Astronomers discover Universe's hottest white dwarf

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Washington, Dec 13 (ANI): 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.

Theories of stellar evolution predict that the stars can be much hotter. However, the probability of catching them in such an extremely hot state is low, because this phase is rather short-lived.

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.

The measured calcium abundance (1-10 times the solar value) in combination with the helium-rich nature of its atmosphere represents a chemical surface composition that is not predicted by stellar evolution models. (ANI)

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