arXiv 1809.021872018-09-06astro-ph.SR

后主序星双星的轨道衰变

Orbital Decay in Binaries Containing Post-Main Sequence Stars

Meng Sun, Phil Arras, Nevin N. Weinberg, Nicholas W. Troup, Steven R. Majewski

备注：: 18 pages, 19 figures, accepted to MNRAS
DOI：: 10.1093/mnras/sty2464

摘要：

研究了包含一颗原亚巨星或红巨星和一颗恒星或亚恒星伴星的双星的轨道衰变。主星被伴星掀起的潮汐导致轨道和恒星自旋之间的角动量交换，导致轨道收缩，主星自旋上升。轨道衰减率的计算，包括平衡潮汐，在对流信封中的湍流粘性阻尼，和动力潮汐，假设是一个旅行的内部重力波的辐射核心。对于密近双星，潮汐强迫期预计将远远短于涡周转时间尺度的对流信封，处方的“约化”粘度是一个重要的考虑因素。动力潮汐倾向于在最近的轨道上占主导地位，而平衡潮汐则在更远的轨道上占主导地位，交叉点取决于恒星质量。主旋转到同步旋转的旋转发生在足够大的秒级，这大大减缓了轨道衰变，直到达尔文不稳定性发生。一个参数调查提出的轨道衰减作为一个功能的主要和次要的质量，以及湍流粘度处方。这些结果与年龄相关的临界分离，$a_{\rm crit}$，其中轨道衰减是快速的，预计将观察到的系统很少的解析公式和数值结果进行了总结。将$a_{\rm crit}$的计算结果与远地点双星以及太阳质量系外行星的主恒星进行了比较。

英文摘要：

The orbital decay of binaries containing a primary sub-giant or red giant star and a stellar or substellar companion is investigated. The tide raised in the primary by the companion leads to an exchange of angular momentum between the orbit and the stellar spin, causing the orbit to contract and the primary to spin up. The rate of orbital decay is computed including both the equilibrium tide, damped by turbulent viscosity in the convective envelope, and the dynamical tide, assumed to be a traveling internal-gravity wave in the radiative core. For close binaries, the tidal forcing period is expected to be much shorter than the eddy turnover timescale in the convective envelope, and the prescription for "reduced" viscosity is an important consideration. The dynamical tide tends to dominate for the closest orbits, while the equilibrium tide dominates for more distant orbits, with the crossover point depending on the stellar mass. The spin up of the primary to synchronous rotation occurs for sufficiently massive secondaries, and this greatly slows the orbital decay until the Darwin instability occurs. A parameter survey is presented for orbital decay as a function of primary and secondary mass, as well as turbulent viscosity prescription. These results are summarized with analytic formulae and numerical results for the age-dependent critical separation, $a_{\rm crit}$, inside of which orbital decay is rapid, and few systems are expected to be observed. The calculations of $a_{\rm crit}$ are compared with APOGEE binaries, as well as solar mass exoplanet host stars.

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英文摘要：

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