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L.E. DeWarf, E.F. Guinan, P. DiTuro, R. Mittal (Villanova University), M. Guedel (PSI, ETHZ), I. Ribas (U. de Barcelona)
We present an aspect of our on-going program of "The Sun in Time" that deals with identifying and studying nearby stars that closely match the Sun in their physical properties. Generally a solar twin is defined to be a star with similar properties to the Sun - such as spectral type, color, Teff, Mv, Fe/H, etc. However, in the definition adopted by us for a true solar twin, we also require the star's age to be close (±1 Gyr) to the Sun's. When possible, the optically selected solar twin candidates are compared to the energy distributions and emission fluxes of the present Sun in the X-ray through NUV wavelength regions. These emissions from the Sun arise chiefly from magnetic-dynamo activity. The magnetic activity and related coronal X-ray and chromospheric emissions vary as function of the star's rotation period, and thus age. Therefore a star's age can be estimated from its levels of magnetic activity by using age-activity relations.
At the present time, the star best matching the Sun in Mv, Teff, age, and chemical abundance [M/H] is 18 Sco (see Porto de Mello & Da Silva 1997, ApJ, 482, L89). Analysis of age-magnetic activity indicators such as Lx, CaII H+K and Mg II h+k indicate levels of magnetic activity closely matched to the Sun. These proxies of age indicate that 18 Sco to be rotating within a few days of the Sun's 25.5 day rotation period. Isochronal fits to the observed Mv and Teff were carried out and indicate an age of 4.8±0.8 Gyr with an inferred mass closely matched to the Sun, M=1.0±0.03 MSun.
This research is supported by NSF/RUI Grant AST93-15365 and NASA Grant NAG5-2160 which we gratefully acknowledge.