A newly discovered white dwarf, which is a companion to the nearby pulsar PSR J2222-0137, is so cool that its carbon has crystallized, forming a giant diamond in space.
This image shows the position of the PSR J2222 system. Image credit: David L. Kaplan et al.
Found in 2013 by Dr Jason Boyles of West Virginia University, PSR J2222-0137 (PSR J2222 for short) was the first object in this pulsar-white dwarf system to be detected.
PSR J2222 lies only 870 light-years from Earth, which makes it one of the closest pulsars in the sky. It is likely the same age as our Milky Way Galaxy, about 11 billion years old.
The first observations revealed that the pulsar was spinning more than 30 times each 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.
By applying Einstein’s theory of relativity, Dr Boyles and his colleagues studied how the gravity of the companion warped space, causing delays in the radio signal as PSR J2222 passed behind it.
These delayed travel times helped the team determine the orientation of their orbit and the individual masses of the two objects.
PSR J2222 has a mass 1.2 times that of the Sun and the white dwarf a mass 1.05 times that of the Sun.
These data strongly indicated that the pulsar companion could not have been a second neutron star; the orbits were too orderly for a second supernova to have taken place.
Artist impression of a white dwarf star in orbit with pulsar PSR J2222. Image credit: B. Saxton / NRAO / AUI / NSF.
“It’s a really remarkable object. These things should be out there, but because they are so dim they are very hard to find,” said Prof David Kaplan from the University of Wisconsin-Milwaukee, who is the first author of a paper published in the Astrophysical Journal (arXiv.org pre-print).
Knowing its location with such high precision and how bright a white dwarf should appear at that distance, the astronomers believed they should have been able to observe it in optical and infrared light.
Remarkably, neither the Southern Astrophysical Research telescope in Chile nor the 10-m Keck telescope in Hawaii was able to detect it.
The astronomers calculated that the white dwarf would be no more than a comparatively cool 2,700 degrees Celsius.
They believe that such a cool, collapsed star would be largely crystallized carbon, not unlike a diamond.
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David L. Kaplan et al. 2014. A 1.05 M ☉ Companion to PSR J2222–0137: The Coolest Known White Dwarf? ApJ 789, 119; doi: 10.1088/0004-637X/789/2/119
