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Exploding star blasts new data towards Earth

Dr Amanda Doyle, Feature writer
Dec 17, 2011, 8:00 UTC

Sen—In August of this year a star exploded in a galaxy 21 million light years away. But stars explode all the time, so what was so special about this supernova?

The cosmic explosion, which was given the name SN 2011fe, was the brightest and the closest supernova observed from Earth in the past 25 years. It occurred on 24 August 2011 in the Pinwheel Galaxy, which is also known as M101. With a vast array of advanced instrumentation available, astronomers were able to observe the event only 11 hours after the explosion, which enabled them to collect some very important data in order to fill in some gaps in our knowledge of supernovae.

SN 2011fe was discovered by the Palomar Transient Factory, a 48 inch telescope which scans millions of objects every night looking for transient events such as supernovae. At the point of discovery the supernova was only 1/1000th of its peak brightness. Follow up observations were quickly made by many telescopes on Earth and in space.

SN 2011fe is classified as a type Ia supernova, which means that it was a white dwarf before it exploded. The white dwarf coexisted with another star in a binary system, and it was this companion that caused the white dwarf’s ultimate demise. However while it has long been theorised that the progenitor for a type Ia supernova was a white dwarf, this is the first time that there is evidence for the fact.

This evidence was gathered mainly due to the proximity of the supernova and due to the early detection. During the hours immediately after the explosion, the luminosity is related to the size of the expanding fireball. “The early-time luminosity is proportional to progenitor radius and we can get an upper limit on the radius of the progenitor,” Mansi Kasliwal from the Carnegie Institution for Science tells SEN. “This is how we directly know that the primary that exploded was a white dwarf.” The radius was found to be only a tenth that of the Sun, meaning that the progenitor had to be a compact star; most likely a white dwarf. Further evidence as to the type of progenitor comes from the detection of unburnt carbon and oxygen in the spectrum, which points to a carbon-oxygen white dwarf.

If the progenitor was a white dwarf, then what type of star was the companion? Type Ia supernovae have two possible origins. The binary can be composed of two white dwarfs, which eventually spiral towards each other, resulting in an explosion. The alternative model is that the white dwarf progenitor siphons gas off a star such as a red giant, helium star or main sequence star. For the latter theory, the white dwarf accretes a very specific amount of material before it explodes. Once it reaches 1.4 times the mass of the Sun, the carbon ignites and the star is no more. This critical value is known as the Chandrasekhar limit, named after the astronomer who first predicted it.

The Pinwheel Galaxy has been imaged many times before the supernova, so why not search through archival pictures and find out what type of star it was? However despite the proximity of the event, historical Hubble images revealed absolutely nothing. But a null result does not necessarily mean a useless result as this enabled astronomers to put constraints on the companion star using the process of elimination.

Infrared Spitzer images from 2004 were analysed, and also showed no sign of the companion star. This means that the star couldn’t possibly be a bright red giant or helium star. The lack of detection of the companion star in radio and x-ray images also imply that it couldn’t be a red giant or helium star. If two white dwarfs collided it is expected that they would produce a spectacle of ultraviolet and optical light, which wasn’t observed. However, a white dwarf companion cannot be completely ruled out because the non detection in the visible part of the spectrum could be consistent with a faint white dwarf. It is hoped that future technology such as the James Webb Space Telescope might have better luck at spotting the companion star.

Despite the immensely destructive force of supernovae, we wouldn’t exist without them. Supernovae created many heavy elements needed for the construction of the Earth and the life it hosts. Understanding exactly how these elements are created, along with how they become mixed together in the explosion is crucial. And SN2011fe decided to lend a helping hand with this too. While the discovery of many intermediate mass elements in the spectrum was expected, the astronomers discovered that there was oxygen moving at 20,000 kilometres per second away from the site of the blast. This was 4,000 kilometres faster than the other elements. However the speed of the oxygen quickly diminished, and it had slowed down to 14,000 kilometres within a matter of hours. The unusual velocity of the oxygen shows that it wasn’t distributed evenly when the white dwarf went supernova. The high velocity oxygen and its strange behaviour are the first such observation in a type Ia supernova.

“This will be the best studied supernova of its type given the youth and proximity of the discovery,” concludes Kasliwal. “Therefore, many new things will be learned and many papers will be written.”