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Merging White Dwarfs Set Off Supernovae
by Nancy Atkinson
New results from the Chandra X-Ray Observatory suggests that the majority of Type Ia supernovae occur due to the merger of two white dwarfs. This new finding provides a major advance in understanding the type of supernovae that astronomers use to measure the expansion of the Universe, which in turns allows astronomers to study dark energy which is believed to pervade the universe. "It was a major embarrassment that we still didn't know the conditions and progenitor systems of some the most spectacular explosions in the universe," said Marat Gilfanov of the Max Planck Institute for Astrophysics, at a press conference with reporters today. Gilfanov is the lead author of the study that appears in the Feb. 18 edition of the journal Nature.
Type Ia supernovae serve as cosmic mile markers to measure expansion of the universe. Because they can be seen at large distances, and they follow a reliable pattern of brightness. However, until now, scientists have been unsure what actually causes the explosions. Most scientists agree a Type Ia supernova occurs when a white dwarf star  -  a collapsed remnant of an elderly star  -  exceeds its weight limit, becomes unstable and explodes. The two leading candidates for what pushes the white dwarf over the edge are the merging of two white dwarfs, or accretion, a process in which the white dwarf pulls material from a sun-like companion star until it exceeds its weight limit.
"Our results suggest the supernovae in the galaxies we studied almost all come from two white dwarfs merging," said co-author Akos Bogdan, also of Max Planck. "This is probably not what many astronomers would expect."
The difference between these two scenarios may have implications for how these supernovae can be used as "standard candles"  -  objects of a known brightness  -  to track vast cosmic distances. Because white dwarfs can come in a range of masses, the merger of two could result in explosions that vary somewhat in brightness. Because these two scenarios would generate different amounts of X-ray emission, Gilfanov and Bogdan used Chandra to observe five nearby elliptical galaxies and the central region of the Andromeda galaxy. A Type Ia supernova caused by accreting material produces significant X-ray emission prior to the explosion. A supernova from a merger of two white dwarfs, on the other hand, would create significantly less X-ray emission than the accretion scenario.
The scientists found the observed X-ray emission was a factor of 30 to 50 times smaller than expected from the accretion scenario, effectively ruling it out. So, for example, the Chandra image above would be about 40 times brighter than observed if Type Ia supernova in the bulge of this galaxy were triggered by material from a normal star falling onto a white dwarf star. Similar results for five elliptical galaxies were found.
This implies that white dwarf mergers dominate in these galaxies.
An open question remains whether these white dwarf mergers are the primary catalyst for Type Ia supernovae in spiral galaxies. Further studies are required to know if supernovae in spiral galaxies are caused by mergers or a mixture of the two processes. Another intriguing consequence of this result is that a pair of white dwarfs is relatively hard to spot, even with the best telescopes.
"To many astrophysicists, the merger scenario seemed to be less likely because too few double-white-dwarf systems appeared to exist," said Gilfanov. "Now this path to supernovae will have to be investigated in more detail."
Source: NASA
About Nancy AtkinsonNancy Atkinson is currently Universe Today's Contributing Editor. Previously she served as UT's Senior Editor and lead writer, and has worked with Astronomy Cast and 365 Days of Astronomy. Nancy is also a NASA/JPL Solar System Ambassador.