Analysis of helium-rich subdwarf O stars. 1: NLTE models, methods, and FITS for 21 Palomar Green survey sdOs

Thejll, Peter; Bauer, F.; Saffer, R. A.; Liebert, J.; Kunze, D.; Shipman, H. L.

doi:10.1086/174691

Cited by 28 publications

(37 citation statements)

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“…In addition deviations from local thermodynamic equilibrium (NLTE) have to be taken into account, because of their high effective temperatures, whereas the LTE assumption is reasonable for sdB stars. Therefore, only a few quantitative spectral analyses have been published (Dreizler et al 1990;Thejll et al 1994), with some conflicting results becoming apparent. The reliability of the NLTE calculation depends strongly on the quality and sophistication of the model atom and the atomic data used.…”

Section: Stellar Atmosphere Modellingmentioning

confidence: 99%

Hot subdwarfs from the ESO supernova Ia progenitor survey

Stroeer

Heber²,

Lisker

et al. 2006

A&A

157

245

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Aims. We address the origin and evolutionary status of hot subdwarf stars by studying the optical spectral properties of 58 subdwarf O (sdO) stars. Combining them with the results of our previously studied subdwarf B (sdB) stars, we aim at investigating possible evolutionary links. Methods. We analyse high-resolution (R > 18 000), high-quality optical spectra of sdO stars obtained with the ESO VLT UVES echelle spectrograph in the course of the ESO Supernova Ia Progenitor Survey (SPY). Effective temperatures, surface gravities, and photospheric helium abundances are determined simultaneously by fitting the profiles of hydrogen and helium lines using dedicated synthetic spectra calculated from an extensive grid of NLTE model atmospheres. Results. We find spectroscopic or photometric evidence for cool companions to eight sdO stars, as well as a binary consisting of two sdO stars. A clear correlation between helium abundances and the presence of carbon and/or nitrogen lines is found: below solar helium abundance, no sdO star shows C or N lines. In contrast, C and/or N lines are present in the spectra of all sdO stars with supersolar helium abundance. We thus use the solar helium abundance to divide our sample into helium-deficient and helium-enriched sdO stars. While helium-deficient sdO stars are scattered in a wide range of the T eff -log (g)-diagram, most of the helium-enriched sdO stars cluster in a narrow region at temperatures between 40 000 and 50 000 K and gravities between log g = 5.5 and 6.0.Conclusions. An evolutionary link between sdB stars and sdO stars appears plausible only for the helium-deficient sdO stars. They probably have evolved away from the extreme horizontal branch; i.e., they are the likely successors to sdB stars. In contrast, the atmospheric properties of helium-enriched sdO stars cannot be explained with canonical single-star evolutionary models. Alternative scenarios for both single-star (late hot flasher) and binary evolution (white-dwarf merger; post-RGB evolution) fail to reproduce the observed properties of helium-enriched sdO stars in detail. While we regard the post-RGB scenario as inappropriate, the white-dwarf merger and the late hot-flasher scenarios remain viable to explain the origin of helium-enriched sdO stars.

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Section: Stellar Atmosphere Modellingmentioning

confidence: 99%

Hot subdwarfs from the ESO supernova Ia progenitor survey

Stroeer

Heber²,

Lisker

et al. 2006

A&A

157

245

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“…In the case of He-rich sdO stars, Dreizler (1993) has used ionization equilibrium techniques with lineblanketed non-LTE models, revealing 44.5kK for LS Iv+10°9, substantially lower than 65kK obtained from UV continuum methods (Sch6nberner & . Temperatures of sdO stars using blanketed models (Dreizler et al 1990) are systematically 5-10kK lower than unblanketed results (Thejll et al 1994).…”

Section: Ob Dwarfs and Supergiantsmentioning

confidence: 87%

The effective temperatures of hot stars

Crowther

1997

Symp. - Int. Astron. Union

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Abstract. We review the effective temperature scale of hot (2: 10 kK) stars, including results from direct, continuum and ionization equilibrium techniques. We discuss the impact of recent developments in theoretical model atmospheres for OB subdwarfs, dwarfs and supergiants, white dwarfs and Wolf Rayet stars and present a revised Teff scale for OB stars. Direct techniques coupled with Kurucz model atmospheres allow Stromgren photometry to be used as a sensitive Teff indicator for normal stars with Teff~25 kK. Reliable Teff determinations for hotter, low surface gravity and H-deficient stars require sophisticated ionization equilibrium techniques, generally considering non-LTE and line-blanketing effects.

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“…Infrared Johnson JHK measurements, listed in Table 3 were collected from the 2MASS All-Sky Catalogue of Point Sources (Cutri et al 2003). For some of the stars there are no published y or V measurements; in such cases the V magnitude has been calculated from the B pg magnitude using the transformation equation given by Thejll et al (1994),…”

Section: Photometrymentioning

confidence: 99%

Physical parameters of helium-rich subdwarf B stars from spectral energy distributions

Ahmad

Jeffery

2003

A&A

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Abstract. We present effective temperatures and angular radii for eleven helium-rich subdwarf B stars. These are measured using spectral energy distributions from archival IUE spectra and existing optical and infrared photometry. The data have been analysed using a grid of high-gravity helium-rich LTE model atmospheres and a χ 2 -minimization procedure. The parameters derived here allow for independent verification of the parameters derived from the analysis of optical spectra.

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Analysis of helium-rich subdwarf O stars. 1: NLTE models, methods, and FITS for 21 Palomar Green survey sdOs

Cited by 28 publications

References 0 publications

Hot subdwarfs from the ESO supernova Ia progenitor survey

Hot subdwarfs from the ESO supernova Ia progenitor survey

The effective temperatures of hot stars

Physical parameters of helium-rich subdwarf B stars from spectral energy distributions

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