arXiv 0908.35182009-08-25astro-ph.SR

多星中的X射线源

X-Ray Sources in Multiple Stars

V. V. Makarov, P. P. Eggleton

摘要：

明亮的X射线星经常在可见双星或多星中发现。周期只有几天的双星在规则的非相对论恒星中拥有最高程度的日冕X射线活动，因为它们的旋转速度很快，潮汐驱动。但是，目视双星的轨道周期太大，以至于伴星之间无法发生任何直接的相互作用。我们认为，大多数最强的X射线组件解决双星是尚未发现的短周期双星，并合并这种双星的残余。短周期活动星，如BY-Dra型双星，在多个系统中的无所不在的解释通过大的相互倾斜的三星的动力学演化。内部对的动力学扰动以循环的方式泵出偏心率，这种现象被称为Kozai循环。在接近近心点的时候，潮汐摩擦会降低双星的角动量，导致它收缩。当内侧双星的轨道周期下降到几天时，伴星的快速表面旋转是由潮汐力驱动的，将活动提高了几个数量级。如果周期进一步下降，合并可能会发生，留下一个快速旋转的活动矮星，只有一个遥远的伴星。

英文摘要：

Luminous X-ray stars are very often found in visual double or multiple stars. Binaries with periods of a few days possess the highest degree of coronal X-ray activity among regular, non-relativistic stars because of their fast, tidally driven rotation. But the orbital periods in visual double stars are too large for any direct interaction between the companions to take place. We suggest that most of the strongest X-ray components in resolved binaries are yet-undiscovered short-period binaries, and that a few are merged remnants of such binaries. The omnipresence of short-period active stars, e.g. of BY-Dra-type binaries, in multiple systems is explained via the dynamical evolution of triple stars with large mutual inclinations. The dynamical perturbation on the inner pair pumps up the eccentricity in a cyclic manner, a phenomenon known as Kozai cycling. At times of close periapsis, tidal friction reduces the angular momentum of the binary, causing it to shrink. When the orbital period of the inner pair drops to a few days, fast surface rotation of the companions is driven by tidal forces, boosting activity by a few orders of magnitude. If the period drops still further, a merger may take place leaving a rapidly-rotating active dwarf with only a distant companion.

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