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

Abstract

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 higher(4), 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) x 10(21) g and a CME kinetic energy of 5. 2(-3.6)(+27.7) x 10(34) 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.