Orbital and atmospheric parameters of two wide O-type subdwarf binaries: BD−11^o162 and Feige 80

¹ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Golm, Germany
² Laboratorio LCH, Centro de Investigación en Química Sostenible (CIQSO), Universidad de Huelva 21007, Spain
e-mail: francisco.molina@ciqso.uhu.es
³ Universidad Internacional de Valencia, Spain
⁴ Astronomical Institute of the Czech Academy of Sciences, 25165 Ondřejov, Czech Republic
⁵ Astroserver.org, Fő tér 1, 8533 Malomsok, Hungary
⁶ Department of Physics, Astronomy, and Materials Science, Missouri State University, Springfield, MO 65804, USA
⁷ Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Av. Gran Bretaña 1111, Playa Ancha, Valparaíso 2360102, Chile
⁸ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

Received: 30 April 2021
Accepted: 14 October 2021

Abstract

Context. There are 23 long-period binary systems discovered to date that contain a B-type hot subdwarf 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.

Aims. 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.

Methods. 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 MS companions. Stellar evolution models (MIST) are fitted to the companion atmospheric parameters to derive masses.

Results. The orbital and atmospheric parameters have been fully solved. Masses of the companions and the sdOs were obtained. The photospheric chemical abundances of the MS stars for elements with available lines in the optical range have been derived. They match general trends expected from Galactic chemical evolution but show a depletion of yttrium in both systems and an enrichment of carbon in the BD−11^o162 MS.

Conclusions. 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^o162 is an exceptional system with a lower mass. It also shows a C overabundance, which could be caused by a higher progenitor mass. The Y depletion in the 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.