The Gobbling Dwarf that Exploded
A unique set of observations, obtained with ESO's VLT, has allowed astronomers to find direct evidence for the material that surrounded a star before it exploded as a Type Ia supernova. This strongly supports the scenario in which the explosion occurred in a system where a white dwarf is fed by a red giant.
Left: artist's impression of the favoured configuration for the progenitor system of SN2006X before the explosion. The White Dwarf (on the right) accretes material from the Red Giant star, which is losing gas in the form of stellar wind (the diffuse material surrounding the giant). Only part of the gas is accreted by the White Dwarf, through a so-called accretion disk which surrounds the compact star. The remaining gas escapes the system and eventually dissipates into the interstellar medium. The Red Giant star has a radius about 100 times larger than our Sun, while the White Dwarf is about 100 times smaller than the Sun.
Right: Once the mass of the White Dwarf has reached a critical limit, a thermonuclear explosion completely disrupts the star, ejecting its material with velocities up to a tenth of the speed of light. Twenty days after the explosion, when the supernova reaches its maximum brightness, the ejected material has reached a size of roughly 450 times the distance from Earth to the Sun. The enormous amount of light emitted by the supernova passes through the surrounding material before being detected by us, thus revealing gas shells which were ejected by the Red Giant in the last few hundred years before the explosion. These density enhancements were produced either by fluctuations in the mass-loss rate of the Red Giant, or by small recurrent explosive episodes on the surface of the White Dwarf in the final phases of its existence.