Three new hot hydrogen-deficient pre-white dwarfs

TL;DR

This paper reports the discovery of three new hydrogen-deficient pre-white dwarfs with unique atmospheric compositions and properties, expanding understanding of stellar evolution in the late stages.

Contribution

It introduces three newly identified hydrogen-deficient pre-white dwarfs, including the first cool PG1159 in earliest pre-WD phase and a rare CO-sdO star, highlighting diverse evolutionary paths.

Findings

Discovered a cool PG1159 star with lowest known surface gravity.

Identified a rare CO-sdO star likely formed by a stellar merger.

Characterized an O(He) star with high temperature and gravity.

Abstract

We have detected three new hydrogen-deficient (H < 0.001 mass fraction) pre-white dwarfs (WDs) with helium-dominated atmospheres. The first object is a relatively cool PG1159 star (effective temperature Teff = 72,000 K) that has the lowest surface gravity of any PG1159 star known (log g = 4.8). It is a PG1159 star in the earliest pre-WD phase. The second object is a hot subdwarf O (sdO) star (Teff = 50,000 K, log g = 5.3) with high carbon and oxygen abundances. It is only the third known member of the recently established CO-sdO spectral class, which comprises stars that are thought to be formed by a merger of a disrupted low-mass CO WD with a higher-mass He WD. The third object is one of the rare stars of spectral type O(He) (Teff = 90,000 K, log g = 5.5).