食激变变星EX Dra轨道周期的双周期变化
Double cyclic variations in orbital period of the eclipsing cataclysmic variable EX Dra
摘要：
EX Dra是一颗长周期食矮新星，具有$\sim2-3$等振幅爆发。从2009年11月到2016年3月，这颗恒星被光度学监测，并获得了29个新的中食时间。利用新资料和已发表的资料，对$O-C$曲线的最佳拟合表明，EX Dra的轨道周期呈向上的抛物线变化，同时经历双周期变化，周期分别为21.4年和3.99年。向上的抛物线变化揭示了一个以$\dot{P}={+7.46}\times10^{-11}{s} {s^{-1}}$的速率长期增加的过程。灾变变星（CV）的进化理论预测，随着CV的进化，轨道周期应该减小而不是增加。长期增加可以解释为次级和初级之间的质量传递，或者可能只是较长周期变化的观察部分。最合理的解释是双周期变化是一对光旅行时间的影响，通过两个同伴的存在。其质量确定为$M_{A}sini'_{A}=29.3(\pm0.6) M_{Jup}$和$M_{B}sini'_{B}=50.8(\pm0.2) M_{Jup}$。当这两个伴星与中央食对的轨道平面共面时，它们的质量将与褐矮星相匹配。
英文摘要：
EX Dra is a long-period eclipsing dwarf nova with $\sim2-3$ mag amplitude outbursts. This star has been monitored photometrically from November, 2009 to March, 2016 and 29 new mid-eclipse times were obtained. By using new data together with the published data, the best fit to the $O-C$ curve indicate that the orbital period of EX Dra have an upward parabolic change while undergoing double-cyclic variations with the periods of 21.4 and 3.99 years, respectively. The upward parabolic change reveals a long-term increase at a rate of $\dot{P}={+7.46}\times10^{-11}{s} {s^{-1}}$. The evolutionary theory of cataclysmic variables (CVs) predicts that, as a CV evolves, the orbital period should be decreasing rather than increasing. Secular increase can be explained as the mass transfer between the secondary and primary or may be just an observed part of a longer cyclic change. Most plausible explanation for the double-cyclic variations is a pair of light travel-time effect via the presence of two companions. Their masses are determined to be $M_{A}sini'_{A}=29.3(\pm0.6) M_{Jup}$ and $M_{B}sini'_{B}=50.8(\pm0.2) M_{Jup}$. When the two companions are coplanar to the orbital plane of the central eclipsing pair, their masses would match to brown dwarfs.
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