New insight has been gained into stellar evolution from the breakthrough of a rare type of white dwarf star system.
Image Credit: Dr. Mark Garlick
White dwarfs are small and dense stars that appear to be the size of a planet. The dwarfs develop when a star of low mass has burnt all its fuel, thereby losing its outer layers. Occasionally called “stellar fossils,” they provide knowledge into various aspects of star formation and evolution.
A unique kind of white dwarf pulsar has been found for the second time in a study headed by the University of Warwick.
White dwarf pulsars consist of a quickly spinning, burnt-out stellar remnant known as a white dwarf, which hits its neighbor—a red dwarf—with strong beams of electrical particles and radiation.
This makes the complete system brighten and then fade considerably over regular intervals. This is a result of the powerful magnetic fields, but researchers are yet to get a clear picture of what causes them.
The “Dynamo model” is a primary theory that describes the strong magnetic fields—that white dwarf consists of dynamos (electrical generators) in their core, as does the Earth, but in a much stronger way. However, for this theory to be tested, there is a need for researchers to search for other white dwarf pulsars to notice if their predictions held.
Having been recently reported in the Nature Astronomy journal, researchers funded by the UK Science and Technology Facilities Council (STFC) explain the newly detected white dwarf pulsar. It is known to be the second time such a star system has been discovered after the breakthrough of AR Scorpii (AR Sco) in 2016.
The white dwarf pulsar had a size similar to the Earth but a mass at least as large as the Sun, which is 773 light years away from Earth and spinning 300 times faster than the planet.
This implies that a teaspoon of white dwarf material would weigh nearly 15 tons. White dwarfs start their lives at extremely hot temperatures before cooling down over billions of years, and the low temperature of J1912−4410 points to an advanced age.
The origin of magnetic fields is a big open question in many fields of astronomy, and this is particularly true for white dwarf stars. The magnetic fields in white dwarfs can be more than a million times stronger than the magnetic field of the Sun, and the dynamo model helps to explain why. The discovery of J1912−4410 provided a critical step forward in this field.
Dr. Ingrid Pelisoli, Science and Technology Facilities Council, Ernest Rutherford Research Fellow, Department of Physics, University of Warwick
Pelisoli added, “We used data from a few different surveys to find candidates, focusing on systems that had similar characteristics to AR Sco. We followed up any candidates with ULTRACAM, which detects the very fast light variations expected of white dwarf pulsars.”
Pelisoli continued, “After observing a couple dozen candidates, we found one that showed very similar light variations to AR Sco. Our follow-up campaign with other telescopes revealed that every five minutes or so, this system sent a radio and X-Ray signal in our direction.
“This confirmed that there are more white dwarf pulsars out there, 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 their old age, the white dwarfs in the pulsar system should be cool,” added Pelisoli.
Pelisoli continued, “Their companions should be close enough that the gravitational pull of the white dwarf was in the past strong enough to capture mass from the companion, and this causes them to be fast spinning. All of those predictions hold for the new pulsar found: the white dwarf is cooler than 13,000K, spins on its axis once every five minutes, and the gravitational pull of the white dwarf has a strong effect on the companion.
Pelisoli concluded, “This research is an excellent demonstration that science works–we can make predictions and put them to test, and that is how any science progresses.”
Dr. Pelisoli is one among the first group of research fellows and Ph.D. students assisted by a £3.5 million private philanthropic donation from a Warwick alumnus. One of the biggest gifts towards the study of astronomy and astrophysics in the UK, the donation is allowing the next generation of astronomers to examine the furthest reaches of the universe.
We are excited to have independently found the object in the X-Ray all-sky survey performed with SRG/eROSITA. The follow-up investigation with the ESA satellite XMM-Newton revealed the pulsations in the high-energy X-Ray regime, thus confirming the unusual nature of the new object and firmly establishing the white dwarf pulsars as a new class.
Axel Schwope, Leibniz Institute for Astrophysics Potsdam
Schwope is heading a complementary study reported as a letter in Astronomy and Astrophysics.
Journal Reference
Pelisoli, I., et al. (2023) A 5.3-min-period pulsing white dwarf in a binary detected from radio to X-Rays. Nature Astronomy. https://doi.org/10.1038/s41550-023-01995-x.