| 摘要 | 热亚矮星是一类特殊的天体,一般认为,它是由一个燃烧的中心氦核和很 薄的外壳组成(外壳质量< 0.02 M⊙)。在赫罗图上,热亚矮星通常位于水平分支 的蓝端,既所谓的极端水平分支,所以也被称为极端水平分支星(EHB,Extreme Horizontal Branch)。通常,热亚矮星分作:B型热亚矮星,O型热亚矮星 OB型热亚矮星。对于天体物理研究,热亚矮星具有重要的意义,例如,脉动 热亚矮星可以用于天体物理中距离的测量;由一颗热亚矮星和白矮星构成的系统可能是Ia型前身星的候选体;特别是热亚矮星可以很好的解释椭圆星系中紫外反转(UV-upturn)的起源,并用于星系演化研究。 为了更好的理解热亚矮星的演化,我们通过剑桥恒星演化程序构造了一系列不同质量和金属丰度的样本并进行演化。这些样本的核心质量从0.33到1.4M⊙,以及相对应的包层质量,从0.000M⊙到最大包层质量。金属丰度的范围从0.0001到0.1,分别是0.0001, 0.001, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08, 0.1。这些新的演化轨迹将用于我们今后的演化星族研究。 质量是天体物理中的重要参量,但也是难于测定的物理量之一。对于热亚矮星,我们仅有少数几颗脉动热亚矮星和双星质量得到了确定。但是,由于这些系统的特殊性,它们样本很少而且观测数据难于获取。在本文中我们发展了一种方便和准确的质量计算方法。根据金属丰度Z = 0.02的热亚矮星演化轨迹库,我们通过详细分析相关物理量,给出质量－有效温度－重力加速度关系，通过这一关系可以简单和准确的对热亚矮星质量进行计算。通过这一方法,我们对ESO Ia超新星前身星巡天(SPY, Lisker et al. 2005)和Hamburg Quasar 巡天(HQS, Edelmann et al.2003)的164颗B型热亚矮星样本和46颗O型热亚矮星的样本的质量进行了计算,我们给出了B型热亚矮星和O型热亚矮星的质量分布情况。结果显示,B型热亚矮星大多数质量在为0.48 到0.52 M⊙,O型热亚矮星大多数质量在0.4 到0.55 M⊙,分析显示大多数低质量(小于∼0.5 M⊙)的O型热亚矮星来自B型热亚矮星 的后期演化(公共包层抛射渠道和并合渠道产生),对大质量O型热亚矮星(大 于∼0.5 M⊙)而言很可能是并合渠道直接产生。 另外,我们对三颗特殊的热亚矮星的质量进行了计算,分别给出其质量 为KPD 1930+2752(0.581M⊙), US 708 (0.541M⊙)和V391 Pegasi(0.500 M⊙)。这些质量的给定,有利于我们对这些系统作更加深入的分析。 |
| 其他摘要 | Hot subdwarfs, including subdwarf O stars, subdwarf OB stars and subd- warf B stars, are defined as stars located below the upper main sequence on the Hertzsprung-Russell diagram(HRD). They are also known as Extreme Horizontal Branch (EHB) stars from the view of their evolutionary stages, i.e. they are be- lieved to be core He-burning objects with extremely thin hydrogen envelopes (< 0.02 M⊙). Hot subdwarfs are an important population in several respects. For example, pulsating subdwarf B (sdB) stars are standard candles in distance de- termination. As well, close binaries composed of a sdB star and a massive white dwarf (WD) are qualified as Supernova Ia progenitors. Moreover, hot subdwarfs are an important source of far-UV light in the galaxy, and they are successfully used to explain the UV-upturn in elliptical galaxies. To understand their evolution better, we constructed a series of zero-age extreme horizontal branch (ZAEHB) stars and calculated their evolutions in de- tails. The model grid contains a wide mass range, that is, the core mass ranges from 0.33 to 1.4 M⊙, and the envelope mass change from 0.000 M⊙ to the max- imum envelope mass, by a step of about 0.005 M⊙. We investigated the cases for ten metallicities, i.e. Z=0.0001, 0.001, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08, 0.1. These evolutionary tracks will be used in our future work of evolutionary population synthesis. Masses are a fundamental parameter, and are not well known for most hot subdwarfs. In general, the mass of a hot subdwarf is derived from asteroseismology or dynamical methods, for which it is often difficult to obtain the necessary data from observations. We intend to find an approach to derive the masses of hot subdwarfs from observational data in the literature. Based on the evolutionary tracks of Population I metallicity (Z=0.02), we obtained a relation between M and log(T^4/g ), where M , T, g are the most probable mass, effective temperature and gravity. This relation is used to study the masses of some observed hot subdwarfs. Using this method, we studied the masses of hot subdwarfs from ESO supernova Ia progenitor survey and Hamburg Quasar Survey. The study shows that most of subdwarf B stars have masses between 0.48 and 0.52 M⊙, whilst most sdO stars are in the range 0.4 ∼ 0.55 M⊙. Comparing our study to the theoretical mass distributions of Han et al. (2003), we found that stars with mass less than ∼ 0.5 M⊙ may evolve from sdB stars, whilst most high-mass(> 0.5 M⊙) sdO stars result from mergers directly. We also obtained the masses of objects which are also of interest for other fields of study, i.e. KPD 1930+2752, US 708 and V391 Pegasi. Their masses are 0.581, 0.541 and 0.500 M⊙, respectively. |
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