Keck/HIRES Spectroscopy of Four Candidate Solar Twins

Abstract

We use high S/N, high-resolution Keck/HIRES spectroscopy of 4 solar twin candidates (HIP 71813, 76114, 77718, 78399) from our Hipparcos-based CaII H & K survey to carry out parameter and abundance analyses of these objects. Our spectroscopic Teff estimates are some 100 K hotter than the photometric scale of the recent Geneva-Copenhagen survey; several lines of evidence suggest the photometric temperatures are too cool at solar $T_{\rm eff}$. At the same time, our abundances for the 3 solar twin candidates included in the Geneva-Copenhagen survey are in outstanding agreement with the photometric metallicities; there is no sign of the anomalously low photometric metallicities derived for some late-G UMa group and Hyades dwarfs. A first radial velocity determination is made for HIP 78399, and UVW kinematics derived for all stars. HIP 71813 appears to be a kinematic member of the Wolf 630 moving group (a structure apparently reidentified in a recent analysis of late-type Hipparcos stars), but its metallicity is 0.1 dex higher than the most recent estimate of this group's metallicity. While certainly ``solar-type'' stars, HIP 76114 and 77718 are a few percent less massive, significantly older, and metal-poor compared to the Sun; they are neither good solar twin candidates nor solar analogs providing a look at the Sun at some other point in its evolution. HIP 71813 appears to be an excellent solar analog of age 8 Gyr. Our results for HIP 78399 suggest the promise of this star as a solar twin may be equivalent to the ``closest ever solar twin'' HR 6060; follow up study of this star is encouraged.