AAS Meeting #193 - Austin, Texas, January 1999
Session 100. The Quiet and Active Sun
Display, Saturday, January 9, 1999, 9:20am-4:00pm, Exhibit Hall 1

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[100.09] In Search of the True Solar Twin

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.

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