Three new hot hydrogen-deficient pre-white dwarfs

¹ Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, Eberhard Karls Universität, Sand 1, 72076 Tübingen, Germany
² Landessternwarte Heidelberg, Zentrum für Astronomie, Ruprecht-Karls-Universität, Königstuhl 12, 69117 Heidelberg, Germany
³ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Straße 24/25, 14476 Potsdam, Germany

^★ Corresponding author; werner@astro.uni-tuebingen.de

Received: 28 November 2024
Accepted: 18 December 2024

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 T_eff = 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 (T_eff = 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) (T_eff = 90 000 K, log g = 5.5).