On the most probable number of flare stars in the Pleiades field

In order to observe flare stars of nearby open clusters and star associations, astronomers of many countries employ the same method. This method of photographic photometry by wide-field telescopes is only partly standardized. Differences in the various steps of the method, in the fields of view of the telescopes and in sky brightness of the observatories participating in the common work can make the statistical evaluation of the results more difficult and can lead to delusive conclusions. A critical evaluation of the problematic points of the observations and reports is given. Statistical results of flare star observations in the Pleiades field made during the past two decades are summarized. It is shown that the estimated number of flare stars in the region observed as published by scientists of the Byurakan Astrophysical Observatory is too low. A model of the flare star system of the field is presented. It is composed of seven differently populated subsystems with various mean flare frequencies. The strength of this model is shown on the basis of the photographic observations of the last decade. The total number of flare stars in the Pleiades field is estimated to be higher than 2500 and the expected number of flare-up observations in order to discover all of them is given (nearly 23000). (author)