Flare-related plasma motions in the outer atmosphere of the RS CVn-type star II Peg

Abstract

Analogous to solar flares, stellar flares are dramatic explosions in the atmosphere, which may be accompanied by prominence eruptions, coronal mass ejections (CMEs), and other forms of plasma motion. Based on time-resolved spectroscopic observations of the RS CVn-type star II Peg, we aim to search for the potential plasma motions associated with flares. In these observations, we detected part of the gradual decay phase of an optical flare, for which we find a lower limit on the energy of the H$ alpha $ line of $6.03 $ erg. Converting this H$ alpha $ energy, we find a bolometric white-light energy of $3.10 $ erg. Moreover, a secondary peak is also observed. After removing a quiescence reference, the H$ alpha $ residual shows an asymmetric behavior, including both a blueshifted and a redshifted emission component. The former component has a bulk velocity of about -180 km s$^ $ and extends its velocity to more than -350 km s$^ $. This phenomenon is likely caused by a prominence eruption event or a chromospheric evaporation process. The latter emission component has a bulk velocity of 130 to 70 km s$^ $ and extends its velocity to nearly 400 km s$^ $. We attribute the redshifted emission component to one or a combination of several possible scenarios: flare-driven coronal rain, chromospheric condensation, backward-directed prominence eruption close to the stellar limb, or falling material in a prominence eruption. The minimum masses of the moving plasmas resulting in the blueshifted and redshifted emission components are estimated to be $0.56 $ g and $1.74 $ g, respectively.