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Astron. Astrophys. 338, 1057-1065 (1998)


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Ultraviolet and optical flares on Gl 866

D. Jevremovi 1, 5, C.J. Butler 1, S.A. Drake 2, D. O'Donoghue 3, 4 and F. van Wyk 4

1 Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland (djc@star.arm.ac.uk; cjb@star.arm.ac.uk)
2 Goddard Space Flight Center, NASA, Greenbelt, MD 20771, USA
3 Department of Astronomy, University of Cape Town, Rondebosch 7700, South Africa
4 South African Astronomical Observatory, P.O. Box 9, Observatory 7935, South Africa
5 Astronomical Observatory, Volgina 7, 11070 Belgrade, Yugoslavia

Received 17 February 1998 / Accepted 2 June 1998

Abstract

We present simultaneous ultraviolet and optical observations of five flares on the very late type M dwarf Gl 866 (dM5.5e). A procedure to estimate the physical parameters of the flaring plasma has been developed using a simplified model of the flare and a comparison of observed and computed Balmer decrements. With this procedure we have determined the optical thickness, electron temperature and electron density of the flaring plasma at flare maximum for three spectroscopically observed flares. The three spectroscopically observed flares cover up to 5 percent of the stellar surface and have areas similar to solar flares.

Key words: stars: activity – binaries: spectroscopic – stars: chromospheres – stars: flare – stars: late-type

Send offprint requests to: D. Jevremovi

© European Southern Observatory (ESO) 1998

Online publication: September 17, 1998

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