In our previous study of low mass stars using TESS, we found a handful which show a periodic modulation on a period <1 d but also displayed no flaring activity. Here we present the results of a systematic search for Ultra Fast Rotators (UFRs) in the southern ecliptic hemisphere which were observed in 2 min cadence with TESS. Using data from Gaia DR2, we obtain a sample of over 13,000 stars close to the lower main sequence. Of these, we identify 609 stars which lie on the lower main sequence and have a periodic modulation <1 d. The fraction of stars which show flares appears to drop significantly at periods <0.2 d. If the periods are a signature of the rotation rate, this would be a surprise, since faster rotators would be expected to have a stronger magnetic field and, therefore, produce more flares. We explore possible reasons for our finding: the flare inactive stars are members of binaries, in which case the stars rotation rate could have increased as the binary orbital separation reduced due to angular momentum loss over time, or that enhanced emission occurs at blue wavelengths beyond the pass band of TESS. Follow-up spectroscopy and flare monitoring at blue/ultraviolet wavelengths of these flare inactive stars are required to resolve this question.

中文翻译：

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TESS 对南方超快旋转低质量恒星的观测

在我们之前使用 TESS 对低质量恒星的研究中，我们发现了少数在 <1 d 的周期内显示出周期性调制但也没有显示出耀斑活动。在这里，我们展示了在南黄道半球系统搜索超快旋转器 (UFR) 的结果，这些结果是用 TESS 以 2 分钟的节奏观察到的。使用来自盖亚 DR2 的数据，我们获得了靠近主序带下部的超过 13,000 颗恒星的样本。其中，我们确定了 609 颗位于主序带下部且周期性调制 <1 d 的恒星。显示耀斑的恒星比例似乎在 <0.2 d 的周期内显着下降。如果周期是旋转速率的标志，这将是一个惊喜，因为更快的旋转器预计会具有更强的磁场，因此会产生更多的耀斑。我们探索了我们发现的可能原因：耀斑非活动恒星是双星的成员，在这种情况下，由于角动量随着时间的推移而导致双星轨道间距减小，恒星的自转率可能会增加，或者在蓝色波长之外发生增强的发射TESS 的通带。需要对这些不活跃恒星的蓝色/紫外线波长进行后续光谱和耀斑监测来解决这个问题。