Orbital and atmospheric parameters of two O-type subdwarf binaries: BD-11°162 and Feige 80

There are 23 long-period binary systems known to date that contain a hot subdwarf whose orbitals parameters have been fully solved. All are B-type subdwarfs (sdB). They evolve into O-type subdwarfs (sdO) once the helium burning transitions from the core to the He-shell. Their study will help to constrain parameters on the formation, evolution and some puzzling features of these binaries.

In this study, we aim to solve orbital and atmospheric parameters of two long-period sdO binaries, and for the first time to take a close look at the chemical composition of their main-sequence companions.

HERMES high resolution spectra are used to obtain radial velocities and solve their orbits. The GSSP code is used to derive the atmospheric parameters and photospheric chemical abundances of the main-sequence companions. Stellar evolution models (MIST) are fitted to the companion atmospheric parameters to derive masses.

The orbital and atmospheric parameters have been fully solved. Masses of the companions and the sdOs were obtained. The photospheric chemical abundances of the main-sequence (MS) stars for elements with available lines in optical range have been derived. They match general trends expected from Galactic chemical evolution but show depletion of yttrium in both systems and an enrichment of carbon in the MS star of BD-11°162.

In the bimodal period-eccentricity diagram, the orbital parameters indicate that Feige 80 matches the same correlation as the majority of the systems, but BD-11°162 is an exceptional system. The observations of Feige 80 suggest a canonical subdwarf mass and a standard formation channel. The C overabundance in BD-11°162 could be caused by a higher progenitor mass. The Y depletion in the MS companions could indicate the existence of a circumbinary disk in past of these systems. Nevertheless, a chemical analysis of a larger sample is necessary to make strong conclusions.