Hungry, hungry white dwarfs: solving the puzzle of stellar metal pollution

Dead stars, also called white dwarfs, have a mass similar to that of the Sun, while being similar in size to Earth. They are common in our Milky Way, as 97% of stars are white dwarfs. When stars reach the end of their lives, their cores collapse into the dense ball of a white dwarf, making our Milky Way resemble an ethereal graveyard.

Despite their prevalence, the chemical composition of these stellar remnants has been a mystery to astronomers for years. The presence of heavy metal elements – such as silicon, magnesium and calcium – on the surfaces of many of these compact objects is a stunning discovery that defies our expectations of stellar behavior.

“We know that if these heavy metals are present on the surface of the white dwarf, the white dwarf is so compact that these heavy metals should sink very quickly to the core,” explains JILA student Tatsuya Akiba. “So you shouldn’t see metals on the surface of a white dwarf unless the white dwarf is actively eating something.”

Although white dwarfs can consume various nearby objects, such as comets or asteroids (known as planetesimals), the intricacies of this process have yet to be fully explored. However, this behavior could be the key to unraveling the mystery of a white dwarf’s metallic composition, potentially leading to exciting revelations about white dwarf dynamics.

In the results reported in a new article in The astrophysical diary letters, Akiba, along with JILA Fellow and University of Colorado Boulder Astrophysical and Planetary Sciences Professor Ann-Marie Madigan, and graduate student Selah McIntyre, believe they have found a reason why these stellar zombies are eating their nearby planetesimals. Using computer simulations, the researchers simulated that the white dwarf received a ‘natal kick’ during its formation (which has been observed) caused by asymmetric mass loss, which changed the motion and dynamics of all surrounding material.

In 80% of their test runs, the researchers found that the orbits of comets and asteroids within a range of 30 to 240 AU from the white dwarf (corresponding to the distance between the Sun and Neptune and beyond) became elongated and aligned from the kick. . Moreover, about 40% of the later eaten planetesimals come from counter-rotating (retrograde) orbits.

The researchers also extended their simulations to investigate the dynamics of the white dwarf after 100 million years. They found that the nearby white dwarf planetesimals still had elongated orbits and moved as one coherent unit, a result never seen before.

“This is something I think is unique about our theory: we can explain why the accretion events are so long lasting,” Madigan says. “While other mechanisms can explain an original accretion event, our simulations kick ass show why this is still happening hundreds of millions of years later.”

These results explain why the heavy metals are found on the surface of a white dwarf, as that white dwarf continually consumes smaller objects in its path.

It’s all about gravity

Because Madigan’s research group at JILA focuses on the dynamics of gravity, looking at the gravity around white dwarfs seemed a natural focus of research.

“Simulations help us understand the dynamics of different astrophysical objects,” says Akiba. “So in this simulation we throw a bunch of asteroids and comets around the white dwarf, which is significantly larger, and see how the simulation evolves and which of these asteroids and comets eats the white dwarf.”

The researchers hope to take their simulations to larger scales in future projects, looking at how white dwarfs interact with larger planets.

As Akiba explains, “Other studies have suggested that asteroids and comets, the small bodies, may not be the only source of metal pollution on the white dwarf’s surface. So the white dwarfs could be eating something bigger, like a planet.”

Discover more about the formation of solar systems

These new findings further reveal more about the formation of white dwarfs, which is important for understanding how solar systems change over millions of years. They also help shed light on the origins and future evolution of our solar system, revealing more about the chemistry involved.

“The vast majority of planets in the universe will eventually orbit a white dwarf,” Madigan says. “It could be that 50% of these systems are eaten by their stars, including our own solar system. Now we have a mechanism to explain why this would happen.”

“Planetesimals can give us insight into other solar systems and planetary compositions beyond where we live in our solar region,” McIntyre adds. “White dwarfs are not only a lens into the past, they are also a kind of lens into the future.”