Orbital and atmospheric parameters of two wide O-type subdwarf binaries: BD$-$11${}^{\mathrm{o}}$162 and Feige 80

Abstract

There are 23 long-period binary systems discovered to date that contain a B-type hot subdwarf(sdB) whose orbital parameters have been fully solved. They evolve into O-type subdwarfs (sdO) once the helium burning transitions from the core to the He shell. Their study will help constraint parameters on the formation and evolution of these binaries and explain some of their puzzling features. In this study, we aim to solve orbital and atmospheric parameters of two long-period sdO binaries and, for the first time, investigate the chemical composition of their main-sequence (MS) companions. HERMES high-resolution spectra are used to obtain radial velocities and solve their orbits. The Grid Search in Stellar Parameter code (GSSP) is used to derive the atmospheric parameters and photospheric chemical abundances of the MS companions. Stellar evolution models (MIST) are fitted to the companion atmospheric parameters to derive masses. In the bimodal period-eccentricity diagram, the orbital parameters indicate that Feige 80 matches the same correlation as the majority of the systems. The analysis suggests that Feige 80 has a canonical subdwarf mass and followed a standard formation channel. However, BD-11${}^{\mathrm{o}}$162 is an exceptional system with a lower mass. It also shows a carbon overabundance, which could be caused by a higher progenitor mass. The yttrium depletion in both MS companions could indicate the existence of a circumbinary disk in these systems' pasts. Nevertheless, a chemical analysis of a larger sample is necessary to draw strong conclusions.

24 pages, 15 figures. Accepted for publication in A&A

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