Ultracool dwarfs consist of lowest-mass stars and brown dwarfs. Their interior is fully convective, different from that of the partly-convective Sun-like stars. Magnetic field generation process beneath the surface of ultracool dwarfs is still poorly understood and controversial. To increase samples of active ultracool dwarfs significantly, we have identified 962 ultracool dwarfs in the latest LAMOST data release, DR11. We also simulate the Chinese Space Station Survey Telescope (CSST) low-resolution slitless spectra by degrading the LAMOST spectra. A semi-supervised machine learning approach with an autoencoder model is built to identify ultracool dwarfs with the simulated CSST spectra, which demonstrates the capability of the CSST all-sky slitless spectroscopic survey on the detection of ultracool dwarfs. Magnetic activity of the ultracool dwarfs is investigated by using the H$α$ line emission as a proxy. The rotational periods of 82 ultracool dwarfs are derived based on the Kepler/K2 light curves. We also derive the activity-rotation relation of the ultracool dwarfs, which is saturated around a Rossby number of 0.12.

翻译：超冷矮星由最低质量的恒星和褐矮星组成。其内部完全对流，不同于部分对流的类太阳恒星。目前对超冷矮星表面以下的磁场生成过程仍知之甚少且存在争议。为显著增加活跃超冷矮星的样本量，我们在LAMOST最新数据发布DR11中识别出962颗超冷矮星。通过降级LAMOST光谱，我们模拟了中国空间站巡天望远镜（CSST）的低分辨率无缝光谱。构建了一种基于自编码器模型的半监督机器学习方法，用于从模拟的CSST光谱中识别超冷矮星，这证明了CSST全天无缝光谱巡天在探测超冷矮星方面的能力。通过以H$α$谱线发射作为示踪指标，研究了超冷矮星的磁活动。基于Kepler/K2光变曲线，推算出82颗超冷矮星的旋转周期。我们还推导出超冷矮星的活动-自转关系，该关系在罗斯贝数约为0.12时达到饱和。