Illuminating Insights into Stellar Evolution Unveiled by the Discovery of Second Fast-Spinning White Dwarf Pulsar

Fast-Spinning White Dwarf Pulsar, Only the Second of Its Kind Discovered, Offers Clues to Magnetic Field Generation and Stellar Evolution

In a significant astronomical breakthrough, astronomers have stumbled upon the second known instance of a white dwarf pulsar, an exceedingly rare type of stellar remnant. Distinguished by its rapid rotation, this celestial object emits powerful beams of charged particles and radiation towards its companion red dwarf star, resulting in a striking cyclic pattern of brightness and dimness across the entire system.

An illustration shows the evolution of a white dwarf pulsar (Image credit: Dr Mark Garlick//University of Warwick)

The mechanism behind this pulsar activity is believed to stem from robust magnetic fields, although the precise cause remains uncertain. One hypothesis proposes a model wherein white dwarfs possess dynamos akin to Earth’s core, generating a magnetosphere, albeit on a much grander scale. Detailed study of this system could help confirm this theory while providing invaluable insights into the evolution of stars.

“The origin of magnetic fields is a major enigma across various branches of astronomy, and this holds particularly true for white dwarf stars,” explained Ingrid Pelisoli, a researcher from the Department of Physics at the University of Warwick. “The magnetic fields in white dwarfs can exceed the sun’s magnetic field by more than a million times, and the dynamo model helps to elucidate this phenomenon. The discovery of J1912−4410 represents a critical advancement in this field.”

White dwarfs form when stars with masses similar to or up to seven times that of the sun exhaust their nuclear fuel, leading their cores to succumb to gravitational collapse. As the stellar cores contract, they shed their outer material, which expands to roughly 100 times the diameter of the original star—an event known as the red giant phase. Over millions of years, the core gradually cools as the dispersed outer shell dissipates, ultimately leaving behind a compact stellar remnant known as a white dwarf.

Our sun will undergo a similar process approximately 4.5 billion years from now, expanding to encompass the orbit of Mars and engulfing the inner planets, including Earth. It will ultimately culminate as a cooling white dwarf in a dying solar system.

Certain white dwarfs exist within systems that retain their dynamic nature even after the tumultuous events that create them, particularly when accompanied by a companion star. One such example is the recently discovered white dwarf pulsar named J191213.72–441045.1 (J1912–4410), following the earlier detection of AR Scorpii (AR Sco) in 2016.

Situated roughly 773 light-years away from our solar system, this white dwarf pulsar contains a mass comparable to that of the sun yet compressed within the dimensions similar to Earth’s. Remarkably, it spins at a staggering rate around 300 times faster than our own planet. Even a mere teaspoonful of material from this white dwarf would weigh approximately 15 tonnes, four times the weight of a hippopotamus. The relatively cool temperature of J1912−4410 indicates an advanced age, as white dwarfs progressively lose heat over time.

Finding a rare white dwarf pulsar 

Using data obtained from various astronomical surveys, the research team employed a systematic search to identify systems exhibiting characteristics similar to AR Sco, ultimately leading to the discovery of J1912–4410. To facilitate the detection of rapid light variations expected of white dwarf pulsars, the team utilized ULTRACAM, an ultra-fast camera capable of capturing 500 images per second. This instrumental choice proved ideal for capturing fleeting astronomical events of immense speed.

Out of the initial batch of several dozen potential candidates, only one displayed light variations highly reminiscent of AR Sco, emanating radio and X-ray signals toward Earth at regular intervals. “This confirmation validates the existence of additional white dwarf pulsars, as previously predicted by theoretical models,” noted Pelisoli.

The dynamo model had further prognostications, several of which were affirmed by the discovery of J1912–4410. The white dwarfs within the pulsar system, owing to their advanced age, were expected to possess lower temperatures. Furthermore, their companion stars should reside in close proximity, enabling the gravitational influence of the white dwarf to have once facilitated the capture of mass from the companion, resulting in rapid spinning.

Indeed, all of these predictions were corroborated by J1912–4410. The white dwarf exhibited a temperature below 23,000 degrees Fahrenheit (13,000 degrees Celsius), significantly cooler than the average temperature of white dwarfs, which typically exceeds 180,000 degrees Fahrenheit (100,000 degrees Celsius). Additionally, J1912–4410 exhibited a rotation period of approximately five minutes, while the gravitational pull of the white dwarf exerted a substantial effect on its companion star, aligning with the anticipated outcomes.

Pelisoli expressed enthusiasm for the research findings, highlighting the exemplification of scientific progress through hypothesis formulation, prediction, and experimental validation. “This research serves as a remarkable demonstration of the scientific method, showcasing how predictions can be put to the test, leading to the advancement of knowledge,” Pelisoli added.

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