Main sequence stars exhibit a clear rotation-activity relationship, in which rapidly rotating stars drive strong chromospheric/coronal ultraviolet and X-ray emission. While the vast majority of red giant stars are inactive, a few percent exhibit strong ultraviolet emission. Here we use a sample of 133 red giant stars observed by SDSS APOGEE and GALEX to demonstrate an empirical relationship between NUV excess and rotational velocity (vsini). Beyond this simple relationship, we find that NUV excess also correlates with rotation period and with Rossby number in a manner that shares broadly similar trends to those found in M dwarfs, including activity saturation among rapid rotators. Our data also suggest that the most extremely rapidly rotating giants may exhibit so-called "super-saturation", which could be caused by centrifugal stripping of these stars rotating at a high fraction of breakup speed. As an example application of our empirical rotation-activity relation, we demonstrate that the NUV emission observed from a recently reported system comprising a red giant with a black hole companion is fully consistent with arising from the rapidly rotating red giant in that system. Most fundamentally, our findings suggest a common origin of chromospheric activity in rotation and convection for cool stars from main sequence to red giant stages of evolution.

中文翻译：

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旋转驱动的红巨星紫外线发射

主序星表现出明显的自转-活动关系，快速自转的恒星驱动强烈的色球/日冕紫外线和X射线发射。虽然绝大多数红巨星都处于非活动状态，但只有少数显示出强烈的紫外线辐射。在这里，我们使用 SDSS APOGEE 和 GALEX 观测到的 133 颗红巨星样本来证明 NUV 过剩与旋转速度 (vsini) 之间的经验关系。除了这个简单的关系之外，我们发现 NUV 过剩还与旋转周期和罗斯比数相关，其趋势与 M 矮星中发现的趋势大致相似，包括快速旋转体中的活动饱和。我们的数据还表明，旋转速度最快的巨星可能会表现出所谓的“超饱和”，这可能是由于这些恒星以高分解速度旋转的离心剥离引起的。作为我们的经验旋转 - 活动关系的一个示例应用，我们证明了从最近报道的系统中观察到的 NUV 发射，该系统包括一个带有黑洞伴星的红巨星，与该系统中快速旋转的红巨星产生的完全一致。最根本的是，我们的研究结果表明，从主序到红巨星演化阶段，冷恒星在旋转和对流中色球活动的共同起源。我们证明，从最近报道的包含一个带有黑洞伴星的红巨星的系统中观察到的 NUV 发射与该系统中快速旋转的红巨星产生的完全一致。最根本的是，我们的研究结果表明，从主序到红巨星演化阶段，冷恒星在旋转和对流中色球活动的共同起源。我们证明，从最近报道的包含一个带有黑洞伴星的红巨星的系统中观察到的 NUV 发射与该系统中快速旋转的红巨星所产生的完全一致。最根本的是，我们的研究结果表明，从主序到红巨星演化阶段，冷恒星在旋转和对流中色球活动的共同起源。