SIMULTANEOUS EXTREME-ULTRAVIOLET EXPLORER AND OPTICAL OBSERVATIONS OFAD LEONIS - EVIDENCE FOR LARGE CORONAL LOOPS AND THE NEUPERT EFFECT IN STELLAR FLARES

We report on the first simultaneous Extreme-Ultraviolet Explorer (EUVE ) and optical observations of flares on the dMe flare star AD Leonis. The data show the following features: 1. Two flares (one large and one of moderate size) of several hours duration were observed in the EUV wavelength range; 2. Flare emission observed in the optical precedes t he emission seen with EUVE; 3. Several diminutions (DIMs) in the optic al continuum were observed during the period of optical flare activity . To interpret these data, we develop a technique for deriving the cor onal loop length from the observed rise and decay behavior of the EUV flare. The technique is generally applicable to existing and future co ronal observations of stellar flares. We also determine the pressure, column depth, emission measure, loop cross-sectional area, and peak th ermal energy during the two EUV flares, and the temperature, area cove rage, and energy of the optical continuum emission. When the optical a nd coronal data are combined, we find convincing evidence of a stellar ''Neupert effect'' which is a strong signature of chromospheric evapo ration models. We then argue that the known spatial correlation of whi te-light emission with hard X-ray emission in solar flares, and the id entification of the hard X-ray emission with nonthermal bremsstrahlung produced by accelerated electrons, provides evidence that flare heati ng on dMe stars is produced by the same electron precipitation mechani sm that is inferred to occur on the Sun. We provide a thorough picture of the physical processes that are operative during the largest EUV f lare, compare and contrast this picture with the canonical solar flare model, and conclude that the coronal loop length may be the most impo rtant factor in determining the flare rise time and energetics.