Asteroids or planets might trigger a supernova

A few well-aimed asteroids or planets might be all it takes to make a star explode.

A handful of impacts from rocky debris in the atmosphere of a white dwarf, the core of a long dead star, could trigger a powerful explosion known as a type 1a supernova. The proposal, reported online February 1 at arXiv.org, suggests a new way to create this type of supernova, whose origins are still heavily debated.

While the exact mechanisms that might destroy the white dwarf aren’t clear, “the idea isn’t pure speculation,” says Rosanne Di Stefano, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics and lead author on the paper. Astronomers know that rocky material rains down on white dwarfs. The atmospheres of many of these stars are laced with elements such as carbon and silicon that should have quickly sunk deep into the star. Since researchers can still see these atoms, there must be a steady supply of material, most likely from asteroids that wander too close.

Most of the asteroids will be torn apart by the white dwarf’s gravity into a disk of whirling debris, which slowly rains down on the star. But Di Stefano and colleagues suggest that occasionally a few intact asteroids, or even a small planet, might hit the white dwarf directly. The influx of material might trigger thermonuclear fusion in the star’s atmosphere that sends a shock wave through the star, leading to its destruction.

“It’s an intriguing idea,” says Christopher Stockdale, an astrophysicist at Marquette University in Milwaukee, Wis.

White dwarfs explode when their mass exceeds a threshold known as the Chandrasekhar limit, which is about 1.4 times the mass of the sun. Beyond that point, the star can no longer support its own weight. But astronomers debate how white dwarfs bulk up past that limit. One idea is that a white dwarf might steal gas from a stellar companion. Alternatively, two white dwarfs locked in orbit around one another could spiral together and collide, destroying themselves in the process. But astronomers can’t find companion stars in preexplosion images. And there don’t seem to be enough binary white dwarfs with the right total mass to trigger a supernova.

Di Stefano says that while it’s possible an asteroid or planet falling onto a white dwarf might be just enough to push the star over the limit, it’s more likely to trigger a thermonuclear explosion in the helium enveloping a lower mass star. Astronomers have been looking for ways a low-mass white dwarf could explode, since there seems to be a dearth of white dwarfs near the Chandrasekhar limit.

 “It’s good to think outside the box,” says Adam Burrows, an astrophysicist at Princeton University. But there are a lot of open questions. For one, it’s not clear that large asteroids or planets will survive the journey toward the white dwarf without being torn apart first. Also, the physics behind what might drive the explosion is not fleshed out. “It’s interesting speculation,” he says, “that’s not yet substantiated.”

Di Stefano acknowledges that there are lots of unknowns. “Our hypothesis is just the beginning point for a lot of work,” she says. Observations with future telescopes, such as the Large Synoptic Survey Telescope, might detect flares from impacts, which could let researchers get a better handle on what types of objects, if any, hit white dwarfs before being torn apart. It wouldn’t take many. Even if only one out every 10 million or 100 million impacts detonates the star, she says, that could contribute substantially to the supernova population.