Scientists solve mystery of the fastest stars in the galaxy

Most stars in the Milky Way move at steady speeds, orbiting the galactic center in regular paths. But astronomers have uncovered rare stars so fast they can escape the galaxy’s gravity altogether. These speedsters are called hypervelocity white dwarfs (HVWDs), the dense remnants left behind when stars like the Sun run out of fuel.

Unlike ordinary white dwarfs, which drift along quietly, HVWDs rocket through space at over 2,000 kilometers per second. That’s fast enough to cross the United States in under two seconds.

The puzzle has long been what force could fling such compact stars into intergalactic space. Traditional theories pointed to powerful supernova explosions, but the details never matched up. A new study published in Nature Astronomy and led by Dr. Hila Glanz of the Technion – Israel Institute of Technology offers an answer.

Using advanced computer simulations, her team discovered that when two unusual white dwarfs smash together, the result can be both a peculiar supernova and the creation of a runaway stellar core moving at record-breaking speeds.

For years, astronomers leaned on a model called the “D6 scenario.” In this setup, two white dwarfs spiral together, and the heavier star siphons helium from the lighter one. The buildup ignites, triggering a type Ia supernova that blasts the lighter star away at high speed. Clever as it sounded, this explanation struggled to line up with real data.

The D6 theory required a rare, very massive white dwarf, greater than one solar mass, to work at all. Even then, it didn’t explain why some runaway stars glow hotter and puffier than expected. And the rate of such explosions predicted by the theory far exceeded the number of HVWDs astronomers have actually found.

Dr. Glanz’s team proposed a new route. Instead of one massive star and one small donor, they modeled a pair of hybrid helium–carbon–oxygen white dwarfs, or HeCO dwarfs. These stars contain layers of helium and carbon-oxygen, making them structurally different from the pure carbon-oxygen dwarfs assumed in older models. When they collide and disrupt one another, the result matches the puzzling brightness and high speeds astronomers see.

To test the idea, the researchers used the Arepo simulation code, designed to model extreme astrophysical events. They created a virtual binary system with two HeCO white dwarfs, one with 0.68 solar masses and the other with 0.62. Over time, the smaller star was stretched by the gravity of its partner and began spilling material onto it.

The helium piled up until a detonation ripped across the larger star’s surface. The shockwave ignited its carbon-oxygen core, triggering a second, more violent blast. The primary star was destroyed in a thermonuclear explosion, while its companion was partly shredded but survived. What remained was a lightweight but blazing-hot stellar core, hurled away at more than 2,000 kilometers per second.

The simulation showed the runaway dwarf shining brighter and larger than a normal one, thanks to the heat and stretching from the explosion. This fits with puzzling observations of stars like J0546 and J1332, which didn’t make sense under older models.

The supernova triggered by this merger was also unlike typical type Ia blasts. The explosion released 1.13 × 10^51 ergs of energy and created about 0.072 solar masses of radioactive nickel-56. This isotope normally powers the bright glow of supernovae, but in this case the smaller amount of nickel would make the blast appear faint and strange compared to normal events.

Such explosions could help explain rare underluminous supernovae, sometimes called “91bg-like” events. These faint outbursts are spotted in older galaxies but have been difficult to trace back to a cause. The debris from the simulated explosion also sped off at high velocities, meaning astronomers might one day spot both a hypervelocity white dwarf and the remnants of its partner’s explosion, confirming the model.

What happens to a hypervelocity white dwarf after its launch into space? To answer that, the researchers used the stellar evolution code MESA to follow its long-term development. At first, the star spins rapidly and shines brightly, inflated by the blast. Over millions of years it cools, shrinks, and slows its rotation, but it remains hotter and puffier than a typical white dwarf of the same mass.

When compared to real stars observed by the Gaia mission, the match was striking. Known hypervelocity dwarfs like J0546 and J0927 lined up neatly with the predictions, including their unusual sizes and speeds. For the first time, a model explained not just how fast these stars move, but also why they look the way they do.

The new work goes beyond explaining a few runaway stars. Type Ia supernovae are vital tools for measuring the expansion of the universe, yet their origins are still debated. If some of them come from HeCO mergers, as this study suggests, then astronomers may need to refine how they interpret supernova data.

Prof. Hagai Perets of the Technion, a co-author of the study, said, “This discovery doesn’t just help us understand hypervelocity stars — it gives us a window into new kinds of stellar explosions.”

Because hybrid white dwarfs may make up a quarter of all white dwarf pairs, this pathway might be more common than once thought. Even if only a fraction of these systems end in hypervelocity remnants, they could explain the handful astronomers have found so far.

Future surveys and Gaia data releases are expected to uncover more of these stellar cannonballs streaking across the galaxy. Each new detection will help confirm the role of HeCO mergers and reveal more about the violent processes shaping stars and galaxies.

Understanding hypervelocity white dwarfs has implications well beyond curiosity about fast stars. These objects serve as test cases for theories of supernova explosions, which are key to measuring cosmic distances and the universe’s expansion rate.

The findings also shed light on rare underluminous supernovae, helping astronomers piece together how elements are forged and spread through galaxies.

By tracing the origins of runaway white dwarfs, scientists gain not only a solution to a long-standing puzzle but also a deeper window into stellar life cycles and the forces that shape the cosmos.

Note: The article above provided above by The Brighter Side of News.

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