APO & SMARTS flare star campaign observations I. Blue wing asymmetries in chromospheric lines during mid M dwarf flares from simultaneous spectroscopic and photometric observation data

TL;DR

The study addresses blue wing asymmetries in Balmer lines during mid-M dwarf flares as potential indicators of prominence eruptions and possible coronal mass ejections (CMEs). It employs a time-resolved, simultaneous multi-wavelength campaign, monitoring YZ CMi, EV Lac, and AD Leo over 31 nights with optical spectroscopy (APO 3.5 m and SMARTS 1.5 m) and photometry (APO ARCSAT and LCOGT), plus TESS coverage and NICER X-ray spectroscopy on select nights. Among 41 detected flares, seven show clear blue-wing asymmetries in Hα, with line-of-sight velocities of $-73$ to $-122$ km s$^{-1}$ and durations from $20$ min to $2.5$ h; all seven also exhibit blue asymmetries in Hβ, while other chromospheric lines vary, and NICER data enable estimates of magnetic field and loop length for one event. The upward-moving plasma mass is estimated to be $10^{15}-10^{19}$ g, broadly consistent with solar-CME scaling, suggesting that these blue-wing signatures could trace prominence eruptions on M-dwarfs that may evolve into CMEs and impact exoplanetary environments.

Abstract

We conducted the time-resolved simultaneous optical spectroscopic and photometric observations of mid M dwarf flare stars YZ CMi, EV Lac, and AD Leo. Spectroscopic observations were obtained using Apache Point Observatory 3.5m and Small \& Moderate Aperture Research Telescope System 1.5m telescopes during 31 nights. Among the 41 detected flares, seven flares showed clear blue wing asymmetries in the H$α$ line, with various correspondences in flare properties. The duration of the blue wing asymmetries range from 20 min to 2.5 hours, including a flare showing the shift from blue to red wing asymmetry. Blue wing asymmetries can be observed during both white-light and candidate non white-light flares. All of the seven flares showed blue wing asymmetries also in the H$β$ line, but there are large varieties on which other chromospheric lines showed blue wing asymmetries. One among the 7 flares was also observed with soft X-ray spectroscopy, which enabled us to estimate the flare magnetic field and length of the flare loop. The line-of-sight velocities of the blue-shifted components range from -73 to -122 km s$^{-1}$. Assuming that the blue-shifts were caused by prominence eruptions, the mass of upward moving plasma was estimated to be 10$^{15}$ -- 10$^{19}$ g, which are roughly on the relation between flare energy and erupting mass expected from solar coronal mass ejections (CMEs). Although further investigations are necessary for understanding the observed various properties, these possible prominence eruptions on M-dwarfs could evolve into CMEs, assuming the similar acceleration mechanism with solar eruptions.