A Hot Jupiter with a Retrograde Orbit around a Sun-like Star and a Toy Model of Hot Jupiters in Wide Binary Star Systems

We report an observation of a transit of the hot Jupiter (HJ) KELT-23A b with the Keck Planet Finder spectrograph and a measurement of the sky-projected obliquity (λ) of its Sun-like (Teff ≈ 5900 K) host star. We measured a projected stellar obliquity of λ ≈ 180◦ , indicating that the orbit of the HJ is retrograde relative to the direction of the stellar spin. Due to the slow sky-projected rotational velocity of the host star (v sin i⋆ ≈ 0.5 km s−1 ), the true orbit of the HJ could be closer to polar. HJs around stars with effective temperatures below the Kraft break – such as KELT-23A – are generally found to have prograde orbits that are well-aligned with the equatorial planes of their host stars (i.e., λ ∼ 0 ◦ ), most likely due to spin-orbit realignment driven by stellar tidal dissipation. This system is therefore a unique outlier that strains migration and tidal theories. The fact that the HJ has a highly misaligned orbit may suggest that the planet arrived at its close-in orbit relatively recently, possibly via interactions with the wide-separation (570 AU) M-dwarf companion in the system, or that it has stalled near an antialigned or polar orientation while realigning. Using Gaia DR3, we determined the orbit of the stellar companion to be moderately face-on (γ = 60 ± 4 ◦ ). We show that the distribution of observed systems in the γ − λ plane can be broadly reproduced using a toy model in which the orbits of the planetary and stellar companions begin aligned with the equatorial plane of the primary star and, upon migrating inwards, the planet preferentially obtains either an aligned or polar orbit.

本研究报道了利用凯克行星探测器光谱仪（Keck Planet Finder spectrograph）对热木星（hot Jupiter, HJ）KELT-23A b的凌星观测，以及对其类太阳（有效温度T_eff≈5900 K）宿主恒星的天球投影倾角（sky-projected obliquity, λ）的测量结果。我们测得该恒星的投影自旋倾角λ≈180°，表明该热木星的轨道相对于恒星自转轴方向呈逆行状态。由于宿主恒星的天球投影自转速度较慢（v sin i⋆≈0.5 km s⁻¹），该热木星的真实轨道可能更接近极轨取向。围绕有效温度处于克拉夫特分界（Kraft break）以下的恒星的热木星，通常被观测到拥有与宿主恒星赤道面高度对齐的顺行轨道（即λ≈0°），这一现象最可能由恒星潮汐耗散（stellar tidal dissipation）驱动的自旋轨道重对齐（spin-orbit realignment）所致。因此该系统是一个独特的异常体，对行星迁移与潮汐理论构成了挑战。该热木星拥有高度轨道错位的事实，可能意味着该行星相对较晚才进入近距轨道：要么是通过与系统中间距达570天文单位（astronomical unit, AU）的M型矮星（M-dwarf）伴星的相互作用，要么是在自旋轨道重对齐过程中停滞于反对齐或极轨取向附近。利用盖亚第三批数据发布（Gaia Data Release 3, Gaia DR3）数据，我们确定该恒星伴星的轨道为适度面迎（γ=60±4°）。我们证明，γ−λ平面内观测到的系统分布，可通过一个玩具模型（toy model）大致重现：该模型假设行星与恒星伴星的轨道最初与主星赤道平面对齐，且在向内迁移过程中，行星优先获得对齐或极轨轨道。