The white dwarfs reach speeds of almost 4x needed to escape the Milky Way’s gravitational pull.
Researchers at the Technion-Israeli Institute of Technology recently discovered the origin of some of the fastest stars ever observed, some found in our very own galaxy: hypervelocity white dwarf stars.
White dwarfs are the extremely hot and dense earth sized core of a star left behind when the star starts to die. Hypervelocity white dwarfs are the term for when these stars blast through the cosmos at incredibly high speeds, though the reason why this occurred was unknown until now.
The findings of this study were published in the peer-reviewed academic journal Nature Astronomy.
How did they do it?
Led by Dr. Hila Glanz of the Technion, an international team performed three dimensional simulations of merging two white dwarfs.
They simulated a merger of two rare helium-carbon-oxygen white dwarfs (HeCO WDs) through a hydrodynamic situation. This simulates sub-atomic particles, the invisible dark matter that makes up ~86% of the universe mass, and the ways it interacts with the modelled stars.
Artist’s impression of two white dwarf stars destined to merge and create a Type Ia supernova. (credit: European Southern Observatory/Flickr)
They demonstrated that hypervelocity white dwarfs (HVWDs) can be produced by two HeCO WDs crashing into each other. The simulations led to the discovery of a dramatic sequence of explosions causing the smaller star to be flung away fast enough to escape the gravitational pull of the Milky Way.
As the smaller, secondary white dwarf comes towards the larger, primary white dwarf it gets deformed. It crashes in to the primary star, triggering an explosion in the primary white dwarf’s shell, leading to a second explosion in its core.
This causes the primary HeCO WD to become a type-1a supernova and propel the core of the secondary white dwarf away at speeds of >2000 kilometers per second – approximately 4x faster than the Milky Way’s escape velocity, creating the HVWD.
“This is the first time we’ve seen a clean pathway where the remnants of a white dwarf merger can be launched at hypervelocity, with properties matching the hot, faint white dwarfs we observe in the halo,” said Glanz.
But what does that actually mean?
The significance of this study is that it gives us new insights into our universe by helping us understand “peculiar” type-1a supernovae: less bright than the typically consistent peak of these standard candles (stellar objects with known luminosity).
Type-1a supernovae can be used to measure the expansion of the universe because we can calculate how far away from us they are and measure the red shift to work out how fast they are moving away from us.
Red shift is when waves are made longer because the source emitting them is moving as it emits. For example, when an ambulance passes you its siren appears higher pitched and faster but slows again as it moves away.
It also resolves the mystery of stellar runaways (stars that are move fast enough to escape their galaxy) as it allows us to understand that the forces used to create this hyper-fast velocity come directly from supernovae.
This helps us to understand how elements formed across galaxies and measure the universe’s expansion. According to co-author Prof. Hagai Perets, “This discovery doesn’t just help us understand hypervelocity stars — it gives us a window into new kinds of stellar explosions.”
Why is this study different?
This is the first study to explore this type of merger in 3D, allowing them to better capture it and the subsequent ejection of the white dwarfs.
Previous models didn’t consider the high velocities and unusual temperatures and luminosities of these known HVWDs such as J0546 and J0927.
The study has great implications for future transient surveys (which cover dynamic and brief moments in space such as supernovae) and broadening our understanding of the HVWDs. Glanz said “This solves the mystery about the origin of stellar run-aways — and also opens up a new channel for faint and peculiar Type-1a supernovae.”
The study was conducted by researchers from the Technion, Universität Potsdam, and the Max Planck Institute for Astrophysics.
