Sleuthing the dynamo: Hubble Space Telescope faint object spectrograph observations of solar-type dwarfs in young galactic clusters

We have used the Faint Object Spectrograph of the Hubble Space Telescope to record the ultraviolet emissions of solar-type [(B-V)(0) similar to 0.6 mag] dwarf stars in young galactic clusters: five in the Hyades (t similar to 600 Myr); three in the Pleiades (t similar to 70 Myr); and two in alpha Persei (t similar to 50 Myr). Despite high levels of scattered light in the G130H (1140-1606 Angstrom) spectra, the key C IV lambda 1549 blend was detected in all of the targets. The 10(5) K emission displays a dramatic decline from the youngest to the oldest stars of the sample, well correlated with rotational velocity; evidence of flaring in the most active of the G dwarfs; a wide spread in intensity among the Pleiades stars; but a small dispersion among the Hyades stars. The Mg II lambda 2800 emission is strongly enhanced in the younger stars, and the 2000 Angstrom continuum is significantly elevated in the Hyades stars compared with the Sun. The hyperactive stars of the sample show an apparent saturation of their C Iv emission at high rotational velocities, analogous to that seen in X-rays. The rotation-activity relations possibly exhibit mild curvature, in addition to the saturation, which nevertheless maps onto a simple power law in f(X)/f(bol) versus f(CIV)/f(bol). The Sun apparently tracks a similar power law over the course of its magnetic activity cycle. We discuss these results in terms of simple magnetic loop models. We show that the nonlinear power-law variation of f(CIV)/f(bol) with rotational velocity (slope similar to 1.5, contrary to the suspected linear dependence of the magnetic filling factor) very likely arises from a systematic increase in the base pressures of the dominant coronal loops compared with their counterparts on the Sun. The nonlinear X-ray/C IV power law (slope similar to 1.7) separately indicates a significant increase in the mean apex temperature of the dominant coronal loops. We argue that the distribution of loop structures might change character from inactive stars like the Sun (dominated by X-ray-bright points and Active Region loops) to the hyperactive cluster stars (dominated by interregion large-scale structures or postflare loops).