dMe flare stars are main sequence stars of spectral type M, exhibiting hydrogen and ionised calcium emission lines. They are smaller and less massive than the Sun, and their rotation period is of the order of days. They are known to be very active stars, with flare energy releases reaching up to 103 times those of the Sun in the visible and radio domains (Haisch 1989). It has been suggested that some dMe stars may exhibit cyclic variations with a period of the order of 10 years (e.g. Phillips & Hartmann 1978; Pettersen et al. 1986; Jetsu 1993; Bondar 1995), and it has been estimated that some dMe stars exhibit about 0.2 to 1.5 visible flares per hour. However, their rate of activity in radio, defined as the number of observed radio bursts per hour, is not well established, and this is a useful parameter to understand the physical processes occurring in a stellar atmosphere.
The numerous publications reporting radio bursts date to the early sixties and might have given the impression that bursts from dMe flare stars could be "easily" detected. However, one can follow, through these publications, that various observing techniques were used and improved to try to distinguish between artificial radio-frequency interference and bursts of stellar origin. This suggests that with the techniques used in the past, some artificial events might have been taken for bursts of stellar origin.
In Sect. 2, we present our critical analysis of the bursts near 21 cm published in the past, and we derive estimates on the rate of bursts one may expect. In Sect. 3, we give the rates inferred from a 200 hour observing campaign performed between 1989 and 1993, using a very reliable technique on the most sensitive single-dish radiotelescope, located in Arecibo (Puerto-Rico). In both Sects. 2 and 3, we focus our study on the bursts detected near 21 cm from the nine dMe flare stars observed during this campaign: AD Leonis, YZ Canis Minoris, YY Gemminorum, EQ Pegasi, TZ Arietis, Wolf 424, Gliese 569, V371 Orionis, and VW Coma Berenices. Some of their properties are recalled in Table 2 (click here). In Sect. 4, we compare the rates predicted from the literature with the rates found with the Arecibo campaign, and we compare them to radio surveys of open clusters and to optical flare rates. In Sect. 5, we summarize our results and conclude.