极短周期激变双星演化的观测与研究

The minimum period cut-off of cataclysmic binaries is the unsolved problem in the field of stellar astrophysics. The standard model proposes that the donor will transit from a star to a degenerate brown dwarf after reaching the period minimum. The change in the structure of the donor leads to a reversal in the direction of orbital period evolution. The systems evolving back towards longer periods are referred to as period bouncers. The evolution theory also predicts that about 70% of cataclysmic binaries should be period bouncers. However, until recently, only a few candidate period bouncers were known, which gives a serious challenge for the standard model. This also make the ultrashort-period cataclysmic binaries around the period minimum into the important objects for studying the evolution of cataclysmic binaries. Using small and medium sized telescopes, coupled with the collected data, we plan to analyze systematically the law of orbital period changes in these binaries, and search for the potential period bouncers with long-term period increase. Meanwhile, the systems with secular period decrease will be studied and the angular momentum loss mechanisms will be discussed. Moreover, the period changes of the candidate bouncers with sub-stellar donors will be detected and the standard model will be tested. These results can enrich our understanding and knowledge on the evolution and outburst of cataclysmic binaries.

激变双星的最短周期截止现象是恒星物理研究领域中没有解决的难题。标准模型认为这类双星演化到最短周期后，次星将从一颗主序恒星变为一颗简并的褐矮星，次星结构上的变化使轨道周期演化由长期减小反转为长期增加。这种反转后轨道增加的天体称为反弹体。理论预言目前观测到的激变双星中约有70%的样本应该是反弹体，然而目前仅有几个候选体被发现，这对标准模型提出了挑战。同时也使得周期截止附近的极短周期双星成为研究激变双星演化的重要天体。本项目计划利用国内外中小口径望远镜对极短周期食激变双星进行长期监测，并结合申请人所在课题组近十年的观测数据积累，系统地分析样本星轨道周期长期变化的规律，寻找轨道周期长期增加的演化反弹体。同时，对轨道周期长期减小的样本进行研究，探讨激变双星角动量损失的规律。另外，对已发现含有褐矮星次星的样本进行轨道周期长期变化分析，检验激变双星的标准模型，丰富人们对这类天体演化和爆发的认识。

激变双星是产生矮新星、新星甚至是Ia型超新星等爆发的重要场所，其演化进程一直是天体物理研究中的热点和难点问题。大量的观测研究表明，激变双星的演化明显比标准模型预言的快，且这类双星演化到最短周期后，轨道周期演化由长期减小反转为长期增加形成反弹体。理论预言目前观测到的激变双星中约有70%的样本应该是反弹体，然而目前仅有几个候选体被发现，这对标准模型提出了挑战。本项目基于LAMOST巡天、TESS巡天等释放的光谱和测光数据，结合地面光学望远镜数据，对一批激变双星进行了深入分析研究。通过对76颗矮新星的宁静态和爆发态的LAMOST光谱特性进行了分类研究，获得了光谱在不同状态的特性和演化特征，并发布了部分源的首次光谱观测结果。另外，提出了LAMOST矮新星爆发光谱地图，发现了7个可能探测到吸积盘旋臂结构的候选体。利用TESS数据发现并证认了IPHAS J0518是一颗特殊类型的掩食矮新星系统（Z Cam型），其爆发存在着双峰分布，并揭示爆发期间吸积盘的复杂行为是由其半径和温度梯度的联合变化引起。通过对短周期系统HT Cas等重要目标的长期监测研究，发现其轨道正在快速衰减，同时还发现其轨道周期呈现出变幅为79.3秒，周期为30年的周期性振荡。快速衰减的轨道揭示了在轨道周期空缺下的矮新星磁滞并未完全停止。而周期性振荡则揭示了在距离HT Cas约为10个天文单位处存在一颗褐矮星伴星。这些研究丰富了人们对这类天体演化和爆发的新认识。