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Blue asymmetries in Balmer lines during mid M dwarf flares

Presentation #515.01 in the session “Stellar Rotation, Variability, and Flares 2”.

Published onJan 11, 2021
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 (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, Nayuta 2m/MALLS (high-dispersion spectroscopy), TESS (space high-precision single-color photometry), and ground-based 0.4-1m telescopes (ground-based photometry). During ~20 nights of observations, we detected more than 30 flares in Balmer lines (e.g. Hα). Among them, at least 6 flare events (including one already reported in Maehara et al. 2020) show clear blue asymmetries, but none show brightening in the continuum. Blue asymmetry durations are different among the 6 events (20min ~ 2hr). These results suggest upward flows of chromospheric plasma during “non-white light” 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 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.


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