Statistical Photometry of Flare Stars in the Ultraviolet and Photographic Bands Provided New and Effective Physical Parameters Characterizing their Energetic Properties

Abstract

Photographic photometry by wide-field optical telescopes offers a unique opportunity for statistical studies of stellar flares especially in star clusters and associations. In the Eta Tauri fields more than 520 flare stars were discovered and approximately 1600 flare ups were recorded in this way. The majority of these objects and phenomena are connected with the famous open cluster the Pleiades but some of the flare stars found in the field are obviously non-members. Statistical investigations described below cover the large group of low-frequency Pleiades flare stars as well as the relatively high-frequency cluster-member CPFS 275, the Hyades member e star II Tau and the probably non-member flare star CPFS 91.

In order to facilitate the comparison of the active dwarf stars with different distance moduli the excess energy released by these objects in the ultraviolet and/or photographic bands during their observed flare ups was expressed in solar units. The ultraviolet or photographic flare overproduction (UFO or PFO) was computed for every single flare phenomenon in this way and the cumulative distribution of these (CUFO or CPFO) was compiled for each of the high-frequency flare stars as well as for the whole group of lazy objects. (In the latter case the distance modulus of the Pleiades cluster was chosen as the most likely value for the low-frequency flare stars involved.) The slope of the CUFO (CPFO) versus flare amplitude graphs — practically that of a regression line computed by the method of least squares — turned out to be an excellent activity parameter. Its values are tabulated for the above-named stars and spectral ranges. The comparison of these figures with others characterizing the luminosity, the mean flare frequency, the excess energy released by the object during an average flare up or the star’s cumulative ultraviolet flare overproduction during 3200 hours of photographic observations demonstrate plainly that the slope values can safely be used as quantitative flare-activity parameters. It is also evident from these studies that — with a view to flare energetics - the mean flare frequency scarcely correlate with other activity indices. On the other hand there seems to be a strong correlation between flare-activity in this sense and absolute brightness.