红亚矮星研究进展

Research Development of Red Subdwarfs

红亚矮星是甚小质量恒星中的贫金属成员,质量从约0.5M⊙(M⊙为太阳质量)到H燃烧的最小质量(0:075M⊙0:085M⊙,取决于金属丰度),其寿命长于哈勃年龄,是银河系结构和化学增丰史的重要示踪体。与银盘上数量最多的恒星成员红矮星不同,红亚矮星在太阳附近非常稀少,并且其运动学特征与盘矮星有较大差异,属于年老银河系星族,即为年老盘星族、厚盘星族或晕星族。观测上,红亚矮星可以根据其不同于红矮星的自行、测光和光谱特征被识别和证认。由于其恒星表面大气温度很低,并且颜色比同质量的矮星更蓝,因此红亚矮星在赫罗图上位于主序带末端的下方,介于矮星与白矮星之间。红亚矮星的光学波段光谱由金属氧化物(如TiO和VO)和氢化物(如CaH和H2O)的分子吸收带占主导。红亚矮星可按其光谱形态和分子带特征分成不同的光谱型和金属丰度等级,其中晚M型到早L型的亚矮星既可能是小质量的恒星,也可能是较大质量的年轻褐矮星。介绍了对红亚矮星研究的历史背景和前沿动态,详细阐述了光谱分析方法在研究亚矮星中的重要性,以及根据光谱特征对亚矮星进行分类的方法。最后,总结了甚小质量恒星大气模型的发展过程,并探讨了如何利用模型对亚矮星的大气参数进行估算等热点问题。

Red subdwarfs are metal-poor very low mass stars(VLMs)with the mass of 0.5M⊙to 0.08M⊙and lifetime longer than the Hubble time.They are supposed to be able to provide crucial information for star formation and metal enrichment history of the Milky Way.Differing from the red dwarfs which are the most dominant members of the Milky Way,red subdwarfs are rare in the solar neighborhood,and they have kinematic properties associated with halo and thick disk.These low mass stars are named as subdwarfs because they are located under the dwarfs of the main sequence on the H-R diagram.In addition,the large proper-motions and characteristic spectral features of red subdwarfs can help distinguish them from red dwarfs.In the optical band,the spectrum of a subdwarf is dominated by the oxide(such as TiO and VO)and hydride(such as CaH).According to the spectral morphology and molecular bands,red subdwarfs can be divided into several spectral types and metallicity subclasses.The masses of the coolest subdwarfs are close to the hydrogen burning limit.A subdwarf with the lowest mass may be a star or a young brown dwarf.Historical background and frontier knowledge of the studies of red subdwarfs are introduced.The recent development of red subdwarf researches is summarized,including samples,kinematic properties,atmospheric parameter estimates,mass and temperature ranges,activity analysis,and the multiplicity.Besides,the importance of the spectral analysis method in the study of subdwarfs is discussed in detail.Finally,the method of classification of subdwarfs based on spectral features and the way to use the latest stellar atmospheric model to estimate the atmospheric parameters of red subdwarfs are introduced.