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arXiv 1409.0639astro-ph.EPastro-ph.SR
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XUV辐射对环联星行星的影响

Effects of XUV radiation on circumbinary planets

J. Sanz-Forcada, S. Desidera, G. Micela

摘要:

最近发现了几颗环联星。一颗行星围绕一个双星系统的轨道构成了几个动力学约束。必须考虑来自主恒星的辐射可能对行星大气层的影响。由于是近距离双星系统,这些恒星有很高的自转速率,这导致了高恒星活动和丰富的XUV辐射的永久状态。累积效应比围绕单个恒星的系外行星更强,并导致其大气层蒸发更快。我们评估恒星辐射对双星系统周围系外行星蒸发和这些行星生存的影响。我们考虑了XUV光谱范围,以解释主要由氢组成的行星大气容易吸收的光子。更复杂的大气成分预计将更有效地吸收这种辐射。我们使用直接X射线观测来评估X射线范围内的能量,并使用日冕模型来计算光谱的(不可检测的)EUV部分。模拟表明,在近轨道上的系外行星将遭受强烈的光蒸发,可能导致在短时间内完全失去大气。由两颗类似太阳的恒星组成的双星系统将高效地蒸发行星的大气。这些系统将很难找到,即使它们是动态稳定的。尽管如此,行星可能会围绕低质量恒星的双星系统运行,以获得更宽的轨道。目前已知的环联星行星基本上不会受到热光蒸发过程的影响,除非开普勒-47 b有膨胀的大气层。环联星行星的轨道周期分布比开普勒行星的平均轨道周期要长得多,这支持了近距离环联星行星存在强烈光蒸发的假设。

英文摘要:

Several circumbinary planets have recently been discovered. The orbit of a planet around a binary stellar system poses several dynamic constraints. The effects that radiation from the host stars may have on the planet atmospheres must be considered. Because of the configuration of a close binary system, these stars have a high rotation rate, which causes a permanent state of high stellar activity and copious XUV radiation. The accumulated effects are stronger than for exoplanets around single stars, and cause a faster evaporation of their atmospheres. We evaluate the effects that stellar radiation has on the evaporation of exoplanets around binary systems and on the survival of these planets. We considered the XUV spectral range to account for the photons that are easily absorbed by a planet atmosphere that is mainly composed of hydrogen. A more complex atmospheric composition is expected to absorb this radiation more efficiently. We used direct X-ray observations to evaluate the energy in the X-rays range and coronal models to calculate the (nondetectable) EUV part of the spectrum. The simulations show that exoplanets in a close orbit will suffer strong photoevaporation that may cause a total loss of atmosphere in a short time. A binary system of two solar-like stars will be highly efficient in evaporating the atmosphere of the planet. These systems will be difficult to find, even if they are dynamically stable. Still, planets may orbit around binary systems of low mass stars for wider orbits. Currently known circumbinary planets are not substantially affected by thermal photoevaporation processes, unless Kepler-47 b has an inflated atmosphere. The distribution of the orbital periods of circumbinary planets is shifted to much longer periods than the average of Kepler planets, which supports a scenario of strong photoevaporation in close-in circumbinary planets.

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