HIPPARCOS Results for Solar Analogs
Giusa Cayrel de Strobel (Observatoire de Paris/Meudon) & Eileen D. Friel (NSF/Boston Univ.)


4. Spectroscopic search for solar analogs

Our spectroscopic search for solar analogs has been directed primarily toward the question of whether photometric solar analogs remain such when subjected to high resolution, high S/N spectroscopic detailed analyses and discussed with reference to internal structure models. In other words, is it possible to find, among the photometric solar analogs, stars with fundamental physical parameters -- mass or effective temperature, age, luminosity, gravity, chemical composition, including lithium-content, atmospheric velocity fields, magnetic fields, chromospheric activity -- very similar, if not identical to those of the Sun? Our spectroscopic work over the years has taken advantage of improved quality and signal-to-noise available in moving from photographic to digital detectors. Our analysis has followed common methods which have lent a uniformity and consistency to the results (Cayrel de Strobel and Bentolila 1989). And we have used a variety of independent spectral indicators to provide simultaneously the stellar temperature, metallicity, and level of chromospheric activity.

The profile in the wings of the H alpha line is used to determine Teff with internal accuracy of typically 30K. The metallicity, [Fe/H], is determined from moderate and weak Fe I and II lines in the spectral region 6650 to 6780 Å. This region also contains the Li line at 6707 Å, and a variety of weak metallic lines, which are used as excellent metal abundance indicators. And, finally, the Ca II IR triplet provides an estimate of the level of chromospheric activity and consequently gives an estimation of the age of each object studied. Note that, although we start with stars selected on photometric criteria, photometric parameters in any system must be calibrated to the fundamental quantities of Teff, log g, and metallicity before being interpreted. In addition, because of the influence the stellar chemical composition has on the observed colors, accurate metallicity parameters can be determined only if an independent, non-photometric based determination of Teff has been made (in our case from the wings of the H alpha line). At the beginning of the search, the stars selected as candidates had to satisfy 3 conditions: 1) their color index, B-V, must be contained in the interval of B-V between 0.59 and 0.69 mag., 2) they must possess a trigonometric parallax, 3) they must have undergone a high resolution detailed spectroscopic analysis.


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