Absolute Properties of the Upper-Main-Sequence Eclipsing
Binary Star MU Cassiopeiae
C. H. S. Lacy1, A. Claret2 and J. A. Sabby1
Department of Physics - University of Arkansas – Fayetteville – USA1
Instituto de Astrofísica de Andalucía-Granada-Spain2
Abstract
We present 6151 differential observations in the V filter measured by a robotic telescope, as well as 29 pairs of radial velocities from high-resolution spectroscopic observations
of the detached EA-type, 9.65 day period, double-lined eclipsing binary star MU Cas. Absolute dimensions of the components are determined with good precision (better than
2% in the masses and radii) for the purpose of testing various aspects of theoretical modeling. We obtain 4.57±0.09 solar masses and 3.67 ± 0.04 solar radii for the hotter, but
smaller, less massive and less luminous photometric primary (star A), and 4.66 ± 0.10 solar masses and 4.19 ± 0.05 solar radii for the cooler, larger, more massive and more
luminous photometric secondary (star B). The effective temperatures and interstellar reddening of the stars are accurately determined from uvby photometry: 15100 ± 500 K for
the primary, 14750 ± 500 K for the secondary, corresponding to spectral types of B5 and B5, and 0.356 mag for Eb-y. The orbits are eccentric, and spectral line widths give
observed rotational velocities that are synchronous with the mean orbital motion for both components. The components of MU Cas are upper-main-sequence stars with an age
of about 63 x 106 years according to models.
Figure 3. Theoretical evolutionary tracks (solid curves) for
the components of MU Cas and the observed values of log g
and log Teff (shown with error bars). The dashed curves on
either side of the tracks correspond to varying the assumed
masses by one standard deviation. The best fit occurs at an
age of about 63 million years for both components.
Figure 1. Differential light curve of MU Cas in the V filter.
9-point normals are shown. The solid curve is the fitted
photometric model. Residuals from the fitted orbit have a
standard deviation of 0.005 mag.
Figure 4. Theoretical evolutionary tracks (solid curves)
for radii of the components of MU Cas. The dashed curves
on either side of the tracks correspond to varying the
assumed masses by one standard deviation. The pairs of
horizontal lines represent the range of radii actually
measured (±1 sigma). The upper pair of horizontal lines
corresponds to the cooler, more massive star which evolves
more quickly. The best fit occurs in an overlap region at a
log age of about 7.8 (63 Myr) for both components.
Conclusions
Figure 2. Spectroscopic observations and adopted orbit
for MU Cas. The open circles correspond to observations
of the photometric primary (by definition, the hotter
star). Phase 0 is primary eclipse.
New photometric and spectroscopic observations of the eccentric eclipsing
binary MU Cas combined with a reanalysis of data from the literature have
allowed us to derive definitive orbital parameters and physical properties of the
component stars. Our determinations have formal errors smaller than 2% in
the masses and radii. MU Cas thus joins the elite of stars with well- measured
absolute properties. At an age of about 100 Myr according to models, the
system is a middle-aged upper main sequence pair of stars with a higher
surface abundance of heavy elements compared to the solar composition. The
two currently favored mechanisms that describe the tidal evolution of binary
star properties disagree in their prediction of whether the rotations of the MU
Cas stars should be synchronous with their orbits, but agree that
the stars’ orbits should be eccentric.