Astronomers detect possibly coolest, dimmest white dwarf

U.S. astronomers said Monday using multiple telescopes they have identified possibly the coldest, faintest white dwarf star ever detected.
The white dwarf, located in the constellation Aquarius, is so cool that its carbon has crystallized. In other words, it's like an Earth-size diamond in space.
"It's a really remarkable object," said lead author David Kaplan, a professor at the University of Wisconsin-Milwaukee. " These things should be out there, but because they are so dim they are very hard to find."
Kaplan and his colleagues found this stellar gem using the National Radio Astronomy Observatory's (NRAO) Green Bank Telescope (GBT) and Very Long Baseline Array (VLBA), as well as other observatories.
White dwarfs are the extremely dense end-states of stars like our Sun that have collapsed to form an object approximately the size of the Earth. Composed mostly of carbon and oxygen, white dwarfs slowly cool and fade over billions of years.
The object in this new study is likely the same age as the Milky Way, approximately 11 billion years old.
The path to this discovery began when the researchers first identified what they refer to as a pulsar in this location.
Pulsars are spinning neutron stars, the collapsed end state of a star many times more massive than our Sun, but only about 30 kilometers across.
The first observations revealed that the pulsar, dubbed PSR J2222-0137, was spinning more than 30 times a second and was gravitationally bound to a companion star, which was initially identified as either another neutron star or, more likely, an uncommonly cool white dwarf. The two were calculated to orbit each other once every 2.45 days.
The following observations showed that the pulsar is about 900 light-years away in the direction of the constellation Aquarius. In addition, it has a mass 1.2 times that of the Sun and the companion a mass 1.05 times that of the Sun.
The researchers said these data strongly indicated that the pulsar companion could not have been a second neutron star, but rather a white dwarf.
"Our final image should show us a companion 100 times fainter than any other white dwarf orbiting a neutron star and about 10 times fainter than any known white dwarf, but we don't see a thing, " said Bart Dunlap, a graduate student at the University of North Carolina at Chapel Hill and one of the team members.
"If there's a white dwarf there, and there almost certainly is, it must be extremely cold," Dunlap said.
The researchers calculated that the white dwarf would be no more than a comparatively cool 3,000 degrees Kelvin (2,700 degrees Celsius).
Astronomers believe that such a cool, collapsed star is composed of mostly carbon and oxygen, which means it might be largely crystallized carbon, or a big diamond.
The researchers said such cool white dwarfs are theoretically not that rare but with a low intrinsic brightness, they can be deucedly difficult to detect.
The findings were published in the Astrophysical Journal.