X-rays from young, late-type stars: quiescent and flare state

The public ROSAT archives offer a unique possibility to study the X-ray emission of large samples of young stars in a consistent way. In this talk a systematic search for X-ray emission from pre-main sequence and zero-age main sequence stars based on pointed ROSAT PSPC observations will be presented.

The comparison of the X-ray properties of sequence and zero-age main sequence stars based on pointed ROSAT PSPC observations will be presented. The comparison of the X-ray properties of T Tauri Stars with those of the Pleiades and Hyades (as representatives of young main sequence stars) provides important information about the evolution of magnetic activity during the early stellar phases.

Next to a statistical evaluation of large X-ray flares leading to the first reliable estimate for flare rates on pre-main sequence stars, the quiescent state of TTS, Pleiades, and Hyades was subject to a detailed analysis. This involves in particular the compilation of X-ray luminosity functions and the investigation of the rotation-activity relation. Spatially overlapping ROSAT exposures are used to examine the longterm variability of the sample stars.