Stellar Cycle and Evolution of Polar Spots in an M+WD Binary

• Published in: The Astrophysical journal Vol. 963; no. 2; pp. 160 - 173
• Main Authors: Zhao, Xinlin, Wang, Song, Li, Xue, Xiang, Yue, Xu, Fukun, Gu, Shenghong, Du, Bing, Liu, Jifeng
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.03.2024; IOP Publishing
• Subjects: Binary stars; Curve fitting; Evolution; Light curve; Magnetic fields; Magnetic induction; Neutron stars; Orbits; Red dwarf stars; Sky surveys (astronomy); Stars; Stellar activity; Stellar evolution; Stellar magnetic fields; White dwarf stars; X-ray emissions
• DOI: 10.3847/1538-4357/ad1e64
• ISSN: 0004-637X

Stellar activity cycles reveal continuous relaxation and induction of magnetic fields. The activity cycle is typically traced through the observation of cyclic variations in total brightness or Ca H&K emission flux of stars, as well as cyclic variations in the orbital periods of binary systems. In this work, we report the identification of a semidetached binary system (TIC 16320250) consisting of a white dwarf (0.67 M ⊙ ) and an active M dwarf (0.56 M ⊙ ). The long-term multiband optical light curves spanning twenty years revealed three repeated patterns, suggestive of a possible activity cycle of about 10 years of the M dwarf. Light-curve fitting indicates the repeated variation is caused by the evolution, particularly the motion, of polar spots. The significant Ca H&K, H α , ultra-violet, and X-ray emissions imply that the M dwarf is one of the most magnetically active stars. We propose that in the era of large time-domain photometric sky surveys (e.g., ASAS-SN, Zwicky Transient Facility, LSST, Sitian), long-term light-curve modeling can be a valuable tool for tracing and revealing stellar activity cycle, especially for stars in binary systems.