The discovery of a rare type of white dwarf star system provides new insight into how stars evolve. The star J1912-4410 – just 773 light-years away from Earth – belongs to one of the rarest categories in the Milky Way.
There are a variety of stars in our galaxy and beyond. Our Sun is a G-type main-sequence star (G2V), informally called a yellow dwarf, though its light is actually white. However, space is full of much more exciting and dynamic stars at various stages of their lives.
What are white dwarfs?
White dwarfs are formed when low mass stars (8 to 30 times the mass of the Sun) burn off all its hydrogen fuel and loses its outer layers. They then become small, extremely dense stars, around the size of a planet. Scientists consider these “stellar fossils” important to understanding different aspects of star formation and evolution.
What are neutron stars?
When a massive star runs out of fuel and collapses, its core also collapses and crushes together every proton and electron into a neutron. “If the core of the collapsing star is between about 1 and 3 solar masses, these newly-created neutrons can stop the collapse, leaving behind a neutron star.”
What are pulsars?
Although white dwarfs are common, white dwarf pulsars are rare. In fact, they are so rare that only one other is known in the entire Milky Way galaxy.
Astronomers found the first white dwarf pulsar in 2016 and named it AR Scorpii (AR Sco). This is a white dwarf in a binary system with a red dwarf star. During AR Scorpii’s spin its beam sweeps past the red dwarf making it brighter across multiple wavelengths. Astronomers detected the effect these pulses had on the red dwarf, and this is how we knew about the presence of the first white dwarf pulsar.
Our recently discovered star, J1912-4410, spins 300 times faster than our planet, is of similar size to the Earth but with a mass at least as large as the Sun.
According to the press release: “A rare type of white dwarf pulsar has been discovered for the second time only, in research led by the University of Warwick. White dwarf pulsars include a rapidly spinning, burnt-out stellar remnant called a white dwarf, which lashes its neighbour – a red dwarf – with powerful beams of electrical particles and radiation, causing the entire system to brighten and fade dramatically over regular intervals. This is owing to strong magnetic fields, but scientists are unsure what causes them.”
“This means that a teaspoon of white dwarf material would weigh around 15 tons. White dwarfs begin 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”, says the statement.
“A key theory which explains the strong magnetic fields is the “dynamo model” – that white dwarfs have dynamos (electrical generators) in their core, as does the Earth, but much more powerful. But for this theory to be tested, scientists needed to search for other white dwarf pulsars to see if their predictions held true.”
Dr Ingrid Pelisoli, University of Warwick’s Department of Physics, 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. 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.
“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. 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. 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 in the companion.
“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.”
Read the paper here https://www.nature.com/articles/s41550-023-01995-x
Image credit: Dr Mark Garlick