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Six of one, half a dozen of the other

posted Mar 16, 2020, 7:22 PM by Jeremy Drake
A couple of earlier postings have discussed the stellar phenomenon known as "coronal mass ejections" (CMEs). CMEs are the result of the sudden release of magnetic energy in the outer atmosphere, or corona, of a star that flings out plasma at speeds of hundreds of kilometers per second. The first direct visible light observation of a CME was of course of one on the Sun, made on 1971 December 14 by the seventh Orbiting Solar Observatory (OSO-7) satellite.

Detecting CMEs on other stars has proved much more problematic and extremely difficult. This is an important problem because some stars can generate magnetic energy at a rate 10,000 times higher than the Sun and CMEs can have a dramatic impact on the atmospheres of exoplanets. This is especially problematic for close-in planets in the habitable zones around stars less massive than the Sun.

We recently analysed a good CME candidate event seen during an X-ray observation of the infamous "Demon Star", Algol, and have since been searching through the literature for other CME candidates. In a study lead by Smithsonian Astrophysical Observatory postdoc Sofia Moschou and published in the 2019 June 1 edition of the Astrophysical Journal, we found 12 of them. While the inferred CME masses as a function of associated flare energy generally were similar or below the extrapolated mean for solar events, the CME kinetic energies were consistently lower than the analogous solar extrapolation by average factors of about 100. These results suggest that the CMEs associated with very energetic flares on magnetically active stars are more limited in terms of the ejecta velocity than the ejecta mass, possibly because of the restraining influence of strong overlying magnetic fields and drag within the stellar wind. One glimmer of potentially good news is that the lower CME velocities present a more optimistic scenario for the effects of CME impacts on exoplanets.