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

Abstract

$\mu$ 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- and space-based astrometry, we determined dynamical masses for both components of $\mu$ Cas: $0.7440\pm 0.0122M_{\odot }$ for the G5 V primary, and $0.1728\pm 0.0035M_{\odot }$ 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, $\mu$ 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, $\mu$ 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 $\mu$ Cas A are $Y=0.255\pm 0.014$ and $12.7\pm 2.7$ Gyr--making $\mu$ 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 $\mu$ Cas A, and its angular diameter would provide tighter constraints.

Accepted by Astrophysical Journal

References in corpus (18)

• Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics
• Synthetic Stellar Photometry - I. General considerations and new transformations for broad-band systems
• Three-dimensional mapping of the local interstellar medium with composite data
• Atomic Diffusion and Mixing in Old Stars I. VLT/FLAMES-UVES Observations of Stars in NGC 6397
• Abundances in the Local Region II: F, G and K Dwarfs and Subgiants
• Isochrones for Old (> 5 Gyr) Stars and Stellar Populations. I. Models for $-2.4\le$ [Fe/H] $\le +0.6$, $0.25\le Y\le 0.33$, and $-0.4\le$ [$\alpha$\Fe] $\le +0.4$
• Atomic Diffusion and Mixing in Old Stars. III. Analysis of NGC 6397 Stars under New Constraints
• Atomic Diffusion and Mixing in Old Stars V: A deeper look into the Globular Cluster NGC 6752
• Accurate effective temperatures of the metal-poor benchmark stars HD 140283, HD 122563 and HD 103095 from CHARA interferometry
• The chemical composition of red giants in 47 Tucanae I: Fundamental parameters and chemical abundance patterns
• Angular Diameters of the G Subdwarf $\mu$ Cassiopeiae A and the K Dwarfs $\sigma$ Draconis and HR 511 from Interferometric Measurements with the CHARA Array
• Differential abundances of open clusters and their tidal tails: chemical tagging and chemical homogeneity
• Constraints on the distance moduli, helium and metal abundances, and ages of globular clusters from their RR Lyrae and non-variable horizontal-branch stars. II. Multiple stellar populations in 47Tuc, M3, and M13
• The Cluster Ages Experiment (CASE). VIII. Age and Distance of the Globular Cluster 47 Tuc from the Analysis of Two Detached Eclipsing Binaries
• Sodium abundances of AGB and RGB stars in Galactic globular clusters II. Analysis and results of NGC 104, NGC 6121 and NGC 6809
• Observations of Binary Stars with the Differential Speckle Survey Instrument. VIII. Measures of Metal-Poor Stars and Triple Stars from 2015 to 2018
• Are the orbital poles of binary stars in the solar neighbourhood anisotropically distributed?
• Cool Subdwarf Investigations III: Dynamical Masses of Low Metallicity Subdwarfs

Cited by in corpus (1)

• The 10 parsec sample in the Gaia era