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

T. Lisker, U. Heber+5 more|ArXiv.org|Sep 6, 2004

Gamma-ray bursts and supernovae31 references81 citations

TL;DR

This study presents a homogeneous analysis of 76 subdwarf B (sdB) stars from the ESO Supernova Ia Progenitor Survey using high-resolution optical spectra to determine atmospheric parameters (Teff, log g, He abundance) via LTE and NLTE synthetic spectrum fitting. The results show that binary population synthesis models best reproduce the observed T_eff–log g distribution, but fail to match the luminosity function, indicating a need for improved modeling of common envelope ejection efficiency and progenitor mass distributions in sdB formation scenarios.

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.

Motivation & Objective

• To determine accurate atmospheric parameters (Teff, log g, He abundance) for a homogeneous sample of 76 sdB stars using high-resolution spectroscopy.
• To test the consistency of observed sdB properties with theoretical evolutionary tracks from single-star and binary population synthesis models.
• To investigate the presence and nature of cool companions in sdB systems through composite spectrum analysis and photometric data.
• To assess the relative contributions of different binary formation channels (e.g., RLOF, CE ejection, He-He mergers) to the observed sdB population.
• To evaluate the limitations of current theoretical models in reproducing the observed luminosity function and distribution in the T_eff–log g plane.

Proposed method

• Acquired high-resolution, high-S/N optical spectra for 76 sdB stars from the ESO SPY survey using the VLT at Paranal Observatory.
• Performed synthetic spectrum fitting using both LTE and NLTE model atmospheres to simultaneously derive Teff, log g, and helium abundance from hydrogen and helium line profiles.
• Conducted optical and infrared photometry to identify and characterize cool main-sequence companions (F–K type) in composite-spectrum objects.
• Compared observed atmospheric parameters and luminosity function with theoretical evolutionary tracks from Dorman et al. (1993) and binary population synthesis models from HPMM (2003).
• Evaluated the consistency of observed He abundances between composite and non-composite sdB stars to test for selection effects.
• Analyzed peculiar Hα profiles in four single-lined sdB stars to search for signs of stellar winds or mass loss.

Experimental results

Research questions

• RQ1 How well do standard single-star evolutionary tracks reproduce the observed distribution of sdB stars in the T_eff–log g plane?
• RQ2 Which binary formation channels (e.g., stable RLOF, common envelope ejection, He-He mergers) best reproduce the observed atmospheric parameter distribution of sdB stars?
• RQ3 To what extent do the observed luminosity functions of sdB stars match predictions from binary population synthesis models?
• RQ4 Are there systematic differences in helium abundance between sdB stars with and without cool companions, suggesting selection biases?
• RQ5 What constraints do the observed properties place on the efficiency of common envelope ejection and the initial mass distribution of progenitor systems?

Key findings

• The Dorman et al. (1993) single-star evolutionary tracks show good agreement in shape with the observed T_eff–log g distribution but exhibit a constant luminosity offset.
• Binary population synthesis models from HPMM (2003) best reproduce the observed T_eff–log g distribution, particularly when considering common envelope ejection and RLOF channels.
• The best-matching simulation sets fail to reproduce the observed cumulative luminosity function, especially for models assuming 100% common envelope ejection efficiency (sets 3, 9, 12).
• There is evidence against an uncorrelated initial mass distribution of progenitor systems, as sets 4, 5, and 6 are rejected by the data.
• No significant difference in helium abundance is found between composite and non-composite sdB stars, indicating no strong selection bias in He abundance measurements.
• Four single-lined sdB stars show peculiar Hα profiles, possibly indicating mass loss or stellar wind activity, though no definitive conclusion is drawn.