Title: Rotationally Driven Ultraviolet Emission of Red Giant Stars II. Metallicity, Activity, Binarity and Sub-subgiants Show Chinese title

Title: 旋转驱动的红巨星紫外线发射 II。金属量、活动性、双星和次次巨星

Abstract: Red Giant Branch (RGB) stars are overwhelmingly observed to rotate very slowly compared to main-sequence stars, but a few percent of them show rapid rotation and high activity, often as a result of tidal synchronizationn or other angular momentum transfer events. In this paper we build upon previous work using a sample of 7,286 RGB stars from APOGEE DR17 with measurable rotation. We derive an updated NUV excess vs $v \sin{i}$ rotation-activity relation that is consistent with our previous published version, but reduces uncertainty through the inclusion of a linear [M/H] correction term. We find that both single stars and binary stars generally follow our rotation-activity relation, but single stars seemingly saturate at $P_{\text{rot}}$/$\sin{i}$ $\sim$10 days while binary stars show no sign of saturation, suggesting they are able to carry substantially stronger magnetic fields. Our analysis reveals Sub-subgiant stars (SSGs) to be the most active giant binaries, with rotation synchronized to orbits with periods $\lesssim$ 20 days. Given their unusually high level of activity compared to other short-period synchronized giants we suspect the SSGs are most commonly overactive RS CVn stars. Using estimates of critical rotation we identify a handful giants rotating near break-up and determine tidal spin up to this level of rotation is highly unlikely and instead suggest planetary engulfment or stellar mergers in a fashion generally proposed for FK Comae stars. Show Chinese abstract

Abstract: 红巨星分支（RGB）恒星相比主序星被观察到旋转非常缓慢，但其中少数百分比的恒星表现出快速旋转和高活动性，这通常是潮汐同步或其他角动量转移事件的结果。 在本文中，我们基于来自APOGEE DR17的7,286颗RGB恒星样本进行先前工作的延伸，这些恒星具有可测量的旋转。 我们推导出一个更新的紫外线过剩与$v \sin{i}$旋转-活动关系，该关系与我们之前发表的版本一致，但通过包含线性[M/H]校正项减少了不确定性。 我们发现单星和双星通常都遵循我们的旋转-活动关系，但单星似乎在$P_{\text{rot}}$/$\sin{i}$$\sim$10天时达到饱和，而双星则没有饱和迹象，表明它们能够携带明显更强的磁场。 我们的分析显示 次次巨星（SSGs）是活动性最强的巨星级双星，其旋转与周期为$\lesssim$ 20天的轨道同步。 鉴于它们与其他短周期同步巨星相比异常高的活动水平，我们怀疑SSGs最可能是过活跃的RS CVn型恒星。 利用临界旋转估计，我们识别出少数接近破裂旋转的巨恒星，并确定潮汐加速至这种旋转水平的可能性极低，而是建议以通常用于FK Comae恒星的方式发生行星吞噬或恒星合并。