Composite-Spectrum Binaries, Paper XXIII: HD 69479/80, a 90-day binary with a cool-giant primary

Abstract

HD~69479/80 is a composite-spectrum binary whose components are a late-G giant and an early-A dwarf. The orbit has a period of only 91 days (which seems short for a system containing a cool giant with a radius of $\sim$13 solar radii), and a very small, but probably significantly non-zero, eccentricity. We separated the component spectra by a procedure of spectral subtraction, using a standard single giant spectrum as a template, and found the closest match to the spectrum of the cool component to be that of 15~Cyg (G8\,III). We measured the radial velocity of the secondary component from each uncovered spectrum, solved the SB2 orbit for the system, and derived a mass ratio $m_1$\,/\,$m_2$ of 1.318. Fitting synthetic spectra to the spectra of the secondary component indicated a \teff~of 9250\,K, {\it log}\,g = 3.75, and a rotational velocity of $\sim$90\,\kms. We determined the difference in absolute magnitude, $δ$V, between the component stars to be 1.07 mag, the late-type component being the brighter; we could thence calculate radii and luminosities for both components, plot their H--R diagram positions, and fit evolutionary tracks. The best-fitting tracks indicated masses of 2.9\,\msun~for the giant and 2.2\,\msun~for the dwarf, which was fully in keeping with the mass ratio given by the SB2 orbit. The track for the dwarf star confirms that this component has begun to evolve away from the ZAMS. Fitting the corresponding isochrone to those H--R diagram positions indicated a log\,(age) of the system of approximately 8.60 Gyr since the cool star evolved from the ZAMS, which is a little younger than the ages deduced for many cool giants. We also detected the $λ$\,6707-Å~lithium line in the spectrum of the giant component, thus adding to the evidence that it is near the start of its primary ascent of the red-giant branch.