Metal abundances in hot white dwarfs with signatures of a superionized wind

TL;DR

This study investigates ultra-high ionized metal lines in hot white dwarfs, suggesting they originate from a superionized wind, with ultraviolet spectral analysis revealing metal abundances consistent with radiative levitation and gravitational settling.

Contribution

It provides the first ultraviolet spectral analysis of three hot white dwarfs with ultra-high ionized metal lines, proposing a multicomponent radiatively driven wind as their origin.

Findings

Light metals have subsolar abundances, heavy metals are solar or oversolar.

Abundance patterns are consistent with gravitational settling and radiative levitation.

Ultraviolet spectra support the presence of a superionized wind.

Abstract

About a dozen hot white dwarfs with effective temperatures Teff = 65,000-120,000 K exhibit unusual absorption features in their optical spectra. These objects were tentatively identified as Rydberg lines of ultra-high excited metals in ionization stages V-X, indicating line formation in a dense environment with temperatures near one million Kelvin. Since some features show blueward extensions, it was argued that they stem from a superionized wind. A unique assignment of the lines to particular elements is not possible, although they probably stem from C, N, O, and Ne. To further investigate this phenomenon, we analyzed the ultraviolet spectra available from only three stars of this group; that is, two helium-rich white dwarfs, HE 0504-2408 and HS 0713+3958 with spectral type DO, and a hydrogen-rich white dwarf, HS 2115+1148 with spectral type DAO. We identified light metals (C, N, O, Si, P, and S) with generally subsolar abundances and heavy elements from the iron group (Cr, Mn, Fe, Co, Ni) with solar or oversolar abundance. The abundance patterns are not unusual for hot WDs and can be interpreted as the result of gravitational settling and radiative levitation of elements. As to the origin of the ultra-high ionized metals lines, we discuss the possible presence of a multicomponent radiatively driven wind that is frictionally heated.