Hot Jupiters in Binary Star Systems
Hot Jupiters in Binary Star Systems
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双星系统中的热木星
DOI：
10.1086/521996
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发表时间：
2007
期刊：
影响因子：
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通讯作者：
J. Michael Ramsahai
中科院分区：
文献类型：
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作者：
Yanqin Wu;Norman W. Murray;J. Michael Ramsahai
径向速度测量发现，在约1%的类太阳恒星周围，半长轴小于0.1 AU的木星质量的行星；算上5天文单位的行星，拥有行星的恒星比例达到~10%（由Marcy et al.和Butler et al.发现）。对半长轴分布的检查表明，轨道周期在3或4天左右的行星明显过量，对应于一个≈0.03 AU，在较短的周期上有一个明显的截断（见图1）。据信，木星质量的行星是在离母恒星很远的地方形成的；然后，一部分会迁移进来，产生短周期的对象。我们认为，相当一部分热木星（a < 0.1 AU）可能出现在双星系统中，其中双星的轨道与行星的轨道高度倾斜。两个轨道之间的相互扭矩降低了行星与其主星之间的最小距离或近日点rp （Kozai机制）。当行星上的潮汐摩擦绕轨道运行的速度超过Kozai扭矩所能激发的速度时，这种近日点崩溃就会停止。同样的摩擦使行星绕轨道运行，产生了半长轴分布的峰值在3天左右的热木星。对于观测到的双星分离、偏心率和质量比分布，大约2.5%的初始半长轴ap≈5 AU的行星将迁移到距离其母星0.1 AU以内。Kozai迁移可能占观测到的热木星的10%或更多。
Radial velocity surveys find Jupiter-mass planets with semimajor axes a less than 0.1 AU around ~1% of solar-type stars; counting planets with a as large as 5 AU, the fraction of stars having planets reaches ~10% (as found by Marcy et al. and Butler et al.). An examination of the distribution of semimajor axes shows that there is a clear excess of planets with orbital periods around 3 or 4 days, corresponding to a ≈ 0.03 AU, with a sharp cutoff at shorter periods (see Fig. 1). It is believed that Jupiter-mass planets form at large distances from their parent stars; some fraction then migrates in to produce the short-period objects. We argue that a significant fraction of the hot Jupiters (a < 0.1 AU) may arise in binary star systems in which the orbit of the binary is highly inclined to the orbit of the planet. Mutual torques between the two orbits drive down the minimum separation or periapsis rp between the planet and its host star (the Kozai mechanism). This periapsis collapse is halted when tidal friction on the planet circularizes the orbit faster than Kozai torque can excite it. The same friction then circularizes the planet orbit, producing hot Jupiters with the peak of the semimajor axis distribution lying around 3 days. For the observed distributions of binary separation, eccentricity, and mass ratio, roughly 2.5% of planets with initial semimajor axis ap ≈ 5 AU will migrate to within 0.1 AU of their parent star. Kozai migration could account for 10% or more of the observed hot Jupiters.