Hot subdwarfs from the ESO Supernova Ia Progenitor Survey - I. Atmospheric parameters and cool companions of sdB stars

TL;DR

This study analyzes 76 sdB stars from the ESO SPY survey, determining their atmospheric parameters and binary companions, comparing observations with theoretical models to understand sdB formation and evolution.

Contribution

It provides detailed atmospheric parameters for a large sdB sample and tests binary evolution models against observed distributions.

Findings

Good agreement with EHB evolutionary tracks in shape, offset in luminosity

Binary formation models reproduce temperature-gravity distribution well

Models do not match the luminosity function, indicating need for theoretical improvements

Abstract

We present the analysis of a high-resolution, high-quality sample of optical spectra for 76 subdwarf B (sdB) stars from the ESO Supernova Ia Progenitor Survey (SPY). Effective temperature, surface gravity, and photospheric helium abundance are determined simultaneously by fitting the profiles of hydrogen and helium lines using synthetic spectra calculated from LTE and NLTE model atmospheres. We perform a detailed comparison of our measurements with theoretical calculations, both for single star evolution and for binary population synthesis models of close binary evolution. The luminosity evolution given by the standard EHB evolutionary tracks from Dorman et al. (1993) shows an overall agreement in shape with our observations, although a constant offset in luminosity exists. The various simulation sets for binary formation channels of sdB stars calculated by Han et al. (2003) are compared individually to our data for testing our current understanding of sdB formation processes and the physical effects involved. The best-matching sets manage to reproduce the observed sdB distribution in the temperature-gravity-plane well. However, they do not match the observed cumulative luminosity function, indicating that theoretical improvement is necessary. We also investigate composite-spectrum objects showing clear signatures of a cool companion with optical and infrared photometry. These stars have cool main sequence companions of spectral types F to K. Typical helium abundances of composite and non-composite sdB stars do not differ.