1997A&A...321..803G

Result. are presented from a monitoring program looking for variations of the equivalent width and flux of Hα, and of the continuum flux, in 18 classical and 18 weak line T Tauri stars, and one Herbig Ae/Be star, on time scales down to 5 minutes. The stars were observed simultaiously for 14 hours using the multiobject-spectrograph FLAIR on the UK Schmidt Telescope. The campaign turned up two events in weak line T Tauri stars. Both events show the characteristics of flares: The increase of the Hα emission is faster than the decline, and the increase of the emission is much larger in Hα than in the continuum. The total energies radiated in Hα are 2.0±0.7x10³³erg and ≥6±2x10³² erg, or roughly 200 to 700 times larger than the largest flares on the Sun. The spectrum of one of these events shows a component of Hα which is blue-shifted by about 600km.s^–1. We interpret this component as an indication of mass loss, and estimate, if the event is a typical one, that the mass loss rate due to flares would be about 10^–13M_☉/yr for a weak line T Tauri star. We derive lower limits to the magnetic flux density of between 10 and 200G. Although a number of events have been observed in the classical T Tauri stars as well, none of these has shown the strong increase within minutes and decrease in an hour characteristic of a flare. We interpret this result as being due to either an optical thickness effect in Hα, or to the absence of optical flares in classical T Tauri stars, or simply due to failing to catch any flares in classical T Tauri stars.