THE FLARE-ONA OF EK DRACONIS
- Center for Astrophysics and Space Astronomy, 389 UCB, University of Colorado, Boulder, CO 80309 (United States)
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 1400 Å (T ∼ 8 × 10{sup 4} 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{sup −1}, 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{sup −1} 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] 1400 Å multiplet line ratios of EK Dra suggest n{sub e} ∼ 10{sup 11} cm{sup −3}, an order of magnitude larger than in low-activity solar twin α 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.
- OSTI ID:
- 22520193
- Journal Information:
- Astronomical Journal (Online), Vol. 150, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 1538-3881
- Country of Publication:
- United States
- Language:
- English
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