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E.F. Guinan, I. Ribas, L.E. DeWarf (Villanova University), G. Harper (CASA), M. Guedel (ETHZ)
We report on the first results of FUSE observations of solar-type (G0-5 V) stars that serve as proxies for the Sun at ages of ~100 Myr to 6.5 Gyr. The observations are carried out during 2000/01 to complete an in depth study of the evolution of the magnetic activity and subsequent FUV line emissions of a ~1 M0 star over its main sequence lifetime. The FUSE observations are an important part of a comprehensive study of the "Sun in Time" across the electromagnetic spectrum that covers the evolution over time of the Sun's chromosphere, transition region (TR), and corona. The FUSE spectrophotometry fills a critical wavelength and energy gap in the "Sun in Time" program and complement observations of the same stars in the X-ray region (corona) with ROSAT, SAX, ASCA, XMM, and Chandra, in the EUV (low corona/TR) with EUVE, and in the UV-NUV (TR and chromosphere) with IUE and HST, and at cm-radio wavelengths (non-thermal corona) with the VLA. The FUSE observations, covering 920 - 1190Å, provide diagnostics for probing the high TR and low corona not covered at other wavelengths. Also, optical photometry of the stars permits the rotation periods, activity cycles, and starspot coverages of the program stars to be determined.
The FUSE spectroscopy of the solar proxies are presented and analyzed. We infer the transition region pressures through the important density sensitive ratio of the C III 977Å/1175Ålines. In the paper we also discuss the crucial question of the influence of the young Sun's expected strong FUV emissions on the developing planetary system - particularly on the chemical evolution and erosion of early planetary atmospheres. The research is supported by NASA FUSE Grants NAG 5-8985 & NAG 5-10387 and EUVE Mini-grant SA2085-26310 which we gratefully acknowledge.