Blue asymmetries in Balmer lines during mid M dwarf flares

Flares are releases of magnetic energy in the solar/stellar atmosphere, and they have strong emissions from radio to X-rays. During some M dwarf flares, chromospheric line profiles show blue asymmetries (Eason et al. 1992; Honda et al. 2018), although red asymmetries are more commonly observed in solar flares. Similar enhancements of the blue wings of Balmer lines may provide clues for investigating the early phases of stellar coronal mass ejections (CMEs) during flares (cf. Vida et al. 2016&2019), but this is still controversial. Thus, we need more flare spectroscopic observations with high time resolution to understand the relationship between mass ejections and flaring events. The latter is helpful for estimating the impact on planets from flares.

We have conducted several simultaneous spectroscopic and photometric observations of mid M dwarf flare stars using APO 3.5m/ARCES, SMARTS 1.5m/CHIRON, TESS, and ground- based 0.4-1m photometric telescopes. During 34 nights of observations, we detected 48 flares in Balmer lines (e.g. H-alpha). Among them, at least 7 flare events show clear blue asymmetries. Blue asymmetry durations are different among the 7 events (20min ~ 2hr).

These results suggest upward flows of chromospheric plasma during flare events. By assuming that the blue asymmetries were caused by prominence eruptions, we estimate the mass and kinetic energy of the upward-moving material to be 10¹⁵ - 10¹⁸ g and 10²⁹ - 10³² erg, respectively. The estimated masses are comparable to expectations from the empirical relation between the flare X-ray energy and mass of upward-moving material for stellar flares and solar CMEs. In contrast, the estimated kinetic energies for these non-white-light flares are roughly 2-3 orders of magnitude smaller than that expected from the relation between flare X-ray energy and kinetic energy for solar CMEs. This could be understood by the difference in the velocity between CMEs and prominence eruptions.