Red Asymmetry of H$_{\alpha}$ Line Profiles during the flares on the active RS CVn-type star II Pegasi

Abstract

Stellar coronal mass ejections (CMEs) have recently attracted much attention for their impacts on stellar evolution and surrounding exoplanets. RS CVn-type stars could produce large flares, and therefore may have frequent CMEs. Here we report the capture of a possible CME or chromospheric condensation on the RS CVn-type star II Pegasi (II Peg) using high-resolution spectroscopic observation. Two flares were detected during the observation, and the low limits of the flare energies are of the order of $10^{33}$ erg and $10^{34}$ erg, respectively. Using mean spectrum subtraction, the H$_{\alpha}$ residual shows red asymmetry during the flares, and the redshifted broad emission components are probably caused by chromospheric condensation or coronal rain. Moreover, a far redshifted extra emission component with a high bulk velocity of 429 km s$^{-1}$ was observed during the second flare and is probably due to a prominence eruption. The velocity greatly exceeds the star's escape velocity, which means that this eruption can develop into a CME. The CME mass is estimated to be 0.83-1.48 $\times 10^{20}$ g, which is slightly larger than the value expected from solar flare-CME extrapolation. The kinetic energy of CME, derived to be 0.76-1.15 $\times 10^{35}$ erg, is less than the kinetic energy extrapolated from solar events. Additionally, we could not completely rule out the possibility of chromospheric condensation resulting in the far redshifted extra emission. Finally, there is a blueshifted broad component in the subtracted H$_{\alpha}$ profile derived using synthesized spectral subtraction when no flare happened, and its behavior is associated with the H$_{\alpha}$ activity features.