Thursday, December 31, 2015

The Hottest White Dwarfs in the Galaxy

White dwarfs are the dense leftover cores of stars that were not massive enough to end their lives in supernova explosions. After a white dwarf forms, it can be extremely hot, with surface temperatures exceeding ~100,000 K. H1504+65 and RXJ0439.8-6809 are two of the hottest white dwarfs known in the galaxy. Both white dwarfs are estimated to have ~0.015 ± 0.01 times the radius of the Sun, which translates to a radius of roughly 10,000 km.

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]