Sleuthing the Dynamo. II.Hubble Space TelescopeGoddard High Resolution Spectrograph Observations of Solar‐Type Dwarfs in Young Galactic Clusters

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Abstract
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-1 (P ~ 3 days), while C IV continues to rise until ~35-50 km s-1 (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.

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