Figure 1: Artist’s impression of a white dwarf.
Observations of H1504+65 and RXJ0439.8-6809 show that both white dwarfs have extreme surface compositions comprised of carbon-oxygen dominated atmospheres that are devoid of hydrogen and helium. It remains unknown as to how the hydrogen-helium envelopes of both white dwarfs can be eroded away to expose their hot carbon-oxygen interiors.
H1504+65 is estimated to have 0.68 to 1.02 times the mass of the Sun and its progenitor was probably a massive main-sequence star with 8 to 10 times the mass of the Sun. The surface temperature of H1504+65 is estimated to be 200,000 ± 20,000 K. H1504+65 is also relatively nearby, located ~2,000 light years away.
RXJ0439.8-6809 is record holder for the hottest white dwarf discovered to date and its surface temperature is estimated to be 250,000 ± 30,000 K. It has 0.73 to 1.02 times the mass of the Sun and it is the leftover core of a relatively massive star that contained several times the mass of the Sun. RXJ0439.8-6809 is located ~30,000 light years away, in the outskirts of the Milky Way galaxy.
The location of RXJ0439.8-6809 is puzzling because its progenitor star is probably too massive to have formed in the outskirts of the galaxy. As a result, the progenitor star of RXJ0439.8-6809 may once have been part of a binary system comprised of two massive stars located within the main disk of the galaxy. The progenitor star of RXJ0439.8-6809 was ejected into the outskirts of the galaxy when its companion star exploded in a supernova.
Figure 2: Positions of H1504+65 and RXJ0439.8-6809 in comparison with other white dwarfs. Werner & Rauch (2015)
Werner & Rauch (2015), “Analysis of HST/COS spectra of the bare C-O stellar core H1504+65 and a high-velocity twin in the Galactic halo”, arXiv:1509.08942 [astro-ph.SR]