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Are Type I Supernovae Progenitors Single or Double Degenerate Systems? - 25th February 2010

WD merger

Type I supernovae occur when a white dwarf, or two combining white dwarfs, reach the Chandrasekhar limit and explode. The Chandrasekhar limit is the maximum possible mass a white dwarf can have: 1.4 times the mass of the sun. New findings by scientists at the Max Planck Institute in Germany have found evidence that up to 95% of type I supernovae in some galaxies are caused by the merger of two white dwarfs, rather than by a white dwarf accreting matter from a companion star.

The scientists have made this claim following x-ray observations of 6 nearby elliptical galaxies. The x-ray luminosities of these galaxies were found to be 30-50 times less than expected if accreting white dwarfs are the source of all type I supernovae. They find that in early type galaxies, which are elliptical galaxies containing old stars formed when the universe was young, only around 5% of type I supernovae are produced by a white dwarf accreting matter until it reaches the Chandrasekhar mass. However, in late type galaxies, which are disk galaxies containing young stars, this percentage may be much higher.

According to the new report, the reason that accreting white dwarfs in late type galaxies can account for a higher percentage of type I supernovae is connected to stellar winds. In the case that the white dwarf is accreting matter from a wind that is being emitted by a red giant companion star, the white dwarf cannot accrete all of the wind. For the wind to contain enough matter for the white dwarf to grow to the Chandrasekhar limit, the mass of the red giant must be at least in the range 1.3 to 1.7 times the mass of the sun, and because stars of this mass have lifetimes between 2 and 5 billion years, only the youngest of early type galaxies can contain these stars. In late type galaxies this is not a problem as the stars within those galaxies will not have reached this age.

It should be noted that there are some factors that this report did not consider: The assumption was made that in the case of a white dwarf accreting matter from a companion star the accreted matter comes from the wind of the companion. However, this is not always the case. In some cases the accretion is via a mechanism known as Roche-lobe overflow. This occurs when the companion star is too large for the outer layers to be gravitationally bound so matter is transferred from these outer layers to the white dwarf. The report also assumes that any nuclear burning on the surface of the white dwarf must be stable, as unstable burning leads to nova events resulting in the white dwarf losing mass. However, not all nova events necessarily cause the white dwarf to lose mass.

This work could have important consequences, as finding the progenitors of type I supernovae is a major area of ongoing research. Learning more about these explosive events will greatly increase our understanding in a number of areas, such as determining the distances to other galaxies and estimating the expansion rate of the universe.


Michael Maxwell

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