Detection of Flare-induced Plasma Flows in the Corona of EV Lac with X-ray Spectroscopy

TL;DR

This study uses high-resolution X-ray spectroscopy to detect and analyze plasma flows during stellar flares on EV Lac, revealing evidence of chromospheric evaporation and differences from solar flare dynamics.

Contribution

First detection of flare-induced plasma flows on EV Lac using Chandra X-ray spectra, providing new insights into stellar flare plasma dynamics and evaporation processes.

Findings

Detected significant Doppler shifts indicating plasma flows during flares.

Observed increase in plasma density and temperature during flares.

Identified transition from redshifts to blueshifts at temperatures above 10 MK.

Abstract

Stellar flares are characterized by sudden enhancement of electromagnetic radiation from the atmospheres of stars. Compared to their solar counterparts, our knowledge on the coronal plasma dynamics of stellar flares and their connection to coronal mass ejections (CMEs) remains very limited. With time-resolved high-resolution spectroscopic observations from the \textit{Chandra} X-ray observatory, we detected noticeable coronal plasma flows during several stellar flares on a nearby dMe star EV Lac. In the observed spectra of O~{\sc{viii}} (3 MK), Fe~{\sc{xvii}} (6 MK), Mg~{\sc{xii}} (10 MK), and Si~{\sc{xiv}} (16 MK) lines, these flare-induced upflows/downflows appear as significant Doppler shifts of several tens to \speed{130}, and the upflow velocity generally increases with temperature. Variable line ratios of the Si~{\sc{xiii}} triplet reveal that these plasma flows in most flares are accompanied by an increase of the coronal plasma density and temperature. We interpret these results as X-ray evidences for chromospheric evaporation on EV Lac. In two successive flares, the plasma flow pattern and a sharp increase of the measured coronal density are highly suggestive of explosive evaporation. The transition from redshifts to blueshifts in such an explosive evaporation occurs at a temperature of at least 10 MK, much higher than that observed in solar flares ($\sim$1 MK). However, in one flare the cool and warm upflows appear to be accompanied by a decreasing plasma density, which might be explained by a stellar filament/prominence eruption coupled to this flare. These results provide important clues to understand the coronal plasma dynamics during flares on M dwarfs.