Sleuthing the dynamo.: II -: Hubble Space Telescope Goddard High Resolution Spectrograph observations of solar-type dwarfs in young galactic clusters

Far-ultraviolet (1150-1670 Angstrom) spectra of three solar-type dwarfs in the young galactic clusters alpha Persei and the Pleiades were obtained with the Hubble Space Telescope Goddard High-Resolution Spectrograph (G140L mode). Time series of Si IV lambda 1393 show erratic variability on timescales of tens of minutes. An X-ray/C IV flux-flux diagram displays a power-law slope of similar to 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 similar to 15 km s(-1) (P similar to 3 days), while C IV continues to rise until similar to 35-50 km s(-1) (P similar to 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.