Exclusion of a luminous red giant as a companion star to the progenitor of supernova SN 2011fe

Type Ia supernovae are thought to result from a thermonuclear explosion of an accreting white dwarf in a binary system (1,2), but little is known of the precise nature of the companion star and the physical properties of the progenitor system. There are two classes of models (1,3): double-degenerate (involving two white dwarfs in a close binary system (2,4)) and single-degenerate models (5,6). In the latter, the primary white dwarf accretes material from a secondary companion until conditions are such that carbon ignites, at a mass of 1.38 times the mass of the Sun. The type Ia supernova SN 2011fe was recently detected in a nearby galaxy (7). Here we report an analysis of archival images of the location of SN 2011fe. The luminosity of the progenitor system (especially the companion star) is 10-100 times fainter than previous limits on other type Ia supernova progenitor systems (8-10), allowing us to rule out luminous red giants and almost all helium stars as the mass-donating companion to the exploding white dwarf.
In a double-degenerate model, the orbit of the two white dwarfs shrinks owing to the release of gravitational radiation (2,4). Ultimately, the lighter object (the secondary) is disrupted and accretion [...]