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

Useless stars often called white dwarfs, have a mass just like the sun whereas being comparable in dimension to Earth. They’re widespread in our galaxy, as 97% of stars are white dwarfs. As stars attain the top of their lives, their cores collapse into the dense ball of a white dwarf, making our galaxy look like an ethereal graveyard.

Regardless of their prevalence, the chemical make-up of those stellar remnants has been a conundrum for astronomers for years. The presence of heavy steel parts—like silicon, magnesium, and calcium—on the floor of many of those compact objects is a perplexing discovery that defies our expectations of stellar conduct.

“We all know that if these heavy metals are current on the floor of the white dwarf, the white dwarf is dense sufficient that these heavy metals ought to in a short time sink towards the core,” explains JILA graduate pupil Tatsuya Akiba. “So, you should not see any metals on the floor of a white dwarf until the white dwarf is actively consuming one thing.”

Whereas white dwarfs can eat varied close by objects, similar to comets or asteroids (often called planetesimals), the intricacies of this course of have but to be totally explored. Nevertheless, this conduct might maintain the important thing to unraveling the thriller of a white dwarf’s steel composition, doubtlessly resulting in thrilling revelations about white dwarf dynamics.

In outcomes reported in a new paper in The Astrophysical Journal Letters, Akiba, together with JILA Fellow and College of Colorado Boulder Astrophysical and Planetary Sciences professor Ann-Marie Madigan and undergraduate pupil Selah McIntyre, imagine they’ve discovered a motive why these stellar zombies eat their close by planetesimals. Utilizing computer simulations, the researchers simulated the white dwarf receiving a “natal kick” throughout its formation (which has been noticed) attributable to uneven mass loss, altering its movement and the dynamics of any surrounding materials.

In 80% of their take a look at runs, the researchers noticed that, from the kick, the orbits of comets and asteroids inside a spread of 30 to 240 AU of the white dwarf (equivalent to the sun–Neptune distance and past) turned elongated and aligned. Moreover, round 40% of subsequently eaten planetesimals come from counter-rotating (retrograde) orbits.

The researchers additionally prolonged their simulations to look at the white dwarf’s dynamics after 100 million years. They discovered that the white dwarf’s close by planetesimals nonetheless had elongated orbits and moved as one coherent unit, a end result by no means seen earlier than.

“That is one thing I feel is exclusive about our concept: we are able to clarify why the accretion occasions are so long-lasting,” states Madigan. “Whereas different mechanisms might clarify an unique accretion occasion, our simulations with the kick present why it nonetheless occurs a whole bunch of tens of millions of years later.”

These outcomes clarify why the heavy metals are discovered on the floor of a white dwarf, as that white dwarf repeatedly consumes smaller objects in its path.

It is all about gravity

As Madigan’s analysis group at JILA focuses on gravitational dynamics, wanting on the gravity surrounding white dwarfs appeared like a pure focus of examine.

“Simulations assist us perceive the dynamics of various astrophysical objects,” Akiba says. “So, on this simulation, we throw a bunch of asteroids and comets across the white dwarf, which is considerably greater, and see how the simulation evolves and which of those asteroids and comets the white dwarf eats.”

The researchers hope to take their simulations to higher scales in future tasks, taking a look at how white dwarfs work together with bigger planets.

As Akiba elaborates, “Different research have steered that asteroids and comets, the small our bodies, won’t be the one supply of steel air pollution on the white dwarf’s floor. So, the white dwarfs would possibly eat one thing greater, like a planet.”

Discovering extra about solar system formation

These new findings additional reveal extra in regards to the formation of white dwarfs, which is vital in understanding how solar methods change over tens of millions of years. In addition they assist make clear the origins and future evolution of our solar system, revealing extra in regards to the chemistry concerned.

“The overwhelming majority of planets within the universe will find yourself orbiting a white dwarf,” Madigan says. “It could possibly be that fifty% of those methods get eaten by their star, together with our personal solar system. Now, we’ve a mechanism to clarify why this could occur.”

“Planetesimals can provide us perception into different solar methods and planetary compositions past the place we dwell in our solar area” McIntyre provides. “White dwarfs aren’t only a lens into the previous. They’re additionally form of a lens into the longer term.”