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arXiv 1008.0955astro-ph.SR
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短周期激变变星的半经验质量损失率

A Semi-empirical Mass-loss Rate in Short-period Cataclysmic Variables

Fedir V. Sirotkin, Woong-Tae Kim

摘要:

激变变星中施主星的质量损失率在短周期激变变星的演化过程中具有重要意义。观测到的施主比相同质量的孤立单星要大,这被认为是质量损失的结果。利用CV的经验质量-半径关系,以及施主的有效温度T_2、轨道周期P和光度随恒星半径变化的同系近似,我们得到CV的半经验质量损失率M2_dot随P的变化,导出的M2_dot为~10^(-9.5)~(-10)Msun/yr,P > 90 min时对P的依赖性较弱,P < 90 min时迅速向最低值下降,符合P-T_2关系。由于由质量损失引起的与热平衡的强烈偏离,半经验M2_dot与之前的数值模型所建议的相比,与P显著不同,并且具有不太明显的转向行为。半经验P-M_2_dot关系与引力波发射引起的角动量损失一致,并强烈地表明0.075 Msun < M_2 < 0.2 Msun的CV秒数小于2 Gyrs。当应用于选定的食变星时,我们的半经验质量损失率与由白矮星的有效温度T_1导出的吸积率吻合得很好,这表明M2_dot可以用来可靠地从T_1推断T_2。基于半经验M2_dot,之前被识别为反弹后CV的SDSS 1501和1433系统尚未达到最小周期。

英文摘要:

The mass-loss rate of donor stars in cataclysmic variables (CVs) is of paramount importance in the evolution of short-period CVs. Observed donors are oversized in comparison with those of isolated single stars of the same mass, which is thought to be a consequence of the mass loss. Using the empirical mass-radius relation of CVs and the homologous approximation for changes in effective temperature T_2, orbital period P, and luminosity of the donor with the stellar radius, we find the semi-empirical mass-loss rate M2_dot of CVs as a function of P. The derived M2_dot is at ~10^(-9.5)-10^(-10) Msun/yr and depends weakly on P when P > 90 min, while it declines very rapidly towards the minimum period when P < 90 min, emulating the P-T_2 relation. Due to strong deviation from thermal equilibrium caused by the mass loss, the semi-empirical M2_dot is significantly different from, and has a less-pronounced turnaround behavior with P than suggested by previous numerical models. The semi-empirical P-M2_dot relation is consistent with the angular momentum loss due to gravitational wave emission, and strongly suggests that CV secondaries with 0.075 Msun < M_2 < 0.2 Msun are less than 2 Gyrs old. When applied to selected eclipsing CVs, our semi-empirical mass-loss rates are in good agreement with the accretion rates derived from the effective temperatures T_1 of white dwarfs, suggesting that M2_dot can be used to reliably infer T_2 from T_1. Based on the semi-empirical M2_dot, SDSS 1501 and 1433 systems that were previously identified as post-bounce CVs have yet to reach the minimal period.

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