Non-LTE spectral analyses of the lately discovered DB-gap white dwarfs from the SDSS

TL;DR

This study uses non-LTE spectral models to analyze SDSS spectra of DB-gap white dwarfs, confirming their existence in the temperature range where LTE models previously suggested they might not be present.

Contribution

It introduces non-LTE spectral analysis for DB-gap white dwarfs, providing more accurate temperature determinations and confirming their existence.

Findings

Non-LTE models confirm DB-gap white dwarfs in the 30-45 kK range.

Re-analysis with non-LTE models refines temperature estimates.

Supports the existence of hydrogen-deficient white dwarfs in the DB-gap.

Abstract

For a long time, no hydrogen-deficient white dwarfs have been known that have effective temperature between 30 kK and < 45 kK, i.e. exceeding those of DB white dwarfs and having lower ones than DO white dwarfs. Therefore, this temperature range was long known as the DB-gap. Only recently, the SDSS provided spectra of several candidate DB-gap stars. First analyses based on model spectra calculated under the assumption of local thermodynamic equilibrium (LTE) confirmed that these stars had 30 kK < Teff < 45 kK (Eisenstein et al. 2006). It has been shown for DO white dwarfs that the relaxation of LTE is necessary to account for non local effects in the atmosphere caused by the intense radiation field. Therefore, we calculated a non-LTE model grid and re-analysed the aforementioned set of SDSS spectra. Our results confirm the existence of DB-gap white dwarfs.