Activity in Flare Stars in the Solar Neighborhood

Activity of flare stars in the vicinity of the Sun is compared in terms of a set of descriptive parameters that minimize problems of bias and noise in measurement. From an examination of U-band observations of about 1000 flares obtained at the McDonald and Cerro Tololo Observatories between 1966 and 1970 we find: a) The distribution of flare amplitudes follows an exponential of the form R(U) = exp[a(U- U0)]. b) Flare durations are greater on stars of higher luminosity, confirming the result of Haro and Chavira. c) Flare durations show considerable spread; in a sample of 140 eVents (or a stellar association of 140 stars of like luminosity) there will occur on the average one event duration that differs from the mean by a factor of 10. d) Incompleteness in sampling is severe. Unique event detection is about 25 percent complete. Practical considerations limit the hope for significant improvement. e) Neither the mean duration of flares nor the dispersion in duration is variable in time. f) By accepting certain assumptions on flare incidence in the unobservable domain, up to 1 percent of stellar energy sources is expended in flare activity (assuming hydrogen recombination), irrespective of stellar luminosity. g) Other than this upper limit, no relation exists between a star's level of activity and its luminosity. Stars of comparatively low but nonzero activity are found at all luminosities. Subject heading: flare stars