Hot White Dwarfs

Abstract

The article covers the physical properties and evolution of single white dwarfs ranging in temperature from 20,000K to 200,000 and higher, the hottest know electron-degenerate stars. After discussing the classification of their spectra, the author reviews the known properties, parameters, evolutionary state, as well as persisting and new puzzles regarding all spectroscopic subclasses of Hot White Dwarfs: the hot DA white dwarfs, the DAO white dwarfs, the PG1159 degenerates, the DO white dwarfs, the DB white dwarfs, the DBA white dwarfs, and the Hot DQ white dwarfs (an entirely new class). The most recent observational and theoretical advances are brought to bear on the topic.The spectroscopic properties of white dwarfs are determined by a host of physical processes which control and/or modify the flow of elements and, hence, surface abundances in high gravity atmospheres: convective dredge-up, mixing and dilution, accretion of gas and dust from the interstellar medium and debris disks, gravitational and thermal diffusion, radiative forces, mass loss due to wind outflow and episodic mass ejection, late nuclear shell burning and late thermal pulses, rotation, magnetic fields, and possible composition relics of prior pre-white dwarf evolutionary states. Virtually all of these processes and factors may operate in hot white dwarfs, leading to the wide variety of observed spectroscopic phenomena and spectral evolution.The basic thrust of research on hot white dwarfs is three-fold: (1) to elucidate the evolutionary links between the white dwarfs and their pre-white dwarf progenitors, whether from the asymptotic giant branch (AGB), the extended horizontal branch, stellar mergers, or binary evolution; (2) to understand the physics of the different envelope processes operating in hot white dwarfs as they cool; and (3) to disentangle and elucidate the relationships between the different spectroscopic subclasses and hybrid subclasses of hot white dwarfs as spectral evolution proceeds. This includes the source of photospheric metals, the chemical species observed, and the measured surface abundances in hot degenerates. The evolutionary significance of certain observed ion species is complicated by the role of radiative forces and weak winds in levitating and ejecting elements at temperatures >20,000 K.

Analysis of cool DO-type white dwarfs from the Sloan Digital Sky Survey data release 10

“…These can be found all along the WD cooling sequence, that is, they have 4 500 ≤ T eff ≤ 170 000 K (Sion 2011). In addition, there are the H-deficient WDs (non-DA WDs), which are usually divided into three subclasses: the DO spectral type (45 000 ≤ T eff ≤ 200 000 K), with the hot DO WDs showing strong He ii lines, whereas in the spectra of cool DO WDs He i lines can also be seen; the DB type (11 000 ≤ T eff ≤ 45 000 K) showing strong He i lines; and the DC (featureless spectra), DQ, and DZ types (T eff ≤ 11 000 K) showing traces of carbon and other metals in their spectra (Sion 2011). The overlap in temperature of the hottest DA and non-DA WDs strongly suggests the existence of a separate evolutionary channel for both classes.…”

Pulsational instability of high-luminosity H-rich pre-white dwarf star

“…Also included are the results of [6] for solar metallicity. Dots of different colors show the DAO and hot DA stars from [4] and hot DA stars from [7]. Dotted lines depict evolutionary tracks corresponding to Z = 0.0001. are no uncertainties in the pulsation stability computations due to convection.…”

Optical Modulation in the X-Ray Binary 4U 1543–624 Revisited

“…This is higher than the effective temperatures of most non-degenerate stars, but hot white dwarfs can have effective temperatures as high as 2 00 000 K, and at least 100 hydrogen-atmosphere white dwarfs have effective temperatures > 60 000 K (e.g., see the review by Sion 2011). Despite its very low mass in comparison with the typical hot white dwarf, the surface properties of the companion to 4U 1543–624 may otherwise be quite similar.…”