The Aligned Orbit of the Warm Jupiter K2-232 b

Measuring the obliquity distribution of stars hosting warm Jupiters may help us to understand theformation of close-orbiting gas giants. Few such measurements have been performed due to practicaldifficulties in scheduling observations of the relatively infrequent and long-duration transits of warmJupiters. Here, we report a measurement of the Rossiter-McLaughlin effect for K2-232b, a warmJupiter on an 11.17-day orbit with an eccentricity of 0.26. The data were obtained with the Automated Planet Finder during two separate transits. The planet’s orbit appears to be well-aligned with the spin axis of the host star, with a projected spin-orbit angle of λ = −11.1 ± 6.6 . Combined with the other available data, we find that high obliquities are almost exclusively associated with planets that either have an orbital separation greater than 10 stellar radii or orbit stars with effective temperatures hotter than 6,000K. This pattern suggests that the obliquities of the closest-orbiting giant planets around cooler stars have been damped by tidal effects.

测量拥有暖木星（warm Jupiters）的恒星的倾角分布，或许有助于我们理解近轨气态巨行星的形成机制。由于暖木星的凌日现象相对罕见且持续时间长，观测调度存在实际困难，因此这类测量的开展较为有限。本文报告了对K2-232b的罗西特-麦克劳克林效应（Rossiter-McLaughlin effect）的测量结果，该行星是一颗暖木星，其轨道周期为11.17天，偏心率为0.26。数据由自动行星搜寻仪（Automated Planet Finder）在两次独立的凌日期间获取。该行星的轨道似乎与宿主星的自转轴高度对齐，其投影自旋-轨道角（spin-orbit angle）λ = −11.1 ± 6.6。结合现有其他数据，我们发现高倾角几乎仅与两类行星相关：一类是轨道间距大于10倍恒星半径的行星，另一类是环绕有效温度超过6000K恒星运行的行星。这一模式表明，较冷恒星周围近轨巨行星的倾角已被潮汐效应所阻尼。