Discovery of hot subdwarfs with extremely high carbon and oxygen abundances

TL;DR

This paper reports the discovery of two hot subdwarfs with extremely high carbon and oxygen abundances, revealing a new class of H-deficient stars and suggesting an alternative white dwarf merger evolutionary pathway.

Contribution

It introduces a new spectroscopic class of hot H-deficient subdwarfs (CO-sdO) with high C and O abundances, expanding understanding of stellar evolution channels.

Findings

Discovered two CO-rich hot subdwarfs (PG1654+322 and PG1528+025).

Spectral analysis shows ~20% C and O abundances in these stars.

Proposes a new evolutionary pathway involving white dwarf mergers.

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.