Hot subdwarfs from the ESO Supernova Ia Progenitor Survey

T Lisker,D Homeier,D Reimers,R Napiwotzki,U Heber,Z Han,N Christlieb

doi:10.1051/0004-6361:20040232

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, [CITE]). 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 [CITE] 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 [CITE] 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.

Highlights

Studies provided evidence that subdwarf B stars are core helium burning stars with a canonical mass of M ≈ 0.5 M, and a very thin hydrogen envelope (Menv < 0.01 M ), placing them on the very hot end of the horizontal branch (HB), the so-called extreme horizontal branch (EHB)
More than 200 subdwarf B (sdB) stars have been analyzed for atmospheric parameters (e.g. Saffer et al 1994; Maxted et al 2001; Edelmann et al 2003), and this interpretation still seems to be valid in general: effective temperature and surface gravity of the observed sdBs mostly lie within the theoretically determined start and end points for core helium burning on the EHB, zero-age EHB (ZAEHB) and terminal-age EHB (TAEHB)
We have presented the results of a spectral analysis of 76 sdB stars from SPY. 24 objects show spectral signatures of a cool companion, which we investigated from optical and infrared photometry

Summary

Introduction

Studies provided evidence that subdwarf B (sdB) stars are core helium burning stars with a canonical mass of M ≈ 0.5 M , and a very thin hydrogen envelope (Menv < 0.01 M ), placing them on the very hot end of the horizontal branch (HB), the so-called extreme horizontal branch (EHB). Unlike the typical post-HB evolution, these objects do not ascend the asymptotic giant branch (AGB) after core helium exhaustion, since hydrogen is not burned continuously in a shell due to the very low envelope mass. Instead, they evolve more or less directly to the white dwarf stage. The extensive theoretical study of binary formation mechanisms of sdB stars by Han et al (2003, hereafter HPMM) shows that a huge number of sdB stars may be missing in observational surveys due to selection effects These are primarily caused by main sequence companions that outshine the sdBs (main sequence spectral type A and earlier) or appear as composite spectrum objects (main sequence type F to K)

Objectives

Findings

Discussion

Conclusion