The Flare-ona of EK Draconis

EK Draconis (HD 129333: G1.5 V) is a well-known young (50 Myr) solar analog. In 2012, Hubble Space Telescope returned to EK Dra to follow up a far-ultraviolet (FUV) SNAPshot visit by Cosmic Origins Spectrograph (COS) two years earlier. The brief SNAP pointing had found surprisingly redshifted, impulsively variable subcoronal "hot-line" emission of Si IV 140 nm (T~ 80,000 K). Serendipitously, the 2012 follow-on program witnessed one of the largest FUV flares ever recorded on a sunlike star, which again displayed strong redshifts (downflows) of 30-40 km/s, even after compensating for small systematics in the COS velocity scales, uncovered through a cross-calibration by Space Telescope Imaging Spectrograph (STIS). The (now reduced, but still substantial) ~10 km/s hot-line redshifts outside the flaring interval did not vary with rotational phase, so cannot be caused by "Doppler Imaging" (bright surface patches near a receding limb). Density diagnostic O IV] 140 nm multiplet line ratios of EK Dra suggest log(Ne)~ 11, an order of magnitude larger than in low-activity solar twin Alpha Centauri A, but typical of densities inferred in large stellar soft X-ray events. The self-similar FUV hot-line profiles between the flare decay and the subsequent more quiet periods, and the unchanging but high densities, reinforce a long-standing idea that the coronae of hyperactive dwarfs are flaring all the time, in a scale-free way; a "flare-ona" if you will. In this picture, the subsonic hot-line downflows probably are a byproduct of the post-flare cooling process, something like "coronal rain" on the Sun. All in all, the new STIS/COS program documents a complex, energetic, dynamic outer atmosphere of the young sunlike star.

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