POSSIBLE DETECTION OF A STELLAR FLARE-GENERATED PARTICLE-BEAM IN POLARIZED-LIGHT

We present broadband linear polarization measurements of the active dK 5e flare star, BD +26-degrees-730, which show a rapid change in polari zation amplitude and direction over a two hour period. We conclude tha t impact polarization resulting from flare-generated particle beams is the most likely cause of the polarization variations. If this interpr etation is correct, this event represents the most direct evidence to date for flare-generated particle beams in a star other than the Sun. With proton beams as the most probable source, energy fluxes of the or der of 10(9-10) erg cm-2 s-1 are required to power the observed polari zation changes. Using the non-flare polarization level (likely produce d by stellar magnetic regions) and simple models, we estimate a lower limit of 7% for the inhomogeneous component of the magnetic region are a filling factor. We compare our results with solar flares and suggest future observational and theoretical efforts.