When astronomers talk about the “end states” of stellar evolution, several categories come to mind: black holes, neutron stars/pulsars, and white dwarfs. What happens when a star lands in two of these states? Such is the case with a cross-genre white dwarf pulsar called J191213.72-441045.1 (J1912-4410 for short). It is part of a binary pair that includes a red dwarf star.
J1912-4410 is the size of Earth but the mass of the Sun. It’s also much cooler than the sun. It is surrounded by an incredibly strong magnetic field that contributes to its pulsar activity. It also rotates on its axis 300 times faster than Earth. In addition, it spits out some material every 5.5 minutes. This is what gives this white dwarf its “pulsar” appearance. But despite some of these properties, J1912-4410 is definitely not a neutron star. It behaves like a pulsar but looks like a white dwarf.
This newly discovered white dwarf pulsar is the second known object of its kind in the galaxy. The first, called AR Sco, was found in 2016 and is the prototypical white dwarf/M star pulsar. With a sample size of now two, astronomers can draw some useful conclusions about what drives them. These rapidly rotating, burned-out, highly magnetic stellar remnants illuminate their red dwarf companions with powerful beams of electrical particles and radiation. This causes the entire system to periodically lighten and fade dramatically. Why? Could the magnetic field play a role?
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According to Ingrid Pelisoli of the University of Warwick, it’s not clear what creates the enhanced magnetic field on a white dwarf pulsar. “The origin of magnetic fields is a big open question in many areas of astronomy, and that’s especially true for white dwarfs,” she said. “Magnetic fields in white dwarfs can be more than a million times stronger than the Sun’s magnetic field, and the dynamo model helps explain why.” The discovery of J1912-4410 was a crucial advance in the field.”
An artist’s rendering of crystallization in a white dwarf star. The two known white dwarf pulsars may have such interiors. Photo credit: Mark Garlick / University of Warwick.
White dwarf dynamos
The white dwarf dynamo model is an attempt to answer the question: How do white dwarfs get their magnetic fields? In general, white dwarfs have fields up to a million times stronger than Earth’s. Recent studies show that the engine that creates a magnetic field in a star is likely similar to the one that creates it inside our planet. Essentially, the movements of material within an object result in electrical currents that create the magnetic fields. In white dwarfs, however, it creates a much stronger field.
Astronomers believe that electric currents are caused by convection movements in the core of the white dwarf. These convection currents are caused by heat escaping from the solidifying core. Because a white dwarf is a cooling remnant of an aging star (like the Sun), its core will eventually “crystallize” as it cools. Because of their old age, the white dwarfs should be pretty cool in both the AR Sco and J1912-4410 systems. The temperature of J1912-4410 is low enough that such crystallization could (or will soon) take place. However, that doesn’t fully explain the activity these two white dwarf pulsars are showing, so they may not be quite at that stage yet.
Illustration of the origin of magnetic fields in white dwarfs in narrow binaries (read counter-clockwise). The magnetic field is created when a crystallizing white dwarf grows away from a companion star and as a result begins to spin rapidly. When the field of the white dwarf connects with the field of the secondary star, mass transport stops for a relatively short period of time. Author: Paula Zorzi
It turns out that the M-dwarf’s star companions also play a role in the action, Pelisoli said. “Their companions should be close enough that the white dwarf’s gravitational pull has historically been strong enough to capture mass from the companion, and this causes them to spin rapidly,” she noted. “All of these predictions apply to the newly found pulsar: the white dwarf is cooler than 13,000 K, spins on its axis every five minutes, and the white dwarf’s gravitational pull has a strong impact on the companion.”
Searching for candidates for the white dwarf pulsar
In a paper describing J1912-4410, Pelisoli and her team say that binary white dwarf pulsars pose a challenge to theoretical models describing white dwarfs. Basically, they wanted to understand why they are magnetically strong and bright across the spectrum. You ask: What is the nature of the dynamo that powers these strange animals? And what’s causing the emissions that make it look so “pulsar-like”?
They examined data from multiple surveys to find candidates similar to AR Sco. “After observing a few dozen candidates, we found one that had very similar light fluctuations to AR Sco,” Pelisoli said. “Our follow-up campaign with other telescopes revealed that this system was sending a radio and X-ray signal our way about every five minutes,” Pelisoli said. “This confirmed that there are more white dwarf pulsars out there, as predicted by previous models. There were other dynamo model predictions that were confirmed by the discovery of J1912-4410.”
After confirming that J1912-4410 was a white dwarf pulsar, the team asked if the “engine” (or dynamo) was the same as AR Sco’s. If it’s similar, that’s strong confirmation that the white dwarf dynamo model works.
More data required
There is still much to be done to understand these strange star beings and many questions to be answered. Does every white dwarf pulsar have exactly the same type of binary pair? Or are there subtle differences that contribute to the activity they show? At what point in its evolution is the white dwarf in such a pair? Does evolutionary level matter, especially given the white dwarf’s cooling temperature profile?
There are challenges. It is still difficult to determine the exact temperatures for white dwarf pulsars. In addition, the very periodic emission pulses that make these white dwarfs look like pulsars need to be studied more closely.
Finally, since the companions of these stellar oddities play a role in their pulsar-like activity, astronomers want to get a better idea of their spectral types and orbits. Now that astronomers have two of these strange systems to study, they want to find more of them in our galaxy and beyond.
For more informations
The discovery of the white dwarf pulsar sheds light on stellar evolution
A pulsating white dwarf with a period of 5.3 minutes in a binary system detected from radio to X-rays
Discovered: The mechanism that creates giant white dwarf magnetic fields