A stellar flare-coronal mass ejection event revealed by X-ray plasma motions

Abstract

Coronal mass ejections (CMEs), often associated with flares<SUP>1-3</SUP>, are the most powerful magnetic phenomena occurring on the Sun. Stars show magnetic activity levels up to ten thousand times higher<SUP>4</SUP>, and CME effects on stellar physics and circumstellar environments are predicted to be substantial<SUP>5-9</SUP>. However, stellar CMEs remain observationally unexplored. Using time-resolved high-resolution X-ray spectroscopy of a stellar flare on the active star HR 9024 observed with the High Energy Transmission Grating Spectrometer onboard the Chandra X-ray Observatory space telescope, we distinctly detected Doppler shifts in S xvi, Si xiv and Mg xii lines that indicate upward and downward motions of hot plasmas (around 10-25 MK) within the flaring loop, with velocities of 100-400 km s<SUP>-1</SUP>, in agreement with a model of a flaring magnetic tube. Most notably, we also detected a later blueshift in the O viii line that reveals an upward motion, with velocity 90 ± 30 km s<SUP>-1</SUP>, of cool plasma (about 4 MK), that we ascribe to a CME coupled to the flare. From this evidence we were able to derive a CME mass of 1 .2<SUB>-0.8</SUB><SUP>+2.6</SUP>×1 0<SUP>21</SUP> g and a CME kinetic energy of 5 .2<SUB>-3.6</SUB><SUP>+27.7</SUP>×1 0<SUP>34</SUP> erg. These values provide clues in the extrapolation of the solar case to higher activity levels in other stars, suggesting that CMEs could indeed be a major cause of mass and angular momentum loss.