1999ApJ...525..240A

Far-ultraviolet (1150-1670 Å) spectra of three solar-type dwarfs in the young galactic clusters α Persei and the Pleiades were obtained with the Hubble Space Telescope Goddard High-Resolution Spectrograph (G140L mode). Time series of Si IV λ1393 show erratic variability on timescales of tens of minutes. An X-ray/C IV flux-flux diagram displays a power-law slope of ∼2, up to the level of the most active cluster stars. These fall systematically below the extrapolation owing to X-ray ``saturation.'' In rotation-activity diagrams, the X-rays plateau above ∼15 km.s<sup>–1</sup> (P∼3 days), while C IV continues to rise until ∼35-50 km.s<sup>–1</sup> (P∼1-1.5 days). Such behavior is significant for understanding the magnetic ``dynamo'' at high rotational velocities and is relevant for predicting the ionizing environment bathing newly formed planets around low-mass main-sequence stars.