In the News this month: the explosion mechanism behind type Ia supernovae
Two white dwarf stars orbit each other and lose energy via gravitational radiation, eventually resulting in a merger between the two stars. CREDIT: NASA/CXC/M.Weiss
Supernova explosions are initially classified by the chemical signatures in their optical spectra. While some are caused by the catastrophic collapse of stars more than eight times as massive as the Sun, others are thought to be caused by white dwarfs, stars like the Sun which have already evolved off the main sequence and shrunk in size. Called Type Ia supernovae, such explosions are thought to have a fixed brightness, allowing them to be used as standard candles to measure distances to galaxies and test cosmological models of the expansion of the universe. There are two possible models for these Type Ia supernovae, both involving the explosion of white dwarf stars. Of these theories, the one thought to be the most likely involves the accumulation of material from a companion star onto the surface of a white dwarf. When the mass of the white dwarf exceeds a certain limit, known as the Chandrasekhar limit, it becomes unstable and explodes. The second theory is that the explosion is caused by the merger of two white dwarfs in orbit around each other. While the first theory was thought to be the most likely explanation, research published in Nature on the 18th February suggests that the second model may, in fact, be far more likely than was previously assumed.
The X-ray signatures of these two different explosion mechanisms are quite different, with far more pre-explosion X-ray emission expected from an accreting white dwarf than from the merger scenario, so two researchers at the Max Planck Institute for Astrophysics in Germany used data from the Chandra X-ray Observatory and the Spitzer Space Telescope to examine several nearby galaxies. The ongoing accretion process prior to a supernova explosion would generate significant amounts of X-ray emission detectable by Chandra, while a binary white dwarf system heading towards a merger would not generate such emission. The infra-red luminosity of a particular galaxy, taken from the Spitzer data, gives an estimate of the number of white dwarfs in the galaxy, leading to an estimate of the expected X-ray luminosity if accretion is the dominant mechanism. The astronomers examined observations of five nearby elliptical galaxies, as well as the bulge of M31, the nearest spiral galaxy to the Milky Way, and found in all cases that the predicted X-ray luminosity was between 30 and 50 times lower than expected if the accretion scenario was the main cause of type Ia supernovae.
The results imply that, at least in elliptical galaxies, the dominant mechanism behind type Ia supernovae is white dwarf merger rather than accretion. The researchers calculate that, in ellipticals, it may be that less than five per cent of type Ia supernovae explosions are caused by accretion. The story is slightly different in spiral galaxies however, where clouds of neutral gas and thick dust lanes typical of star formation in spiral galaxies could be obscuring the X-ray radiation created in the pre-explosion phase of the accretion scenario.
These new results may have implications for cosmological studies, since the assumed standard luminosity of type Ia supernovae is used to calculate the expansion velocity of the universe. Since the two merging stars may have slightly different masses in different systems, the total explosion luminosity may not be as standard as thought.
This blog post is a news story from the Jodcast, aired in the March 2010 edition.
Gilfanov, M., & Bogdan, A. (2010). An upper limit on the contribution of accreting white dwarfs to the type Ia supernova rate Nature, 463 (7283), 924-925 DOI: 10.1038/nature08685