Magnetic activity of ultracool dwarfs in the LAMOST DR11

TL;DR

This paper identifies and analyzes magnetic activity in ultracool dwarfs using LAMOST data, simulates CSST spectra with machine learning, and explores the activity-rotation relationship, revealing saturation at a specific Rossby number.

Contribution

It introduces a semi-supervised autoencoder approach for identifying ultracool dwarfs in simulated spectra and investigates their magnetic activity and rotation properties.

Findings

962 ultracool dwarfs identified in LAMOST DR11

Magnetic activity correlates with rotation, saturating at Rossby number 0.12

Demonstrates CSST slitless spectra's capability to detect ultracool dwarfs

Abstract

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$\alpha$ 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.