Hubble Space Telescope ultraviolet spectroscopy of the hottest known helium-rich pre-white dwarf KPD0005+5106

TL;DR

This study analyzes ultraviolet spectra of the hottest known helium-rich pre-white dwarf KPD0005+5106, refining its temperature and composition measurements, and supports its origin as a binary-white dwarf merger descendant.

Contribution

It provides a detailed atmospheric analysis of KPD0005+5106, improving temperature and abundance estimates, and confirms its evolutionary link to R Coronae Borealis stars.

Findings

Refined effective temperature to 195,000±15,000 K.

Confirmed helium-rich composition with trace metals.

Supported origin as a binary-white dwarf merger descendant.

Abstract

We present a model-atmosphere analysis of ultraviolet echelle spectra of KPD0005+5106 taken with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. The star is the hottest known pre-white dwarf (Teff = 200,000+-20,000 K, log g = 6.7+-0.3; Wassermann et al. 2010). Its atmosphere is composed of helium with trace amounts of metals. It is of the so-called O(He) spectral type that comprises very hot helium-rich pre-white dwarfs whose origin is debated. From neon and silicon ionisation balances, we derive tighter constraints on the effective temperature (195,000+-15,000 K) and improve previous abundance determinations of these elements. We confirm the idea that KPD0005+5106 is the descendant of an R Coronae Borealis (RCB) star and, hence, is the outcome of a binary-white dwarf merger. We discuss the relation of KPD0005+5106 to other O(He) and RCB stars.