今天是二人组，但曾经是三人组？双白矮星HS 2220 $+$ 2146可能是后蓝离散双星
Today a Duo, But Once a Trio? The Double White Dwarf HS 2220$+$2146 May Be A Post-Blue Straggler Binary
摘要：
对于足够宽的轨道间距{ \it a}，双星的两个成员独立演化。这意味着在宽的双白矮星（DWD）中，质量更大的WD应该总是首先产生，当它的质量更大的祖先结束了它的主序星生命时，因此应该比它的同伴更老和更冷。结合在一起的宽DWD HS 2220 $+$ 2146（$a\approx500$ AU）不符合这幅图：更大质量的WD是这对中更年轻、更热的。我们表明，这种差异是不太可能是由于过去的传质阶段或存在一个未解决的同伴。相反，我们建议，HS 2220 $+$ 2146通过一个新的宽DWD进化通道，涉及合并的内部二进制在一个分层的三重系统形成。由此产生的蓝色离散星和它的宽伴星随后独立进化，形成了今天看到的WD对。虽然我们不能排除其他的可能性，但最有可能的形成通道是内部双星合并，而两颗恒星仍然在主序星上。这为我们提供了一个诱人的可能性，即Kozai-Lidov振荡可能在内部双星的合并中发挥了作用。{ \it Gaia}可以发现数百个更宽的DWD，从而导致识别其他系统，如HS 2220 $+$ 2146。已经有迹象表明，其他WD系统可能是通过不同的，但相关的，分层的三重进化方案形成的。表征这些人口可能允许彻底测试的效率与KL振荡诱导恒星合并。
英文摘要：
For sufficiently wide orbital separations {\it a}, the two members of a stellar binary evolve independently. This implies that in a wide double white dwarf (DWD), the more massive WD should always be produced first, when its more massive progenitor ends its main-sequence life, and should therefore be older and cooler than its companion. The bound, wide DWD HS 2220$+$2146 ($a\approx500$ AU) does not conform to this picture: the more massive WD is the younger, hotter of the pair. We show that this discrepancy is unlikely to be due to past mass-transfer phases or to the presence of an unresolved companion. Instead, we propose that HS 2220$+$2146 formed through a new wide DWD evolutionary channel involving the merger of the inner binary in a hierarchical triple system. The resulting blue straggler and its wide companion then evolved independently, forming the WD pair seen today. Although we cannot rule out other scenarios, the most likely formation channel has the inner binary merging while both stars are still on the main sequence. This provides us with the tantalizing possibility that Kozai-Lidov oscillations may have played a role in the inner binary's merger. {\it Gaia} may uncover hundreds more wide DWDs, leading to the identification of other systems like HS 2220$+$2146. There are already indications that other WD systems may have been formed through different, but related, hierarchical triple evolutionary scenarios. Characterizing these populations may allow for thorough testing of the efficiency with which KL oscillations induce stellar mergers.
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