Supernova observations yield new answers

Posted by Emma Woollacott

The discovery of a supernova just hours after its explosion has given scientists an unprecedented amount of information about such cosmic explosions.

PTF 11kly, a type Ia supernova, was spotted in August in the Pinwheel galaxy, and was the closest to Earth in 40 years. Located 21 million light-years away, it was visible in early September with binoculars, and gave scientists their best chance yet to study a thermonuclear supernova up close, with modern instruments.

Now, an international team of scientists has discovered that it was a white dwarf star, and that its companion star couldn't have been a red giant, as previously suspected.

Over the past 50 years, astrophysicists have discovered that type Ia supernovae are part of binary systems: two stars orbiting one other. The identification of PTF 11kly as a white dwarf bears out current theories.

Such white dwarfs would normally be dead forever - but if one has a companion star, then the white dwarf can steal its matter, and return to life. If it steals too much matter, the carbon atoms will fuse so rapidly that the burning cannot be stopped, leading to a Type Ia supernova.

Scientists haven't yet established the type of the companion star. However, they've ruled out the type they expected, a red giant. A red giant would have made the supernova brighter by several orders of magnitude early on.

"This is the first time through direct imaging of the explosion site, we were able to rule out certain types of stars as the companion to a Type Ia supernova," says  Weidong Li, a research scientist at the University of California, Berkeley. "The second star couldn't have been a massive red giant."

As was expected, early spectra showed many intermediate-mass elements spewing out of the expanding fireball, including ionized oxygen, magnesium, silicon, calcium, and iron, traveling at 16,000 kilometers a second. However, some oxygen was traveling much faster, at over 20,000 kilometers a second.

"The high-velocity oxygen shows that the oxygen wasn't evenly distributed when the white dwarf blew up, indicating unusual clumpiness in the way it was dispersed," says Peter Nugent of Berkeley Lab.

The explosion showed a 'tremendous' amount of mixing, he says, with some radioactive nickel mixed all the way to the photosphere.

"Understanding how these giant explosions create and mix materials is important because supernovae are where we get most of the elements that make up the Earth and even our own bodies – for instance, these supernovae are a major source of iron in the universe," says  Mark Sullivan of the University of Oxford. "So we are all made of bits of exploding stars."