A newly discovered star just 773 light years away belongs to one of the rarest categories in the Milky Way.

J1912-4410 is a white dwarf pulsar, a type of star so rare that only one other is known to exist in the entire galaxy. Its discovery confirms that these stars exist in a class of their own and gives us a new tool to interpret not only the evolution of stars, but also the strange signals detected throughout the Milky Way that defy conventional explanations.
The discovery seems to confirm that the magnetic field of a white dwarf is generated by an internal dynamo similar to how the Earth's liquid core generates its magnetic field, but much stronger.

"The origin of magnetic fields is a big open question in many areas of astronomy, and this is especially true for white dwarf stars," says astrophysicist Ingrid Pelisoli from the University of Warwick in the UK.
"Magnetic fields in white dwarfs can be a million times stronger than the Sun's magnetic field, and the dynamo model helps explain why. The discovery of J1912-4410 provides a critical step forward in this field."

Traditionally, pulsars are a type of dead star called neutron stars. They are the remains of massive stars, about 8 to 30 times as massive, that have run out of hydrogen fuel to fuse at their cores. The star ejects its outer material and the core, no longer supported by the outward pressure of fusion, collapses under gravity into an ultra-dense object.

In the case of a pulsar, as the neutron star spins rapidly down to the millisecond scale, beams of electromagnetic radiation produced by the rapid rotation and strong magnetic field burst from the magnetic poles. As the star spins, these beams pass through our field of view like a cosmic lighthouse, causing the star to appear to pulsate.
White dwarfs are a similar type of stellar remnant. They are the collapsed cores of dead stars below about 8 solar masses. They are less dense than neutron stars and have larger radii. As far as we knew until a few years ago, they didn't turn into pulsars either.

Then, in 2016, astronomers found the first white dwarf pulsar, AR Scorpii. AR Scorpii is a little different from a traditional pulsar. It is a white dwarf in a binary system with a red dwarf star. As it rotates, its rays pass past the red dwarf, causing it to glow at multiple wavelengths in regular, 1.97-minute time slices; the pulsations we see are not directly from the white dwarf's rays, but from the effect of those rays on its red dwarf companion.

AR Scorpii's system, however, challenges our understanding of white dwarfs and has a spin speed that is usually only achieved through a mass transfer from the red dwarf that causes the white dwarf to spin faster. However, the white dwarf's rotation speed implies a strong magnetic field, which would require a significant amount of mass to be transferred to achieve the white dwarf's dizzying rotation speed.
One possible explanation is the changes white dwarfs undergo as they cool and crystallise. It is possible that the AR Scorpii white dwarf started out without a magnetic field, allowing its spin rate to increase as it slowly steals mass from the red dwarf.
However, as the white dwarf cooled, internal density changes combined with convection as heat escaped may have initiated a dynamo. This rotating, conducting and convecting fluid converts kinetic energy into magnetic energy, which spins outward from the object as a magnetic field.

We don't really know what happens inside white dwarf stars; we know that they are incredibly dense, that the mass of the Sun is packed into a body the size of the Earth, and that only the refusal of electrons to occupy the same state below a certain critical threshold prevents it from collapsing further, but what this looks like and how it behaves is purely hypothetical. AR Scorpii could mean that the interior of a white dwarf could produce a dynamo.
But the sample size of a single star makes it impossible to confirm this, so Pelisoli and colleagues looked for more. By combing through survey data, they looked for stars with similar properties to AR Scorpii. Then they followed up on their candidates to see if they matched.
"After observing a few dozen candidates, we found one that showed light variations very similar to AR Scorpii. Our follow-up campaign with other telescopes revealed that this system sends a radio and X-ray signal towards us every five minutes," says Dr. Gonzalez.
"This confirmed that there are more white dwarf pulsars out there, as predicted by previous models."

The newly discovered J1912-4410 also fits some other features of the dynamo model. White dwarf pulsars should be relatively cold, indicating that crystallisation is occurring inside and close enough to binary companions that mass transfer may have taken place in the past to enhance the white dwarf's rotation. J1912-4410 fits these properties perfectly.
A second study, led by astrophysicist Alex Schwope of the Leibniz Institute for Astrophysics Potsdam in Germany, independently found J1912-4410 in data from the X-ray space observatory eROSITA. They concluded that the object is a white dwarf pulsar like AR Scorpii, strongly suggesting that there are more of these objects out there.

And it could help astronomers solve ongoing mysteries. For example, something near the galactic centre regularly emits radio waves in 18.18-minute pulses. It could be a white dwarf pulsar, perhaps not a binary companion because it doesn't tick all the boxes seen in AR Scorpii and J1912-4410. But this discovery gives us a new tool to understand the strange things we detect in the Milky Way galaxy.

"We are excited to have found the object independently in the X-ray all-sky survey performed with SRG/eROSITA," says Schwope. "A follow-up survey 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."
The two papers were published in Nature Astronomy and Astronomy & Astrophysics.

 

Source: https://www.sciencealert.com/