White dwarf pulsar discovered sheds light on stellar evolution

Discovery of a rare type of white dwarf star system provides new understanding of stellar evolution.

White dwarfs are small, dense stars typically the size of a planet. They form when a low-mass star burns up all its fuel and loses its outer layers. Sometimes referred to as „stellar fossils,” they provide insight into various aspects of star formation and evolution.

A rare white dwarf pulsar has been discovered for the second time in research conducted by the University of Warwick. In white dwarf pulsars, the fast-spinning, burnt-out stellar remnant called a white dwarf pelts its neighbor — a red dwarf — with powerful electrical particles and radiation, causing the entire system to brighten and dim at regular intervals. This is due to strong magnetic fields, but scientists don’t know what causes it.

A major theory explaining strong magnetic fields is the „dynamo model” – white dwarfs have dynamos (electrical generators) at their cores, similar to Earth’s, but much more powerful. But to test this theory, scientists need to look for other white dwarf pulsars to see if their predictions hold true.

Published today in the journal Nature Astronomy, scientists funded by the UK Science and Technology Facilities Council (STFC) describe the newly discovered white dwarf pulsar, J191213.72-441045.1 (J1912-4410 for short). This is the second time such a star system has been discovered, following the discovery of AR Scorpii (AR Sco) in 2016.

At 773 light-years from Earth and orbiting 300 times faster than our planet, the white dwarf pulsar is about the same size as Earth, but at least as massive as the Sun. This means that one teaspoon of white dwarf matter would weigh about 15 tonnes. White dwarfs begin their lives at extremely hot temperatures before cooling over billions of years, and J1912−4410’s low temperature indicates an advanced age.

Dr Ingrid Belisoli, STFC Ernest Rutherford Research Fellow in the Department of Physics, University of Warwick, said: “The origin of magnetic fields is a big open question in many fields of astronomy, and this is particularly true for white dwarf stars. White dwarfs’ magnetic fields can be a million times stronger than the Sun’s, and the dynamo model helps explain why. The discovery of J1912−4410 provided an important breakthrough in this field.

“We used data from a few different surveys to identify candidates, focusing on organizations with similar characteristics to AR Scow. We followed up any candidates with ultracom, which detects very fast light variations expected in white dwarf pulsars. After looking at two dozen candidates, we found one that showed light variations very similar to AR Sco. Every five minutes or so of our follow-up campaign with other telescopes, the system sent a radio and X-ray signal in our direction.

„This confirmed the abundance of white dwarf pulsars, as predicted by previous models. There were other predictions made by the dynamo model, which were confirmed by the discovery of J1912−4410. Due to aging, white dwarfs in the pulsar system should be cool. Being strong, their companions must be close enough that they spin fast. All of those predictions apply to the newly discovered pulsar: the white dwarf is cooler than 13,000K, spins on its axis every five minutes, and the white dwarf’s gravitational pull has a strong effect on the companion.

„This research is a great demonstration that science works—we can make predictions and test them, and that’s how any science progresses.”

Dr Belisoli is one of the first group of research fellows and PhD students to be supported by a £3.5 million private donation from a Warwick alumnus. One of the largest gifts to astronomy and astrophysics research in the UK, the donation will enable the next generation of astronomers to explore the far reaches of our universe.

Axel Swope of the Leibniz Institute for Astrophysics Potsdam (AIP), who leads a complementary study published as a letter in Astronomy and Astrophysics, added: “We are delighted to have independently discovered the object in the X-ray All-Sky Survey. SRG/eROSITA. A follow-up investigation with the ESA satellite XMM-Newton revealed pulsations in the high-energy X-ray regime, thus confirming the unusual nature of the new object and firmly establishing white dwarf pulsars as a new class.

Read the paper here https://www.nature.com/articles/s41550-023-01995-x