Astronomers have caught a white dwarf, the type of stellar remnant the sun will leave when it dies, in a rare stage of its evolution. The white dwarf orbits a "cosmic lighthouse" in the form of a neutron star spinning hundreds of times a second sweeping twin beams of radiation across the universe, a so-called millisecond pulsar.
The binary system, designated NGC 362D, serves as a rare laboratory to study what happens after a star has spent millions of years donating stellar material to a neutron star before it "dies" to birth a white dwarf. NGC 362D was discovered recently using the MeerKAT radio telescope in South Africa, with the white dwarf companion found in archival data from the Hubble Space Telescope collected between 2006 and 2016. This data also helped to reconstruct the evolutionary history of this system.
This revealed that the white dwarf in this binary system is the youngest white dwarf ever seen to be partnered with a millisecond pulsar. Additionally, this stellar ember also seems to be emitting less ultraviolet radiation than it should. This indicates that when the white dwarf stopped transferring matter to its neutron star companion around 600 million years ago, it was left in an envelope of ejected material that is now blocking UV radiation.
"What makes this system particularly interesting is the possibility of observing it at a very early stage of its evolution," team leader Greta Ettorre of the University of Bologna, Italy, said in a statement sent to Space.com. "Moreover, the attenuation of the signal that we observed in the ultraviolet could indicate the presence of material still surrounding the white dwarf."
A tale of two dead stars
The young white dwarf isn't the only dead star in this system. Like all neutron stars, the pulsar component of NGC 362D was born when a star with between eight and 12 times the mass of the sun died in a massive supernova explosion.
This sees the star's core, with a mass between one and two times that of the sun, crush down to a width of around 12 miles (20 kilometers). The resulting neutron star is therefore composed of matter so dense that if a teaspoon of it were scooped out and brought to Earth, it would weigh around 10 million tons. That is the equivalent of stacking 85,000 adult blue whales on a spoon.
There is another extreme consequence of this rapid stellar core shrinkage. Think of an ice skater here on Earth drawing in their arms to increase their rate of spin, which happens due to the conservation of angular momentum. The same happens with a newborn neutron star but on a much grander scale; some pulsars have been measured to spin in excess of 700 times per second.
This speed can be boosted in systems like NGC 362D, in which a neutron star is snatching material from a donor star because that material carries with it angular momentum. This is referred to as "pulsar recycling."
The theft of this material results in the companion star losing its outer layers, pushing it toward transforming into a white dwarf. This seems to have happened relatively recently for the white dwarf in NGC 362D.
"NGC 362D currently offers us a unique opportunity to study what happens in the immediate aftermath of the millisecond pulsar recycling process," team member and National Institute for Astrophysics (INAF) researcher Emanuele Dalessandro said. "In this context, the possible presence of material around the white dwarf is particularly interesting.
"In fact, it could produce signatures at radio wavelengths in addition to the optical, similar to those observed in systems with non-degenerate companions, and could therefore help us correctly interpret the properties of other young systems."
That means that the team's findings could be of great value in the understanding of other similar systems, especially those at these rare evolutionary stages. The results could also be of use in the continuing effort to use populations of pulsars as "cosmic clocks" in so-called "pulsar timing arrays."
"Millisecond pulsars rotate with extraordinary regularity and can be used as true natural clocks. Measuring the arrival times of their pulses with extreme precision allows us, among other things, to study gravity under extreme conditions and to carry out increasingly precise tests of general relativity," Ettorre concluded. "Understanding their surroundings and reconstructing their evolutionary history is therefore essential to fully exploit what these remarkable cosmic laboratories can tell us."
The findings were published on Wednesday (Sept. 30) in the journal Astronomy & Astrophysics Letters.
