A binary system of two dead stars has revealed the coldest, dimmest white dwarf discovered to date. White dwarfs are the end stage for low or intermediate mass stars such as our Sun. The outer layers of the star are evicted, leaving behind only the core.
Energy is no longer produced in this core, which causes it to start collapsing. The core will contract until all of the electrons are squashed together as close as they can possibly be. The core can’t collapse any further, and is given the term “degenerate”. The end result is a star the size of the Earth or smaller, which is gradually fading away into oblivion.
A neutron star is another type of dead star, but this time it is the remnant of a more massive star that has exploded in a supernova. A neutron star is even more dense than a white dwarf, and will typically have a radius of a mere 15 kilometres. When neutron stars spin rapidly, they are given the name pulsars, and these pulsars emit beams of radio waves like a lighthouse.
The pulsar PSR J2222-0137 was observed for over two years, allowing astronomers to deduce that it is around 900 light years from Earth, making it one of the closest pulsars to us.
They used radio telescopes to time the pulses, revealing delays in the signal caused when the pulsar passed behind an unseen companion. This delay was used to map the orbit of the pair and to estimate their masses. The result was quite surprising, as the companion was found to be 1.05 times the solar mass, which meant it was either an unusually low mass neutron star, or an extremely faint white dwarf. Either possibility is exciting, as only a handful of each type of system is known.
The orderly orbits indicated that the companion could not be another neutron star, as a second supernova explosion would have caused a more chaotic orbit. “If they were both neutron stars we would expect an ellipse,” David Kaplan tells the writer. “And in fact we see a number of neutron star-neutron star binaries, which are always ellipses.”
This mass is quite high for a white dwarf. Massive white dwarfs are rare, and only eight per cent of all known white dwarfs have a mass greater than 0.9 times the mass of the Sun. Massive white dwarfs are much smaller than low mass white dwarfs. This is because the increased pressure in the massive star causes the electrons to become more tightly packed. In addition, less than 20 neutron stars with white dwarf companions have been detected, so overall this is a very remarkable find.
Attempts to observe the white dwarf in visible and infrared light with the Southern Astrophysical Research (SOAR) telescope in Chile and the 10-metre Keck telescope in Hawaii failed, much to astronomers surprise. Despite the non-detection, it is still possible to place limits on the brightness of the white dwarf and these limits indicate that the dead star is much colder than any other that we know of. In fact, it is so cool that is likely that the carbon in the star has become crystallised, essentially making it a diamond in space.
Is it possible that this faint white dwarf could ever be observed directly? “We hope so,” says Kaplan. “We are working on some Hubble Space Telescope observations to try and pin it down.”