Astronomers have detected an ancient stellar remnant that's 10 times fainter than the dimmest white dwarf ever discovered. Fortuitously orbited by a pulsar, this cold and collapsed star consists of crystallized carbon — essentially making it an Earth-sized diamond in space.
Indeed, this white dwarf would have never been discovered if it hadn't been for the pulsar that spins around it.
White dwarfs are stars in their end-state — extremely dense and compact objects that have collapsed to form an object approximately the size of Earth. Packed with carbon and oxygen, they burn at an agonizingly slow rate, requiring billions of years to fade away. In fact, this particular white dwarf is estimated to be about 11 billion years old, which is the same age as the Milky Way. On their own, these objects are incredibly difficult to detect owing to their low intrinsic brightness.
Artistic impression of an unrelated but similar duo, PSR J0348 +0432. ESO / L. Calçada.
Pulsars, on the other hand, are like signal beacons just asking to be discovered. They're rapidly spinning neutron stars — the super-dense remnants of massive stars that have exploded as supernovas. They spin like crazy, spewing powerful beams of radio waves from their poles into space.
First You Find the Pulsar
This pulsar, dubbed PSR J2222-0137, was detected by Jason Boyles of West Virginia University in Morgantown using the Green Bank Telescope (GBT). The object is about 900 light-years away and it spins more than 30 times each second.
Further analysis showed that it wasn't alone; this pulsar was gravitationally bound to something. Astronomers figured that it was another neutron star, or more likely a white dwarf. The two were calculated to orbit one another every 2.45 days.
Einstein Brought in to Help
To confirm that it was a white dwarf, the researchers applied Einstein's theory of relativity. They studied how the gravity of the companion star warped space, causing delays in the radio signal as the pulsar passed behind it. Delays in travel times allowed the astronomers to determine the precise orientation of their orbit and the individual masses of the two stars; the pulsar has a mass 1.2 times that of our Sun, while the companion has a mass 1.05 times that of the Sun.
So the pulsar's companion object couldn't possibly be a neutron star — the orbits appear far too orderly for a second supernova to have occurred.
Fascinatingly, the astronomers figured that the white dwarf should be detectable in optical and infrared light. So they tried to use the Southern Astrophysical Research (SOAR) telescope in Chile and the 10-meter Keck telescope in Hawaii to detect it, but those surveys yielded nothing. It's that dim.
"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," noted Bart Dunlap, a graduate student at the University of North Carolina at Chapel Hill. "If there's a white dwarf there, and there almost certainly is, it must be extremely cold."
And by cold they mean a comparatively cool 3,000 degrees Kelvin (2,700 degrees C), which is 5,000 times colder than our Sun, which blazes away at 15 million degrees Kelvin.
Like a Diamond in the Sky
This cool, collapsed star should largely be comprised of crystallized carbon, which is for all intents-and-purposes a diamond. Other diamond-like stars have been discovered, so this isn't the first — nor are these objects considered rare. It's just that owing to their low intrinsic brightness, they're exceptionally hard to find. Relatedly, our galaxy may also contain diamond planets — super-Earths with a mass-to-volume ratio comparable to lead.
The study appears at the Astrophysical Journal, but you can read it at the arXiv pre-print archive: "A 1.05 M⊙ Companion to PSR J2222-0137: The Coolest Known White Dwarf?".
Image: B. Saxton (NRAO/AUI/NSF).
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