Astronomers discover Universe's hottest white dwarf

A team of German and American astronomers have discovered the hottest white dwarf in the Universe, using NASA's space-based Far-Ultraviolet Spectroscopic Explorer (FUSE).

Known as white dwarf KPD 0005+5106, it is among the hottest stars ever known with a temperature of 200,000 K at its surface.

It is so hot that its photosphere exhibits emission lines in the ultraviolet spectrum, a phenomenon that has never been seen before.

These emission features stem from extremely ionized calcium (nine-fold ionized, i.e., CaX), which is the highest ionization stage of a chemical element ever discovered in a photospheric stellar spectrum.

Stars of intermediate mass (1-8 solar masses) terminate their life as an Earth-sized white dwarf after the exhaustion of their nuclear fuel. During the transition from a nuclear-burning star to the white dwarf stage, the star becomes very hot.

Many such objects with surface temperatures around 100,000 Kelvin are known.

Since its discovery as a faint blue star in 1985, KPD 0005+5106 attracted much attention because optical spectra taken with ground-based telescopes suggested that this white dwarf is very hot.In addition, it belongs to a particular class of rare white dwarfs whose atmospheres are dominated by helium.

A detailed analysis of these spectra, combined with ultraviolet observations performed with the Hubble Space Telescope (HST), had led to the conclusion that KPD 0005+5106 has a temperature of 120,000 Kelvin, which made it the hottest member of its class.

Although theory predicted the existence of such hot white dwarfs, the star nevertheless represents a challenge to the concepts of stellar evolution because of its composition.