Composite hot-subdwarf binaries -- I. The spectroscopically confirmed sdB sample

TL;DR

This study analyzes a sample of composite hot subdwarf-B binary systems to understand their atmospheric properties and binary interaction history, revealing minimal mass transfer during subdwarf formation.

Contribution

It provides the first detailed spectral analysis of nine composite sdB binaries using independent methods, enhancing understanding of binary interactions during RLOF.

Findings

Good agreement between GSSP and XTgrid codes for atmospheric parameters

Minimal mass transfer to companions during sdB formation

Estimated duration of RLOF phase based on rotational velocities

Abstract

Hot subdwarf-B (sdB) stars in long-period binaries are found to be on eccentric orbits, even though current binary-evolution theory predicts these objects to be circularised before the onset of Roche-lobe overflow (RLOF). To increase our understanding of binary interaction processes during the RLOF phase, we started a long term observing campaign to study wide sdB binaries. In this article we present a composite-binary-sdB sample, and the results of the spectral analysis of 9 such systems. The grid search in stellar parameters (GSSP) code is used to derive atmospheric parameters for the cool companions. To cross-check our results and also characterize the hot subdwarfs we used the independent XTgrid code, which employs Tlusty non-local thermodynamic equilibrium models to derive atmospheric parameters for the sdB component and Phoenix synthetic spectra for the cool companions. The independent GSSP and XTgrid codes are found to show good agreement for three test systems that have atmospheric parameters available in the literature. Based on the rotational velocity of the companions, an estimate for the mass accreted during the RLOF phase and the miminum duration of that phase is made. It is found that the mass transfer to the companion is minimal during the subdwarf formation.