Absolute Properties of the Main-Sequence Eclipsing Binary Star
V885 Cygni
C. H. S. Lacy1, L. P. R. Vaz2, A. Claret3 and J. A. Sabby1
Department of Physics - University of Arkansas – Fayetteville – USA1
Departmento de Fisica - ICEx – Belo Horizonte – Brazil2
Instituto de Astrofísica de Andalucía-Granada-Spain3
Abstract
We present 4179 differential observations in the V filter measured by a robotic telescope, as well as 25 pairs of radial velocities from high-resolution spectroscopic observations
of the detached EB-type, 1.69 day period, double-lined eclipsing binary star V885 Cyg. Absolute dimensions of the components are determined with high precision (better than
1.5% in the masses and radii) for the purpose of testing various aspects of theoretical modeling. We obtain 2.005±0.029 solar masses and 2.345 ± 0.012 solar radii for the
hotter, but smaller, less massive and less luminous photometric primary (star A), and 2.234 ± 0.026 solar masses and 3.385 ± 0.026 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:
8375 ± 150 K for the primary, 8150 ± 150 K for the secondary, corresponding to spectral types of A3m and A4m, and 0.058 mag for Eb-y. The metallic-lined character of the
stars is revealed by high-resolution spectroscopy and uvby photometry. The orbits are circular, and spectral line widths give observed rotational velocities that are synchronous
with the orbital motion for both components. The components of V885 Cyg are main sequence stars with an age of about 5 x 108 years according to models.
Figure 1. Differential light curve of V885 Cyg in the V filter. The
solid curve is the fitted photometric model. The residuals from the
fit (O-C) are shown below the main graph. The dotted vertical lines
delimit the eclipse phases.
Figure 3. Upper panel: Theoretical evolutionary tracks (solid curves)
for the components of V885 Cyg 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. Lower panel: Theoretical evolution of the components’ radii
with age (solid curves). The dashed curves correspond to varying the
assumed masses by one standard deviation. The pairs of horizontal
lines correspond to the measured radii of the components (±1 standard
deviation from the fitted value). The best fit for both components
occurs at an age of about 500 million years. The solid vertical lines
correspond to the adopted range of ages (±40 million years in the
overlap region), while the vertical dash-dotted lines correspond to the
maximum possible observational error in the masses. The adopted
chemical composition was (X, Z) = (0.61, 0.03), α = 1.68 and αOV = 0.20.
Conclusions
Figure 2. Spectroscopic observations and adopted orbit for
V885 Cyg. The filled circles correspond to observations of the
photometric primary (by definition, the hotter star). Eclipse
phases are designated by the vertical dotted lines. Phase 0 is
primary eclipse.
New photometric and spectroscopic observations of the
eccentric eclipsing binary V885 Cyg 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 1.5% in the masses
and radii. V885 Cyg thus joins the elite of stars with the
best measured absolute properties. At an age of about
500 Myr according to models, the system is a relatively
old main sequence pair of stars with a high 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
orbits of V885 Cyg should be circularized, but agree
that the stars’ rotations should be synchronous.