New research shows that some old stars known as white dwarfs might be held up by their rapid spins, and when they slow down, they explode as Type Ia supernovae. Thousands of these “time bombs” could be scattered throughout our galaxy. In this artist’s conception, a supernova explosion is about to obliterate an orbiting Saturn-like planet. Image: David A. Aguilar (CfA)
In the Hollywood blockbuster
“Speed,” a bomb on a bus is rigged to blow up if the bus slows down
below 50 miles per hour. The premise—slow down and you explode—makes for a
great action movie plot, and also happens to have a cosmic equivalent.
shows that some old stars might be held up by their rapid spins, and when they
slow down, they explode as supernovae. Thousands of these “time
bombs” could be scattered throughout our galaxy.
haven’t found one of these ‘time bomb’ stars yet in the Milky Way, but this
research suggests that we’ve been looking for the wrong signs. Our work points
to a new way of searching for supernova precursors,” says astrophysicist
Rosanne Di Stefano of the Harvard-Smithsonian
Center for Astrophysics
type of stellar explosion Di Stefano and her colleagues studied is called a
Type Ia supernova. It occurs when an old, compact star known as a white dwarf
A white dwarf
is a stellar remnant that has ceased nuclear fusion. It typically can weigh up
to 1.4 times as much as our sun—a figure called the Chandrasekhar mass after
the astronomer who first calculated it. Any heavier, and gravity overwhelms the
forces supporting the white dwarf, compacting it and igniting runaway nuclear
fusion that blows the star apart.
There are two
possible ways for a white dwarf to exceed the Chandrasekhar mass and explode as
a Type Ia supernova. It can accrete gas from a donor star, or two white dwarfs
can collide. Most astronomers favor the first scenario as the more likely
explanation. But we would expect to see certain signs if the theory is correct,
and we don’t for most Type Ia supernovae.
we should detect small amounts of hydrogen and helium gas near the explosion,
but we don’t. That gas would come from matter that wasn’t accreted by the white
dwarf, or from the disruption of the companion star in the explosion.
Astronomers also have looked for the donor star after the supernova faded from
sight, without success.
Di Stefano and
her colleagues suggest that white dwarf spin might solve this puzzle. A
spin-up/spin-down process would introduce a long delay between the time of
accretion and the explosion. As a white dwarf gains mass, it also gains angular
momentum, which speeds up its spin. 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.
stops, the white dwarf will gradually slow down. Eventually, the spin isn’t
enough to counteract gravity, leading to a Type Ia supernova.
is new because we show that spin-up and spin-down of the white dwarf have
important consequences. Astronomers therefore must take angular momentum of
accreting white dwarfs seriously, even though it’s very difficult
science,” explains Di Stefano.
process could produce a time delay of up to a billion years between the end of
accretion and the supernova explosion. This would allow the companion star to
age and evolve into a second white dwarf, and any surrounding material to
In our galaxy,
scientists estimate that there are three Type
Ia supernovae every thousand
years. If a typical super-Chandrasekhar-mass white dwarf takes millions of
years to spin down and explode, then calculations suggest that there should be
dozens of pre-explosion systems within a few thousand light-years of Earth.
supernova precursors will be difficult to detect. However, upcoming wide-field
surveys conducted at facilities like Pan-STARRS and the Large Synoptic Survey
Telescope should be able to spot them.
know of any super-Chandrasekhar-mass white dwarfs in the Milky Way yet, but
we’re looking forward to hunting them out,” says coauthor Rasmus Voss of
Radboud University Nijmegen, The Netherlands.
This research appears in a paper in The Astrophysical Journal Letters
and is available online.