Discovery of hot subdwarfs with extremely high carbon and oxygen abundances

Abstract

Helium rich subdwarf O stars (sdOs) are hot compact stars in a pre-white dwarf evolutionary state. Most of them have effective temperatures and surface gravities in the range Teff = 40,000-50,000 K and log g = 5.5-6.0. Their atmospheres are helium dominated. If present at all, C, N, and O are trace elements. The abundance patterns are explained in terms of nucleosynthesis during single star evolution (late helium core flash) or a binary He-core white dwarf merger. Here we announce the discovery of two hot hydrogen-deficient sdOs (PG1654+322 and PG1528+025) that exhibit unusually strong carbon and oxygen lines. A non-LTE model atmosphere analysis of spectra obtained with the Large Binocular Telescope and by the LAMOST survey reveals astonishingly high abundances of C (~20%) and O (~20%) and that the two stars are located close to the helium main sequence. Both establish a new spectroscopic class of hot H-deficient subdwarfs (CO-sdO) and can be identified as the remnants of a He-core white dwarf that accreted matter of a merging low-mass CO-core white dwarf. We conclude that the CO-sdOs represent an alternative evolutionary channel creating PG1159 stars besides the evolution of single stars that experience a late helium-shell flash.

Accepted for publication in MNRAS Letters. This Letter is accompanied by a paper entitled "An evolutionary channel for CO-rich and pulsating He-rich subdwarfs" by Miller Bertolami et al., appearing in the same issue of MNRAS Letters and on arxiv

References in corpus (10)

• The UKIRT Infrared Deep Sky Survey (UKIDSS)
• The Zwicky Transient Facility: System Overview, Performance, and First Results
• Estimating distances from parallaxes. V: Geometric and photogeometric distances to 1.47 billion stars in Gaia Early Data Release 3
• The Zwicky Transient Facility: Data Processing, Products, and Archive
• The unWISE Catalog: Two Billion Infrared Sources from Five Years of WISE Imaging
• A selection of hot subluminous stars in the GALEX survey II. Subdwarf atmospheric parameters
• Hot subdwarf Atmospheric parameters, Kinematics, and Origins -- Based on 1587 hot subdwarf stars observed in Gaia DR2 and LAMOST DR7
• The SALT survey of helium-rich hot subdwarfs: methods, classification, and coarse analysis
• Inflation of Migrated Hot Jupiters
• Non-local thermodynamic equilibrium spectral analysis of five hot, hydrogen-deficient pre-white dwarfs

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• How an overweight and rapidly rotating PG 1159 star in the Galactic halo challenges evolutionary models
• Stars on the ascending helium giant branch I. From white dwarf merger to helium giant: the evolutionary state of the rapidly rotating hot subdwarf HE 1518-0948
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