Solving a supernova mystery

14 December 2011

The star that exploded to create the nearest supernova of its type to be discovered since 1986 has been revealed by an international team including Oxford University scientists. New observations reported in two papers in this week's Nature show that a very dense, very small white dwarf star made of carbon and oxygen, orbiting another star, triggered the explosion. The observations also rule out previously popular models of what the second 'companion' star might look like.

The supernova (SN 2011fe), which was first spotted on 24 August 2011 by scientists from Oxford University and the Palomar Transient Factory (PTF) collaboration, appeared in the Pinwheel Galaxy, M101, 21 million light years distant. The discovery was particularly important because it was a type 1a supernova – the kind used by scientists to measure the accelerating expansion of the Universe and dark energy.

By studying how the supernova’s brightness evolved, and the light spectra it emitted, the team were able to calculate the size of the exploding star, what it was made of, and what kind of stellar system it originated from.

‘This is the first direct confirmation that this very important type of supernova is triggered by a white dwarf star made of carbon and oxygen,’ said Dr Mark Sullivan of Oxford University’s Department of Physics, who led the Oxford team and is an author of both Nature papers.

The star that caused the supernova started out life as a star not unlike our Sun. Over a lifetime of tens to hundreds of millions of years its hydrogen fuel began to run out and it bloated to become a giant red star. While the core of the star remained hot enough to burn helium into carbon and then oxygen, the upper layers were eventually stripped away so that only the core remained – forming a roughly Earth-sized carbon-oxygen white dwarf.

‘The key to the supernova explosion is that the white dwarf star did not live alone in space, but with a second star in a binary system,' said Dr Sullivan. ‘As the white dwarf orbited it Hoovered up additional material from its companion and began to grow, but there is a limit to how massive a white dwarf can get – about 1.4 times the mass of our Sun – before it can no longer support itself. As it nears this limit it becomes hot enough to trigger a thermonuclear explosion.

‘This creates a gigantic expanding fireball in space with a brightness powered by radioactive decay – the supernova that was visible from Earth. Spotting this supernova so early, and being able to examine it with modern instruments, ensures that it will become the best-studied supernova in history: we suspected carbon-oxygen white dwarf stars were responsible for type 1a supernova but for the first time we’ve got a close-up of one in the act of blowing up.’

Even more information can be gleaned from how the supernova’s brightness changes over time: if material ejected from the initial explosion collides with the companion star it changes the appearance of the supernova (its light curve) shortly after the explosion. ‘Because we caught this supernova very early, our observations are able to rule out some types of companion to the white dwarf,’ said Dr Sullivan. ‘For example, we can rule out a companion red giant star; the second star cannot have been much bigger than the Sun.’

A separate analysis of Hubble Space Telescope images taken before the explosion, conducted by the team, has provided additional evidence that the companion star cannot have been a red giant star.

Whilst the observations support existing models of supernovae formation they have thrown up some surprises: in particular the uneven or ‘clumpy’ way in which oxygen was distributed within the explosion and the high degree of mixing of materials, such as radioactive nickel, throughout the fireball.

Dr Sullivan added: ‘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, so we are all made of bits of exploding stars.’

The Palomar Transient Factory is an international collaboration of scientists and engineers from the California Institute of Technology, DOE’s National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory, NASA’s Infrared Processing and Analysis Center, the University of California at Berkeley, Las Cumbres Observatory Global Telescope Network, the University of Oxford, Columbia University, the Weizmann Institute of Science in Israel, and Pennsylvania State University.