VLBI OBSERVATIONS OF SINGLE MAIN-SEQUENCE M-STARS

Single dMe stars have been observed at 18 cm wavelength by intercontinental very long baseline interferometry (VLBI). Here we report on two stars, EQ Peg B and AD Lee, that have been detected during flares, and one of them also at a relatively low ('quiescent') state. There are no indications that any of the stars has been spatially resolved. Upper limits on the size of the radio sources are less than 1.9 stellar diameters for flares and less than 3.7 stellar diameters for quiescence. This yields directly observed brightness temperatures beyond 2 10(10) K for flares and 2 10(9) K in quiescence. The observations suggest that the size of the closed corona is less than predicted from pressure equilibrium between plasma and magnetic field. An extent of the order of one density scale height is compatible with the observations.