Separated at Birth -- When is a Solar Twin Really a Twin:
The Coronal and Chromospheric Properties of Optically Selected Solar Analogs and Twins

Edward F. Guinan

Villanova University, Villanova, PA 19085\

Manuel Guedel

Paul Scherrer Institute

Larry E. DeWarf

Villanova University

We have examined the X-ray, EUV, FUV, and UV properties of optically selected analogs of the Sun. The optical selection of solar analogs and twins comes chiefly from spectral and photometric criteria such as spectral types, B-V indices, and absolute magnitudes. In selecting stars as analogs of the Sun, known members of interacting binaries are excluded, but stars that are members of a wide (non-interacting) binary or multiple star system (such as alpha Cen A) are included. For the most part, the solar proxies in our sample have spectral types that range from about G0 V to G5 V (or have B-V indices between about +0.60 and +0.70). Several of these stars appear in lists of stars well-matched to the Sun -- including the well-known nearby ``solar twin'' candidates HD 44592 (=HR 2290), 18 Sco, 16 Cyg A & B, and alpha Cen A. When possible, the stars are compared to the energy distributions and emission fluxes of the present Sun in the X-ray, EUV, FUV, UV, and also in the microwave region in a few cases.

Stars that appear well-matched to the Sun in the optical sometimes show stronger coronal and chromospheric emission than the Sun. This enhanced coronal and chromospheric activity arises from the more rapid rotation due to younger ages. However, the study of time-dependence of solar activity using solar proxies with known ages and rotation rates provides a "time machine" to investigate and understand the past, present, and expected future behavior of the Sun and solar magnetic activity. It also puts tighter constraints on the present Sun's place among solar type stars.