A search for microflaring activity on dMe flare stars. II. Observations of YZ Canis Minoris

We report on time-series photometric observations of the dM4.5e flare star YZ Canis Minoris obtained in 1993 November with the High Speed Photometer on board the Hubble Space Telescope. The data consist of five 30 minute time sequences with a sampling rate of 0.01 a that were taken through the F240W filter (centered at 240 nm with an 80 nm width). At these wavelengths the stellar photospheric background is small, so relatively small flares can be detected. The observations show a stellar background of 120 counts s(-1) on which are superposed 54 flare events ranging in integrated flux from 2.0 x 10(28) to 3.0. 10(30) ergs, as well as longer term variations with an amplitude of up to 50% of the average continuum intensity and timescales ranging from several minutes to hours. A statistical analysis of this background suggests that it may be composed of unresolved microflaring activity that has an energy distribution considerably steeper than that deduced for the larger flare events. This is consistent with previous observations as well as the self-organized criticality and reconnecting current sheet flare theories. These results are compared with data from the dM8e flare star CN Leonis, which was obtained earlier with the same experimental setup. CN Leo has both a smaller stellar background and a lower hare occurrence rate than YZ CMi The fact that CN Leo also has a quiescent X-ray flux that is less than 10% of the YZ CMi emission suggests a Link between chromospheric and coronal heating.