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.

arXiv commentAccepted by Astrophysical Journal

References in corpus (48)

• Maps of Dust IR Emission for Use in Estimation of Reddening and CMBR Foregrounds
• Gaia Data Release 2. Summary of the contents and survey properties
• Measuring Reddening with SDSS Stellar Spectra and Recalibrating SFD
• Planck 2013 results. XVI. Cosmological parameters
• Validation of the new Hipparcos reduction
• Nine-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Final Maps and Results
• Gaia Data Release 2: The astrometric solution
• The Geneva-Copenhagen survey of the Solar neighbourhood: Ages, metallicities, and kinematic properties of 14,000 F and G dwarfs
• Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics
• Big Bang Nucleosynthesis: 2015
• Long-Term Stability of Planets in Binary Systems
• An absolutely calibrated effective temperature scale from the InfraRed Flux Method
• Intrinsic iron spread and a new metallicity scale for Globular Clusters
• The Ages of 55 Globular Clusters as Determined Using an Improved delta V_TO^HB Method Along with Color-Magnitude Diagram Constraints, and Their Implications for Broader Issues
• NLTE line formation of Fe in late-type stars II: 1D spectroscopic stellar parameters
• Gaia FGK Benchmark Stars - Metallicity
• Gaia FGK Benchmark Stars: Effective temperatures and surface gravities
• Synthetic Stellar Photometry - I. General considerations and new transformations for broad-band systems
• The Sirius System and its Astrophysical Puzzles: Hubble Space Telescope and Ground-Based Astrometry
• Three-dimensional mapping of the local interstellar medium with composite data
• Hubble Space Telescope Astrometry of the Procyon System
• Atomic Diffusion and Mixing in Old Stars I. VLT/FLAMES-UVES Observations of Stars in NGC 6397
• The PASTEL catalogue: 2016 version
• Abundances in the Local Region II: F, G and K Dwarfs and Subgiants
• Towards stellar effective temperatures and diameters at one per cent accuracy for future surveys
• Detailed Abundances for a Large Sample of Giant Stars in the Globular Cluster 47 Tucanae (NGC 104)
• Abundances and possible diffusion of elements in M67 stars
• 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
• Observations of Binary Stars with the Differential Speckle Survey Instrument. VI. Measures During 2014 at the Discovery Channel Telescope
• Interferometric diameters of five evolved intermediate-mass planet-hosting stars measured with PAVO at the CHARA Array
• The Gaia-ESO Survey: Evidence of atomic diffusion in M67?
• Distances to the globular clusters 47 Tucanae and NGC 362 using Gaia DR2 parallaxes
• 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
• The age of 47Tuc from self-consistent isochrone fits to colour-magnitude diagrams and the eclipsing member V69
• 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
• An Examination of Recent Transformations to the BV(RI)_C Photometric System from the Perspective of Stellar Models for Old Stars
• Chemical abundances in the multiple sub-giant branch of 47 Tucanae: insights on its faint sub-giant branch component
• 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
• The Gaia FGK benchmark stars version 2.1
• Are the orbital poles of binary stars in the solar neighbourhood anisotropically distributed?
• Cool Subdwarf Investigations III: Dynamical Masses of Low Metallicity Subdwarfs

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• The 10 parsec sample in the Gaia era
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