April 5, 2005
Clues to Supernova Origin Found in Dusty Stellar Wind
RAS -- Scientists from Imperial College London have detected a dusty wind emitted by a star that, at the end of its life, turned into a white dwarf and then exploded as a supernova. This is the first time that a wind from this type of supernova precursor has been observed and it is also the first time that associated dust has been detected.
The properties of the wind hold vital clues to the kind of star that exploded. Dr Rubina Kotak, from Imperial College London, will be presenting the discovery at the RAS National Astronomy Meeting at the University of Birmingham on Tuesday 5th April.
The wind was detected around a Type Ia supernova, which is the kind of supernova used to measure the rate of expansion of the Universe. Type Ia supernova are sometimes referred to as "standard candles" because they have a predictable peak luminosity, which means that their observed brightness can be used to work out their distance from Earth.
Although Type Ia supernovae have shown us that the expansion of the universe is accelerating, there is still much we do not know about their nature and origin. To find out more about how supernovae explosions occur, scientists study the debris left behind and compare the observed concentrations of chemical elements with theoretical predictions.
The team from Imperial, including Dr Kotak and Professor Peter Meikle, selected supernova SN 2002ic for study because it is the first Type Ia supernova in which hydrogen has been observed. Using the Very Large Telescope facility in Chile, the scientists precisely measured the speed at which the hydrogen was moving. They discovered that it was expanding much slower than expected, which indicated that they were seeing the undisturbed wind emitted by the star, prior to its supernova explosion.
The scientists then obtained an infrared image of SN 2002ic taken with the United Kingdom Infrared Telescope on Mauna Kea (Hawaii) and found that it was incredibly bright. This infrared luminosity appears to be due to the dustiness of the wind from the star, and the subsequent heating of dust grains by the supernova explosion.
The team from Imperial are continuing to monitor the behaviour of SN 2002ic using both ground-based telescopes as well as the Spitzer Space Telescope. Although it is not a typical Type Ia supernova, it should help scientists understand more about this important group of supernovae.
Types of Supernovae
Supernovae come in two main flavours: Type Ia and Type II. The former are believed to result from the explosion of a white dwarf. Most low mass stars, such as our Sun, end up as white dwarves but these very compact objects only explode if their mass reaches a critical value of about 1.4 solar masses. The general consensus is that this critical mass can only be attained if the white dwarf has a companion star. Under certain conditions, matter from the companion star flows onto the white dwarf making it denser and smaller until it can no longer support any more material. At this point the white dwarf starts to collapse under its own weight and this ignites the nuclear fuel, leading to a gigantic thermonuclear explosion. Type II supernovae occur when a star that is about 8 times or more massive than our Sun has exhausted its nuclear fuel and can no longer support itself against the inward pull of gravity. The core then collapses, bounces on itself and causes the outer layers to explode, possibly due to the effects of neutrino deposition or the generation of jets.
Until recently the presence of hydrogen in Type Ia supernovae had been predicted but never observed. In 2003, Mario Hamuy and colleagues announced that they had discovered hydrogen in Type Ia supernova, SN 2002ic. The supernovaÃÂ¿s spectrum bore all of the hallmarks of a Type Ia supernova but also showed a very strong and distinct signature of hydrogen. SN 2002ic is about 1000 million light years from Earth.
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