Hubble Space Telescope Astrometry of the Metal-poor Visual Binary μ Cassiopeiae: Dynamical Masses, Helium Content, and Age

μ Cassiopeiae is a nearby, high-velocity, metal-poor ([Fe/H] =-0.81) visual binary. We have used high-resolution imaging with the Hubble Space Telescope (HST), obtained over nearly two decades, to determine the period (21.568 yr) and precise orbital elements. Combining these with published ground- A nd space-based astrometry, we determined dynamical masses for both components of μ Cas: 0.7440 ± 0.0122 M o for the G5 V primary and 0.1728 ± 0.0035 M o for its faint dM companion. We detect no significant perturbations in the HST astrometry owing to a third body in the system. The primary aim of our program was to determine, with the aid of stellar models, the helium content and age of the metal-deficient primary star, μ Cas A. Although we now have a precise mass, there remain uncertainties about other parameters, including its effective temperature. Moreover, a reexamination of archival interferometric observations leads to a suspicion that the angular diameter was overestimated by a few percent. In the absolute magnitude versus color plane, μ Cas A lies slightly cooler and more luminous than the main sequence of the globular cluster 47 Tucanae; this may imply that the star has a lower helium content, and/or is older, and/or has a higher metallicity, than the cluster. Our best estimates for the helium content and age of μ Cas A are Y = 0.255 ± 0.014 and 12.7 ± 2.7 Gyr-making μ Cas possibly the oldest star in the sky visible to the naked eye. Improved measurements of the absolute parallax of the system, the effective temperature of μ Cas A, and its angular diameter would provide tighter constraints.