The Sun in Time: Identification of the 'Solar Twin'

Guinan, DeWarf, and McCook, with Villanova junior R. Hamilton and FIT student L. Seward, along with I. Ribas (Univ. de Barcelona) and M. Güdel (PSI, Switzerland) continue their ongoing investigation of the coronal (X-ray; Einstein/ROSAT), transition region (FUV; IUE/FUSE), and chromospheric (FUV-UV; IUE) emissions of single solar-type stars. By considering only main-sequence stars in a restricted range of spectral types ranging from F8 V to G8 V and stars with measured rotation periods, they have focused on the role of rotation in determining activity levels. The selection of solar-like stars significantly limits the range of variation of stellar properties. In this sample, however, there is still a wide spread of rotation rates and ages, ranging from about 1.5 to 37 days, with corresponding ages from 100 Myr to 8.5 Gyr respectively. This has given an adequate cross section of both age and rotation period, with a resulting wide range of magnetic dynamo induced activity. These stars thus constitute a test of the effect of varying rotation rates (and age) on the stellar dynamo, keeping all other stellar parameters approximately constant.

An important component of this ``Sun in Time'' project focuses on the identification of a star whose properties are most like the Sun -the much coveted solar twin. Comparison of the XUV emission fluxes of the present Sun with solar twin candidates has revealed that in addition to properties used to historically define a solar twin (spectral type, color, $T_{\rm eff}$, $M_{\rm V}$, [Fe/H], etc.); age is also crucial. Because these emissions arise primarily from the magnetic-dynamo, they vary as a function of rotation period, which is related to the stellar age. They have developed correlations between X-ray (corona), FUV (transition region), and UV (chromosphere) emissions and rotation period relations obeyed by the entire group and those of the solar twin candidates. They have also introduced a Mg2 h+k (2800 Å) ``index'' that corrects for varying continuum levels and instrumental sensitivities present in the IUE/LWP spectra. From all the available data, the closest match to our Sun in all of its properties (including age), is 18 Sco. The preliminary results of this study are being presented at the $203^{rd}$ meeting of the American Astronomical Society in Atlanta, GA.

This research is supported by grants from NSF/RUI (AST00-71260), NASA/FUSE (NAG5-12125), and the Delaware Space Grant College Consortium through the Undergraduate Summer Research Assistance program (NTG5-40024).