Photometric and spectroscopic monitoring, radial velocities and evolutionary status of the chromospherically active, close eclipsing binaries ST Centauri and V0775 Centauri

Abstract

We have combined photometric and spectroscopic observations of two very close eclipsing binary systems, ST Centauri and V775 Centauri, to determine their evolutionary state from calculations of masses and radii and other properties. Spectral types were determined and radial velocities calculated from spectra obtained with the Australian National University’s 2.3 m telescope and Wide Field Spectrograph. The spectral type of the ST Cen primary component is F8 IV and the secondary is F8-8.5 IV. The ST Cen mean masses and radii of $1.40 \pm 0.05\ \mathrm{M}_{\odot}$ and $1.38 \pm 0.10\ \mathrm{M}_{\odot}$ and $2.2 \pm 0.1\ \mathrm{R}_{\odot}$ and $2.13 \pm 0.08\ \mathrm{R}_{\odot}$ for the primary and secondary components respectively, indicate that they are close to terminal age on the Main Sequence. Although the spectra of V775 Cen appeared to be single lined, the broadening function method enabled us to determine the radial velocities of the faint secondary component as well as those of the primary star. The mean masses and radii of the V775 Cen components are $1.65 \pm 0.31\ \mathrm{M}_{\odot}$ and $0.72 \pm 0.29\ \mathrm{M}_{\odot}$ and $1.68 \pm 0.11\ \mathrm{R}_{\odot}$ and $1.24 \pm 0.08\ \mathrm{R}_{\odot}$ for components 1 and 2 respectively. Our data show that V775 Cen is a close Algol binary system, with a secondary component that has evolved beyond terminal age Main Sequence, has filled its inner critical equipotential surface and is transferring mass to the primary star. The masses and radii of both components of V775 Cen are smaller than those of other Algol binary systems. Although the primary component of V775 Cen is on the Main Sequence, its spectral type is F0/F1 class IV. Both systems displayed episodes of photospheric and chromospheric activity. This was evident in the H$\alpha$ lines of ST Cen and the Na I D lines of the components in both systems. Narrow Na I D lines between those of the binary components suggest that circumbinary gas is present.