Abstract
The recent systematic surveys providing enormously massive datasets of white dwarfs show that there is still a deficit of a factor of 2.5 in the DA/non-DA ratio within the temperature range of 30 000 K eff < 45 000 K, which has been regarded as the “DB gap” meaning a range with almost no helium atmosphere white dwarfs. Since all white dwarfs have to evolve through this temperature range along almost the identical sequence on the color-magnitude diagram, this implies that most of the helium atmosphere DO stars once evolve into hydrogen atmosphere hot DA stars in the temperature range of the DB gap and then back to helium atmosphere DB stars. Possible scenarios for this chameleon-like disguises of white dwarfs with helium dominant atmospheres are described and a new class of pulsating white dwarfs, named the hot-DAV stars, is predicted from these scenarios. One pulsating DA white dwarf, being consistent with the prediction, has been discovered indeed.
Highlights
Spectroscopic observations indicate that most of the white dwarfs show only either hydrogen absorption lines or helium lines
Since all white dwarfs have to evolve through this temperature range along almost the identical sequence on the color-magnitude diagram, this implies that most of the helium atmosphere DO stars once evolve into hydrogen atmosphere hot DA stars in the temperature range of the DB gap and back to helium atmosphere DB stars
The former, constituting ∼ 85 per cent of all white dwarfs, are classified as DA white dwarfs by analogy with main sequence A stars whose visible spectra are dominated by Balmer absorption lines of hydrogen
Summary
Spectroscopic observations indicate that most of the white dwarfs show only either hydrogen absorption lines or helium lines. The majority of white dwarfs are divided in terms of spectroscopic features into two major subgroups: those showing hydrogen Balmer lines only and those showing helium lines only The former, constituting ∼ 85 per cent of all white dwarfs, are classified as DA white dwarfs by analogy with main sequence A stars whose visible spectra are dominated by Balmer absorption lines of hydrogen. The latter are classified as DB white dwarfs, and a smaller fraction as DO white dwarfs by analogy with the main sequence O and B stars whose visible spectra show strong absorption lines of ionized and neutral helium, respectively The puzzle to be solved still remains: why such chameleon-like disguises, from helium atmosphere to hydrogen atmosphere and again back to helium atmosphere, are seen?
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