红巨星中长副周期变星的起源

The Origin of Long Secondary Periods Red Variables

长副周期变星是一类特殊的半规则变星,大量存在于银河系和大小麦哲伦云中,约占渐近巨星分支的25%～30%。该类变星有两个光变周期,短的主周期为30～200 d,由径向脉动的低阶模式导致;长的副周期长达400～1500 d。在长周期变星的周光关系中,长副周期变星处在D序列,平行于径向脉动的A、B和C序列,且与属于双星系统的E序列部分重叠。长副周期现象的起源一直是个谜,现存的解释模型有:径向脉动、非径向脉动、双星、磁活动、带有一个巨大黑子的转动红巨星、转动的椭球红巨星、κ机制等,但这些模型没有一种能完美地解释长副周期变星的起源。

Among the variable red giant stars one sub-type called long secondary periods（LSPs） star is unique.The light curves of these stars exhibit not only a short primary period but also a long secondary period,which is approximately nine times longer than the short one.The short primary period is caused by the first or second radial overtone,with a period of 30-200 d,while the origin of the long secondary period is unclear.The long secondary period variables follow a distinct period-luminosity relation which is parallel to the radial pulsation sequences A,B and C, and overlap with the binary sequence-E.Moveover,the LSPs stars show infrared excess according to the Spitzer/SAGE observation,which indicates the existence of mass ejection.However,the property of the dust envelope is yet to be clarified.The spectroscopic observations show that the LSPs stars have a radial variation with a full amplitude of 5 km·s^（-1）,and with a period comparable to the photometric long-period variations.The minimum light usually has about 0.2 phase lag in comparison with the mean rising velocity.The radial amplitude of LSPs stars is large, amounting to 20%-30%of the mean stellar radius.Additionally,the stars show large variations in the strength of the Ha absorption line,with a period equal to that of the light variations.The absorption line indicates the presence of a chromosphere which covers up to 50%of the star at some phases of the LSP.The origin of the LSPs star is a mystery up to now,although there are many models,such as radial pulsation,non-radial pulsation,binary,magnetic activity,rotation red giants with a large spot,rotation ellipsoidal red giants and k mechanism.All these models have problems to explain all the observational facts and new models are called for.