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

Coronal mass ejections (CMEs), often associated with flares^1-3, are the most powerful magnetic phenomena occurring on the Sun. Stars show magnetic activity levels up to ten thousand times higher⁴, and CME effects on stellar physics and circumstellar environments are predicted to be substantial^5-9. 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^-1, 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^-1, 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_-0.8^+2.6×1 0²¹ g and a CME kinetic energy of 5 .2_-3.6^+27.7×1 0³⁴ 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.