HAT-P-11：第二颗行星的发现和了解系外行星倾角的线索
HAT-P-11: Discovery of a Second Planet and a Clue to Understanding Exoplanet Obliquities
摘要：
HAT-P-11是一颗中K矮星，是在太阳系外发现的第一颗海王星大小的行星。HAT-P-11b的轨道与恒星的自旋不一致-这是少数已知的行星围绕质量小于太阳的恒星旋转的情况之一。我们发现一个额外的行星系统的基础上，十年的精确径向速度（RV）测量从凯克/HIRES。HAT-P-11 c的质量（$M_P \sin{i} = 1.6\pm0.1$ $M_J$）和轨道周期（$P = 9.3^{+1.0}_{-0.5}$年）与木星相似，但轨道（$e=0.60\pm0.03$）更加偏心。在我们对RV和恒星活动的联合建模中，我们发现了$\sim$ 7 m s $^{-1}$的活动诱导RV信号，与其他活动K矮星一致，但明显小于HAT-P-11 c引起的31 m s $^{-1}$反射运动。我们研究了HAT-P-11b和c之间的动力学耦合，作为HAT-P-11b未对准轨道的可能解释，发现行星-行星Kozai相互作用不能由于广义相对论进动而使行星b的轨道倾斜;然而，在百万年时间尺度上运行的节点进动是解释HAT-P-11b高轨道性的可行机制。这留下了一个问题，为什么HAT-P-11 c可能有这样一个倾斜的轨道。在38 pc的距离上，HAT-P-11系统通过天体测量和直接成像提供了进一步描述系外行星特征的丰富机会。
英文摘要：
HAT-P-11 is a mid-K dwarf that hosts one of the first Neptune-sized planets found outside the solar system. The orbit of HAT-P-11b is misaligned with the star's spin --- one of the few known cases of a misaligned planet orbiting a star less massive than the Sun. We find an additional planet in the system based on a decade of precision radial velocity (RV) measurements from Keck/HIRES. HAT-P-11c is similar to Jupiter in its mass ($M_P \sin{i} = 1.6\pm0.1$ $M_J$) and orbital period ($P = 9.3^{+1.0}_{-0.5}$ year), but has a much more eccentric orbit ($e=0.60\pm0.03$). In our joint modeling of RV and stellar activity, we found an activity-induced RV signal of $\sim$7 m s$^{-1}$, consistent with other active K dwarfs, but significantly smaller than the 31 m s$^{-1}$ reflex motion due to HAT-P-11c. We investigated the dynamical coupling between HAT-P-11b and c as a possible explanation for HAT-P-11b's misaligned orbit, finding that planet-planet Kozai interactions cannot tilt planet b's orbit due to general relativistic precession; however, nodal precession operating on million year timescales is a viable mechanism to explain HAT-P-11b's high obliquity. This leaves open the question of why HAT-P-11c may have such a tilted orbit. At a distance of 38 pc, the HAT-P-11 system offers rich opportunities for further exoplanet characterization through astrometry and direct imaging.
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