Beta Hydri (G2 IV): a revised age for the closest subgiant

Abstract

The secular evolution of solar-type atmospheres may be studied through comparisons of the current Sun with old solar-type stars of known age. Among the few such stars in the solar Galactic neighborhood, beta Hydri (G2 IV) stands out as a normal single star with an advanced age. Previous age determinations ( =~ 9.5 Gy) were based on the old ground-based parallax of 153 mas. The new Hipparcos value of 133.78+/- 0.51 mas implies an absolute magnitude M_V=3.43 +/- 0.01, 0.3 mag brighter than previously believed. New evolutionary calculations produce best-fit models with ages around 6.7 Gy. Although the Hipparcos data thus lead to a significant reduction of its estimated age, beta Hyi remains an old star. Based on observations made with the ESA Hipparcos astrometry satellite