Hungry white dwarf stars eat heavy metals

White dwarfs, the remnants of stars like our Sun but only about the size of Earth, make up 97% of our galaxy’s stars. These dead stars, dense and compact, represent a common endpoint for stellar evolution, transforming the Milky Way into a sort of celestial necropolis.

One long standing puzzle has been the composition of these stars, particularly the unexpected presence of heavy metals like silicon, magnesium, and calcium on their surfaces. This phenomenon is at odds with the typical behavior expected from such dense objects, where heavy elements should sink rapidly.

White dwarfs absorb heavy metals

“We know that if these heavy metals are present on the surface of the white dwarf, the white dwarf is dense enough that these heavy metals should very quickly sink toward the core. So, you shouldn’t see any metals on the surface of a white dwarf unless the white dwarf is actively eating something,” said lead author Tatsuya Akib, a graduate student in planetary sciences at the University of Colorado Boulder (CU Boulder).

This “eating” refers to the absorption of nearby objects such as comets or asteroids, also known as planetesimals. This process has intrigued astronomers as a potential key to understanding the metallic surface composition of white dwarfs.

“Natal kick” may be responsible

In a recent publication, the experts offer a new explanation for this behavior. They propose that a “natal kick” – a displacement during formation due to asymmetric mass loss observed in white dwarfs – might be responsible for the dynamics that lead to these celestial bodies consuming nearby planetesimals.

The team’s computer simulations showed that in 80% of scenarios, this kick resulted in the elongation and alignment of the orbits of comets and asteroids within 30 to 240 astronomical units of the white dwarf. Remarkably, about 40% of the consumed planetesimals originated from retrograde, or counter-rotating, orbits.

100 million years of simulations

Extending their simulations over 100 million years, the team observed that the elongated orbits of nearby planetesimals persisted and moved in unison, a phenomenon previously undocumented.

“This is something I think is unique about our theory: we can explain why the accretion events are so long-lasting,” explained senior author Anne-Marie Madigan, an astrophysicist at CU Boulder. “While other mechanisms may explain an original accretion event, our simulations with the kick show why it still happens hundreds of millions of years later.”

These findings suggest that the presence of heavy metals on a white dwarf’s surface can be attributed to the continuous accretion of smaller celestial bodies it encounters.

White dwarf interactions

Madigan’s team, which specializes in gravitational dynamics, explored in more detail the interactions between white dwarfs and their gravitational environments.

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

The researchers plan to expand their simulations to include interactions with larger planetary bodies, anticipating that white dwarfs may also consume larger objects like planets.

White dwarfs are a lens to the past and future

These discoveries not only provide insights into the life cycle of white dwarfs but also illuminate the broader processes of solar system evolution and the chemical complexities involved.

“The vast majority of planets in the universe will end up orbiting a white dwarf. It could be that 50% of these systems get eaten by their star, including our own solar system. Now, we have a mechanism to explain why this would happen,” Madigan said.

“Planetesimals can give us insight into other solar systems and planetary compositions beyond where we live in our solar region. White dwarfs aren’t just a lens into the past. They’re also kind of a lens into the future,” concluded co-author Sarah McIntyre, an undergraduate student at CU Boulder.

More about white dwarf stars

White dwarf stars are fascinating remnants of stars similar to our Sun, found in the final stages of their stellar life cycle. When a star has exhausted the nuclear fuel at its core, it sheds its outer layers and leaves behind a dense core, which we observe as a white dwarf.

These stars are incredibly dense and compact; despite being similar in size to Earth, they contain about as much mass as the Sun.

Cooling process

The surface of a white dwarf is characterized by its extreme temperatures, initially very hot but gradually cooling over billions of years. This cooling process is slow due to the star’s small surface area relative to its mass, which makes it less efficient at radiating heat away.

Black dwarfs

White dwarfs are typically composed of carbon and oxygen, which were generated by the fusion of helium in the star’s previous evolutionary stages. The fate of a white dwarf is to continue to cool and fade away, eventually becoming what is known as a “black dwarf,” although the universe is not old enough for any white dwarfs to have reached this stage yet.

Life cycle of stars

These stars are also significant in the study of astrophysics because they serve as one of the possible endpoints of stellar evolution and play a key role in our understanding of the life cycle of stars.

Additionally, white dwarfs are often involved in exotic phenomena such as type Ia supernovae, which occur when a white dwarf accretes matter from a companion star to the point where it undergoes a catastrophic explosion, playing a critical role in measuring cosmic distances.

The study is published in The Astrophysical Journal Letters.

Image Credit: NASA and H. Richer (University of British Columbia)

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