IUE OBSERVATIONS OF SOLAR-TYPE STARS IN THE PLEIADES AND THE HYADES

We have made the first extensive set of IUE observations of solar-type stars (spectral types F5-G5) in the Pleiades. Spectra were obtained in 1988 January and August for both the transition region (C IV 1550 angstrom; low-resolution) and chromospheric (Mg II 2800 angstrom; high-resolution) emission wavelength regions, respectively. The Pleiades solar-type stars lie at the extreme limit of what IUE can observe in very long exposures. We were able to detect Mg II fluxes for two out of three Pleiades stars and interesting C IV upper limits for two of these stars. Long-wavelength, high-resolution spectra were also obtained for previously unobserved solar-type stars in the Hyades. With the inclusion of spectra of additional Hyades stars obtained from the IUE archives, we have calculated surface fluxes and fractional luminosities for both clusters' solar-type stars. These values provide a better estimate for the Mg II saturation line for single (as opposed to contact-binary) stars. Future observations with more sensitive instruments like HST are needed to determine whether the Pleiades lie at the saturation limit or somewhat below the limit for the most active single stars. In addition, the spectra of such young stars (Pleiades approximately 10(7.8) yr; Hyades approximately 10(8.8) yr) extend the main-sequence age baseline for activity-age relations over more than two orders of magnitude and, hence, allow for further discussion of proposed evolutionary scenarios for solar-type stars.