恒星演化对星团主序拐点扩展的影响研究

The extended main-sequence turnoffs has been observed in the color-magnitude diagram (eMSTO) of the intermediate-age and young-age star clusters in the Magellanic Cloud. One of interpretation of the eMSTO is that the star cluster has experienced an extended star formation history (eSFH), which disagrees with classical understanding of star clusters being simple stellar populations. An alternative interpretation of the eMSTOs is the effects of star rotation and interacting binary stars. Yang et al. show that the extension of MSTO caused by star rotations is related to the rotation rate of stars and the age of star clusters. They predict that the equivalent age spread caused by rotation has a peak at age about 1.7 Gyr and a relatively high rotation rate and a high efficient rotational mixing are required to explain the eMSTOs of young clusters. Observations have confirmed their predictions. However, star rotation cannot explain the eMSTO of young star clusters. A new mechanism or the extended star formation histories is needed to explain the eMSTO of young star clusters. The origin of the eMSTO is still an open question. Solar-like oscillations were observed by Kepler in many stars. The characteristics of the oscillations of many stars are different from that of the Sun. The method of determining the size of the convective core from the observed oscillation frequencies was developed by Yang. It was found that the value of overshooting parameters is variable for stars with an approximately equal mass. The overshooting of the convective core can bring more hydrogen-rich material into H-burning core, which significantly prolongs the life-time of the burning of core hydrogen and enhances the He-core mass left behind. If the parameter varies between stars in a cluster, the observed eMSTOs of young- and intermediate-age star clusters can be explained well by the effects of overshooting. Moreover, Asteroseismology would confirm or rule out eSFH scenario, rotation scenario, or overshooting scenario in the explanations of the eMSTO. This project mainly focuses on the effects of star rotations and core overshooting on the evolutions of stars and the eMSTO of star clusters.

哈勃望远镜在麦哲伦云中观测到星团CMD中存在主序拐点扩展eMSTO现象。其中一种解释认为星团由多星族构成,挑战了星团由单一星族构成的基本概念也与大量的观测事实不符。另一种解释认为由恒星演化因素：恒星转动和相互作用双星等造成。转动模型预测了eMSTO和星团年龄有关、在约1.7G年处扩展有一个峰值、转动不足以导致年青星团出现明显的eMSTO。前两点已被观测证实。最近观测发现了年青星团中存在明显的eMSTO，还证实转动不足以解释年青星团的eMSTO。eMSTO起源仍然是未解的难题。Kepler观测到了大量恒星的星振。我们已经建立起了利用观测频率确定恒星对流核大小的理论，并发现恒星的对流核超射是变化的。这一发现开启了星团eMSTO起源的新研究。本项目研究转动和对流核超射对恒星演化的影响以及这些恒星演化对星团MSTO的影响，研究利用星振学检验解释eMSTO的多星族模型、转动模型、超射模型的正确性。

在过去的十几年，哈勃望远镜发现在大麦哲伦云的大质量中等年龄星团的颜色星等图中存在主序拐点扩展现象。同时，人们还发现太阳的金属丰度远低于之前认为的值。此后，又在大麦哲伦云的年青星团的颜色星等图中发现了主序拐点扩展和双主序现象。这些星团的恒星通常被认为形成于同一时期，它们有近似相同的年龄和化学丰度。因此，这些发现挑战了恒星形成和演化理论。此外，在低金属丰度下标准太阳模型完全无法解释观测。许多研究指出这是由恒星演化造成的。这些问题一直困扰着恒星物理学者，挑战了恒星演化理论。. 我们研究了年青星团主序拐点扩展及双主序的形成原因，解释了年青星团中的一般恒星在颜色星等图中形成一支主主序支，并和双星和相互作用双星演化形成一支亮蓝的次主序支及主序拐点扩展。我们的理论模型很好地重现了观测的双主序结构，解释了亮蓝支星族数量较少和恒星形成有关，因为只有一定的质量比和间距下的双星才能并和和形成相互作用双星。这个理论首次预言了星团中存在许多白矮星+氦或氢主序，氦星+氢主序的双星系统。它们在颜色星等图上有特殊结构。这些研究为理解星团恒星形成和演化提供了重要依据，揭示出双主序及主序拐点扩展是恒星形成及演化的共同结果。. 我们研究了转动、对流超射等对恒星演化的影响，并利用这种演化模型来解释太阳模型/元素丰度问题。我们的研究揭示太阳模型问题是因为人们忽略了转动和对流超射对恒星演化的影响造成的。我们得到转动演化模型比标准太阳模型更好，能更好地解释日震学反演和中微子流量等观测。利用贝叶斯、蒙特卡洛模拟，我们研究了中子星黑洞的质量分布，结合星振学确定了KIC2837475等恒星对流区大小。我们研究还发现C、O元素增丰会导致计算的恒星年龄和主序拐点处的有效温度偏低等。这些研究是理解主序拐点扩展、太阳模型问题及中子星恒星级黑洞来源演化的基础，在恒星演化方面有重要科学意义.