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White dwarfs, remnants of stars like the Sun, pack a mass equivalent to the Sun's but are as small as Earth, making up 97% of our galaxy's stars. The transformation of a star into this dense state marks the stellar lifecycle's end, rendering our galaxy a celestial necropolis.
The chemical composition of white dwarfs, especially the unexpected presence of heavy metals like silicon, magnesiu by Clarence Oxford
Los Angeles CA (SPX) May 06, 2024
White dwarfs, remnants of stars like the Sun, pack a mass equivalent to the Sun's but are as small as Earth, making up 97% of our galaxy's stars. The transformation of a star into this dense state marks the stellar lifecycle's end, rendering our galaxy a celestial necropolis.
The chemical composition of white dwarfs, especially the unexpected presence of heavy metals like silicon, magnesium, and calcium on their surfaces, has puzzled astronomers for years. The discovery challenges prevailing theories of stellar evolution.
"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, explains JILA graduate student Tatsuya Akiba. "So, you shouldn't see any metals on the surface of a white dwarf unless the white dwarf is actively eating something.
White dwarfs may accrete material from their surroundings, such as comets or asteroids, although the specifics of this process remain under investigation. This activity is central to understanding their metallic surface composition.
Recent findings published in The Astrophysical Journal Letters by Akiba, JILA Fellow and University of Colorado Boulder professor Ann-Marie Madigan, and undergraduate student Selah McIntyre, explore why these stellar remnants consume nearby planetesimals. Their computer simulations reveal a "natal kick" during formation, causing changes in motion and material dynamics around the white dwarf. In 80% of simulations, nearby comets and asteroids adopt elongated, aligned orbits, with 40% of eaten planetesimals originating from counter-rotating orbits.
The research also indicates that these elongated orbits persist, forming a coherent unit even 100 million years later, suggesting a prolonged accretion phase. "This is something I think is unique about our theory: we can explain why the accretion events are so long-lasting, states Madigan.
The continuous accretion of planetesimals explains the presence of heavy metals on white dwarf surfaces over extended periods.
The group's focus on gravitational dynamics aids their understanding of these accretion events. "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 bigger, and see how the simulation evolves and which of these asteroids and comets the white dwarf eats.
These findings not only offer insights into the behavior of white dwarfs but also into the broader dynamics of solar system formation and evolution. As Madigan asserts, most planetary systems are destined to orbit a white dwarf, with many potentially consumed by their central star.
"Planetesimals can give us insight into other solar systems and planetary compositions beyond where we live in our solar region, McIntyre adds. "White dwarfs aren't just a lens into the past. They're also kind of a lens into the future.
Research Report:Tidal Disruption of Planetesimals from an Eccentric Debris Disk Following a White Dwarf Natal Kick
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Stellar Chemistry, The Universe And All Within It