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

Abstract

µ Cassiopeiae is a nearby, high-velocity, metal-poor ([Fe/H] = −0.81) visual binary. We have used highresolution 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-and space-based astrometry, we determined dynamical masses for both components of µ Cas: 0.7440 ± 0.0122 M ⊙ for the G5 V primary, and 0.1728 ± 0.0035 M ⊙ for its faint dM companion. We detect no significant perturbations in the HST astrometry due 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 re-examination 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.