smaller than the maximum period obtained by Han et al. (2002) [1]. This difference is mainly due to the difference in the definition of the nuclear boundary. (3) the blue hook star in the globular cluster. The blue hook star is very high temperature in the horizontal branch, and they are higher than the ordinary extreme water. The two mechanism is proposed to explain the formation of the blue hook star, the helium self augmented mechanism and the delayed helium flash mechanism.Lei et al. (20152016) [2,3] study the formation of the blue hook star through the tidal enhanced star wind in the double stars. We found that the hot sub dwarf of the common cladding channel can also produce blue because of the delay of helium flash. Because the delayed helium flicker is the physical cause of the formation of the blue hook star, it is not related to the prior material loss mechanism, and the extreme horizontal branch star formed by the partial stable material transfer channel may also be the blue hook star.
【学位授予单位】：中国科学院大学(中国科学院云南天文台)
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P144

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