[论文解读] Evolution of Cool Close Binaries - Approach to Contact

K. Stȩpień|arXiv (Cornell University)|May 13, 2011

Stellar, planetary, and galactic studies参考文献 1被引用 12

一句话总结

本文建模了由磁制动驱动的冷却密近双星演化，表明通过磁化风造成的角动量损失可自然解释轨道周期小于2天的分离双星的观测周期分布。关键结果是磁制动是W UMa型碰撞双星的主要形成机制，这些双星由初始周期为2–3天的分离双星演化而来，经过6–10 Gyr后达到碰撞配置。

ABSTRACT

A set of 27 evolutionary models of cool close binaries was computed under the assumption that their evolution is influenced by the magnetized winds. Initial periods of 1.5, 2.0 and 2.5 d were considered. For each period three values of 1.3, 1.1 and 0.9 solar mass were taken as the initial masses of the more massive components. Here the results of the computations of the first evolutionary phase are presented, which starts from the initial conditions and ends when the more massive component reaches its critical Roche lobe. In all considered cases this phase lasts for several Gyr. For binaries with the higher total mass and/or longer initial periods this time is equal to, or longer than the main sequence life time of the more massive component. For the remaining binaries it amounts to a substantial fraction of this life time. From the statistical analysis of models, the predicted period distribution of detached binaries with periods shorter than 2 d was obtained and compared to the observed distribution from the ASAS data. An excellent agreement was obtained under the assumption that the period distribution in this range is determined solely by the mass and angular momentum loss due to the magnetized winds. This result indicates, in particular, that virtually all cool detached binaries with periods of a few tenths of a day, believed to be the immediate progenitors of W UMa-type stars, were formed from detached systems with periods around 2-3 d and that magnetic braking is the dominant formation mechanism of cool contact binaries. It operates on the time scale of several Gyr rendering them rather old, with age of 6-10 Gyr. The results of the present analysis will be used as input data to investigate the subsequent evolution of the binaries, through the mass exchange phase and contact or semi-detached configuration till the ultimate merging of the components.

研究动机与目标

• 研究磁制动在冷却密近双星向碰撞配置演化过程中的作用。
• 确定仅靠磁制动是否能重现周期小于2天的分离双星的观测周期分布。
• 评估在这些系统中质量转移和洛希瓣溢出的时间尺度与条件。
• 为后续研究碰撞双星中的质量交换及最终合并提供初始演化模型。

提出的方法

• 计算了27个初始周期为1.5、2.0和2.5天的冷却密近双星演化模型。
• 采用以磁化风驱动角动量损失（磁制动）为主要机制的恒星演化模型。
• 假设大质量分量的初始质量为1.3、1.1和0.9 M☉，并调整次星质量以避免极端质量比。
• 追踪从零龄主序到大质量分量开始洛希瓣溢出（RLOF）的演化过程。
• 将预测的分离双星周期分布与ASAS巡天数据进行比较，以验证磁制动模型。
• 通过模型的统计分析推断碰撞双星从分离前体形成的过程。

实验结果

研究问题

• RQ1 仅靠磁制动是否能解释周期短于2天的分离双星的观测周期分布？
• RQ2 在磁制动作用下，冷却密近双星的洛希瓣溢出时间尺度是多少？
• RQ3 初始质量和轨道周期如何影响通过磁制动形成碰撞双星？
• RQ4 为何周期极短（P < 0.45 d）的碰撞双星常见，而该周期范围内的半分离系统却稀少？
• RQ5 从分离双星到W UMa型碰撞双星的演化路径是什么？

主要发现

• 仅靠磁制动即可与ASAS数据导出的观测周期分布对周期小于2天的分离双星产生极佳匹配。
• 所有周期为零点几大周期的冷却分离双星，很可能均通过磁制动从初始周期2–3天的前体演化而来。
• 从分离到碰撞配置的阶段持续6–10 Gyr，表明碰撞双星通常是古老系统。
• 对于初始周期为2.5天或更长的双星，达到洛希瓣溢出的时间超过大质量分量的主序星寿命，因此在红巨星阶段膨胀会触发RLOF。
• 初始周期为1.5–2.0天的双星在主序演化期间达到RLOF，导致A型质量转移，并迅速形成周期小于0.5天的碰撞系统。
• 最终演化状态为合并，由持续的磁制动驱动，当质量转移缓慢时，角动量损失与之平衡。