By Mark Brown, Wired UK
Astronomers have discovered that unstable, fast-spinning white dwarf stars might be ticking time bombs that explode into Type Ia supernovae if they slow down. Like a cosmic, billion-year equivalent of the movie Speed.
Type Ia supernovae — rare astronomical explosions that occur just three times every thousand years — happen when an compact white dwarf star — a brittle remnant of a star that’s ceased nuclear fusion — destabilises.
White dwarf stars explode when they exceed the Chandrasekhar mass, which is about 1.4 times the mass of our Sun. Any bigger and gravity overwhelms the forces supporting the white dwarf, compacting it and igniting runaway nuclear fusion that blows the star apart. Cue a violent, colourful explosion.
There’s two ways for white dwarfs to go over Chandrasekhar mass: when they are plumped up with gas from a nearby donor star or two white dwarfs collide and combine mass. Astronomers favor the first scenario as the more likely explanation, but the forensic evidence of Type Ia supernovae shows that the latter is way more prevalent.
We should see leftover hydrogen and helium gas that wasn’t sucked up by the white dwarf, and we should see remnants of the donor star in the stellar explosion. But we don’t. However, physicist Rosanne Stefano from the Harvard-Smithsonian Center for Astrophysics says her research suggest, “we’ve been looking for the wrong signs.”
Stefano suggests that the white dwarf’s frenetic spin may offset the forces of gravity and allow a star to exceed the Chandrasekhar mass for as long as it keeps on spinning. “If the white dwarf rotates fast enough, its spin can help support it, allowing it to cross the 1.4-solar-mass barrier and become a super-Chandrasekhar-mass star,” a spokesperson for the astrophysics centre writes.
It could give the star an extra billion years to live, before it finally slows down and explodes. This would give enough time for the surrounding helium and hydrogen gases to dissipate and the donor star to evolve into a second white dwarf, hiding all tracks of its pre-supernova events.
These supernova precursor signs will be difficult to spot, but upcoming wide-field surveys should help spot them. Co-author Rasmus Voss of Radboud University Nijmegen, in the Netherlands said:”We don’t know of any super-Chandrasekhar-mass white dwarfs in the Milky Way yet, but we’re looking forward to hunting them out.”
Written by Mark Brown
Edited by Duncan Geere
David A. Aguilar (CfA)