Broadening and redward asymmetry of H$\alpha$ line profiles observed by LAMOST during a stellar flare on an M-type star

TL;DR

This study reports spectroscopic observations of a stellar flare on an M-type star, revealing broadening and redward asymmetry in the Hα line, indicating plasma motions and physical conditions during the flare.

Contribution

First spectroscopic detection of Hα line broadening and red asymmetry during a stellar flare on an M-type star using LAMOST data, linking line profile changes to plasma dynamics.

Findings

Hα line intensity increased impulsively and then decreased.

Red wing enhancement indicates plasma moving away at 100-200 km/s.

Estimated plasma mass involved is around 10^15 kg.

Abstract

Stellar flares are characterized by sudden enhancement of electromagnetic radiation in stellar atmospheres. So far much of our understanding of stellar flares comes from photometric observations, from which plasma motions in flare regions could not be detected. From the spectroscopic data of LAMOST DR7, we have found one stellar flare that is characterized by an impulsive increase followed by a gradual decrease in the H$\alpha$ line intensity on an M4-type star, and the total energy radiated through H${\alpha}$ is estimated to be on the order of $10^{33}$ erg. The H$\alpha$ line appears to have a Voigt profile during the flare, which is likely caused by Stark pressure broadening due to the dramatic increase of electron density and/or opacity broadening due to the occurrence of strong non-thermal heating. Obvious enhancement has been identified at the red wing of the H$\alpha$ line profile after the impulsive increase of the H$\alpha$ line intensity. The red wing enhancement corresponds to plasma moving away from the Earth at a velocity of 100$-$200 km s$^{-1}$. According to the current knowledge of solar flares, this red wing enhancement may originate from: (1) flare-driven coronal rain, (2) chromospheric condensation, or (3) a filament/prominence eruption that either with a non-radial backward propagation or with strong magnetic suppression. The total mass of the moving plasma is estimated to be on the order of $10^{15}$ kg.