外赋AGB星星风吸积的角动量守恒模型

CONSERVATION OF ANGULAR MOMENTUM FOR THE WIND ACCRETION OF EXTRINSIC AGB STARS

用整个系统的角动量守恒条件代替切向动量守恒条件，推导出星风质量吸积及轨道参量变化方程．在新的轨道参量变化方程的基础上，计算了外赋AGB星系统的星风质量吸积及轨道参量的变化．将星风吸积模型同内禀AGB星核合成模型结合起来，通过逐次脉冲吸积质量和混合，自洽地计算外赋AGB星的重元素超丰，并给出计算结果与观测值的比较．对初始质量较大的Ba星（M2．0＝2．5M⊙），当系统轨道周期大于1300天时，属于星风吸积，小于600天成为共同包层双星或灾变双星．对初始质量较小的Ba星（M2．0＝13M⊙），当系统轨道周期大于1600天时，属于星风吸积，小于600天时成为灾变双星，由此可以解释Ba星的重元素超丰和轨道参量的观测事实，并有利于解释观测到的外赋S星轨道周期的600天下限．随着星风吸积过程的进行，轨道偏心率逐渐增大，这对解释Ba星轨道偏心率平均值大于外赋S星和CH星平均值的观测事实有利．

The equations of wind accretion and orbital parameters' variations are rededuced in the condition of the conservation of total angular momentum instead of the tangenhal momentum. Subsequently, for the extrinsic AGB stars, the wind mass accrehon and orbtal parameters' variahons are calculated, and their heavy elements overabundances are also computed in a consistent way. We combine the wind accrehon model with the nucleosynthesis of the intrinsic AGB stars and adopt the model of progressive pulses and mixing. The calculated results are compared with observations. This shows that, for the Ba stars with higher initial mass (M2,0 = 2.5M⊙),the formahon of those with orbital periods longer than 1300 days is due to wind accretion, and that with periods shorter than 600 days may give rise to the components of common envelopes or cataclysmic binaries. For the barium stars with lower initial mass (M2,0 = 1.3M⊙), the coresponding values of orbital period are 1600 and. 600 days, respechvely. So the observahon of heavy elements' overabundances and the orbital parameters of Ba stars may be interpreted, and the orbital periods'lower limit (600 days) of the extrinsic S stars can be interpreted too. Moreover, the higher eccentricity of Ba stars can be explained with the eccentricity's increase during the wind accretion. The average eccentricity of Ba stars is higher than those of extrinsic S stars and CH stars.