How rare runaway dead stars are tricky clues for a cosmic mystery

Astronomers call a special kind of supernova a "cosmic yardstick" for good reason: The so-called Type Ia supernova gives off a predictable amount of light, making it a handy tool for measuring distances in space. 

These particular supernovas are useful but still puzzling, with scientists still unsure of what triggers their blasts. NASA estimates they happen only twice per millennium in the Milky Way.

A leading idea has been that in an orbiting pair of white dwarfs — the remains of dead sun-size stars — one bursts apart almost immediately, while the other survives, perhaps hurtling away at breakneck speed because it's no longer tethered by gravity to its companion. 

But that can't explain all Type Ia supernovas, according to researchers led by Technion — Israel Institute of Technology. By studying what could cause a dead-star remnant to suddenly go rogue — flying so fast it could escape the galaxy — the international team discovered a new scenario for a white dwarf explosion.

Based on how often Type Ia supernovas occur, scientists can infer how many runaway white dwarfs they should see overall, said Hagai Perets, who co-led new research published in Nature Astronomy. 

"When you do that, it turns out there are not enough," Perets told Mashable. "If any kind of Type Ia supernova explodes and produces this kind of hyper-velocity white dwarf, then you should have about 100 times more of those than what we actually see."

The European Space Agency's Gaia telescope discovered that high-velocity white dwarfs exist in 2018. Credit: ESA / ATG medialab / Gaia / DPAC / A. Moitinho illustration

A key mystery about Type Ia supernovas is that, in the dominant theory known as D6, a white dwarf would need a companion star in order to explode. This so-called D6 scenario is shorthand for "dynamically driven double-degenerate double-detonation." But so far, no one has found such a companion. 

When the European Space Agency's Gaia telescope discovered the existence of high-velocity white dwarfs in 2018, it became a possible answer for where those elusive companions went, said Samuel Boos, a UC Santa Barbara researcher not involved in the Technion study. It also supported the prediction of extreme speeds for these white dwarfs — the fastest stars in the galaxy.

"It seemingly provided the smoking gun," he told Mashable in an email.

But while Gaia revealed that runaways are indeed real, scientists debated how they were created. That's where the new Technion study comes in.

In this new study, researchers used a 3D supercomputer simulation to explore what happens when two "hybrid" white dwarfs crash into each other. These rare white dwarfs are lighter than usual, with a carbon-oxygen core wrapped in a much thicker helium layer. 

The video above shows a computer simulation for how two hybrid white dwarfs collide, trigger a supernova, and create a high-velocity white dwarf.

The team found that as the lighter of the two spirals into the heavier one, it gets "partly eaten," spilling helium onto its companion. That triggers a two-step explosion: first the outer helium, then the inner carbon. The blast completely destroys the heavier white dwarf but launches the lighter one out at around 4.5 million mph — fast enough to skip right out of the Milky Way. 

"The fact that they have such a large velocity means that they should have been created by something that is very violent, and that's why people [have thought] they were formed by some kind of supernova explosion," Hila Glanz, the other lead author of the study, told Mashable.  "So first of all, how do they survive, and why don't we see the remnant of the explosion itself?"

The team argues their hybrid collision model is a better fit for real runaway white dwarfs. The explosion is far fainter than a classic Type Ia supernova, and the ejected material spreads out thinly in space. That could explain why astronomers haven't spotted the bright, dense debris they'd normally expect after a supernova. 

Here, both stars are smaller, with one partly destroyed before the other blows up. Because they're closer together, the survivor gets kicked out faster and with more force.

The surviving white dwarf's reduced size — after being partially torn apart — also could explain why it appears hotter and puffier, consistent with three real runaway examples. The standard theory, by contrast, can't account for those properties.

"Sort of sold their skin in order to save their life," Perets said.

Astronomers recently got the first photographic evidence of a double-detonation supernova, which occurred in the Large Magellanic Cloud. Credit: ESO / Priyam Das et al. / Hubble / K. Noll et al.

Though this new idea shows how dead stars could become fugitives, it also points to a previously unknown way to create a dimmer white dwarf explosion. Understanding this diversity of Type Ia supernovas and their behavior is crucial, the researchers say, because these events are the universe's main source of iron. 

One potential takeaway is that some missing companions in classic Type Ia supernovas may arise if both white dwarfs explode, said Boos, whose work primarily focuses on the D6 scenario. He recently published a paper in The Astrophysical Journal showing that a "quadruple detonation" — where one double detonates after the other — can produce the expected brightness and chemical fingerprints just as well as a single explosion.

"It’s very possible there are few or no truly-D6 runaways (as they exploded along with the primary)," Boos said in an email, "and that all of these candidates indeed come from these hybrid mergers."

As researchers ponder the implications, they can also imagine what's next for those super-fast runaway white dwarfs. After fleeing the Milky Way, they could cross into other galaxies, but only to pass through. They're likely too fast to ever be captured again. 

"They go with a boom, go all the way, become the fastest ones," Perets said, "but then they just continue in loneliness."