Ultraviolet and Chromospheric Activity and Habitability of M Stars

Abstract

M-type stars are crucial for stellar activity studies because they cover two types of magnetic dynamos and are particularly intriguing for habitability studies due to their abundance and long lifespans during the main-sequence stage. In this paper, we used the LAMOST DR9 catalog and the GALEX UV archive data to investigate the chromospheric and UV activities of M-type stars. All the chromospheric and UV activity indices clearly show that the saturated and unsaturated regimes, and the well-known activity–rotation relation, are consistent with previous studies. Both the FUV and NUV activity indices exhibit a single-peaked distribution, while the Hα and Ca II H&K indices show a distinct double-peaked distribution. The gap between these peaks suggests a rapid transition from a saturated population to an unsaturated one. The smoothly varying distributions of different subtypes suggest a rotation-dependent dynamo for both early-type (partly convective) to late-type (fully convective) M stars. We identified a group of stars with high UV activity above the saturation regime (log ${R}_{\mathrm{NUV}}^{{\prime} }\gt -2.5$ ) but low chromospheric activity, and the underlying reason is unknown. By calculating the continuously habitable zone and the UV habitable zone for each star, we found that about 70% stars in the total sample and 40% stars within 100 pc are located in the overlapping region of these two habitable zones, indicating that a number of M stars are potentially habitable. Finally, we examined the possibility of UV activity studies of M stars using the China Space Station Telescope.