2026ApJ...997..244X

Ultracool dwarfs consist of lowest-mass stars and brown dwarfs. Their interior is fully convective, in contrast to that of the partly convective Sun-like stars. The 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 Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) data release, DR11. We also simulate the Chinese Space Station Survey Telescope (CSST) low-resolution slitless spectra by degrading the LAMOST spectra. A semisupervised 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 in the detection of ultracool dwarfs. The magnetic activity of the ultracool dwarfs is investigated by using the Hα 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.