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Session 13 - Stellar Spectra.
Display session, Wednesday, January 07
Solar magnetic activity is of special interest to solar-system physicists who study its influence on planetary atmospheres, and to stellar astronomers who study its analogs on other stars. The stars, in particular, tell us what solar activity might have been like in the Sun's youth, or its distant future; and provide insights into the fundamental stellar parameters---and underlying physical mechanisms---that drive it. I describe recent HST\, GHRS low-resolution (\sim1 Åfar-ultraviolet spectra, and archival ROSAT\/ X-ray photometry, of two G-type dwarf stars in the young open cluster \alpha Persei (t\sim50 Myr), and of the Pleiades G dwarf H II 314 (t\sim70 Myr). The young galactic cluster stars provide convenient surrogates for the hyperactive neonatal Sun, corresponding to the Hadean era when planetary surfaces were forming. I compare the new results with previous measurements of field and cluster stars of a range of activity levels, including earlier FOS spectra of \alpha Per, Pleiades, and Hyades members. The solar-type stars follow a nonlinear (\alpha=2 power law) relation in an X-ray/C IV flux-flux diagram, although the most active members of the sample fall away from the main trend, showing a saturation in X-rays. Stellar rotation is a key factor setting the activity level, presumably via the dynamo. The decline of rotation with advancing age in single stars (owing to wind-induced spindown) leads to the age--activity relation. The subcoronal f_C IV/f_bol flux ratio apparently follows a simple power law up to v_\mboxrot\sim 50 km s^-1, and only the very fast rotating \alpha Per star HE 699 appears to deviate. In contrast, saturation (away from the \alpha\sim 3 power law) occurs in the X-ray diagram at only \sim 20 km s^-1, affecting all of the \alpha Per stars, and the Pleiad H II 314. Unlike the previous FOS measurements of H II 314, there did not appear to be any dramatic flare activity during the more recent GHRS exposures. This work was supported by grant GO-06795.01-95A from STScI.
Program listing for Wednesday