"The question of what causes a Type Ia supernova is one of the great unsolved mysteries in astronomy,"
Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics (CfA).
A massive supernova variety - Type Ia - brightens and dims so predictably that astronomers use them to measure the universe's expansion -called a "standard candle.". The resulting discovery of dark energy and the accelerating universe rewrote our understanding of the cosmos. Yet theorigin of these supernovae, which have proved so useful, remains unknown.
There are two leading scenarios for the intermediate step from stable white dwarf to supernova, both of which require a companion star. In the first possibility, a white dwarf swallows gas blowing from a neighboring giant
star. In the second possibility, two white dwarfs collide and merge. To establish which option is correct (or at least more common), astronomerslook for evidence of these binary systems.
Given the average rate of supernovae, scientists can estimate how many pre-supernova white dwarfs should exist in a galaxy. But the search forthese progenitors has turned up mostly empty-handed.
To hunt for accreting white dwarfs, astronomers looked for X-rays of a particular energy, produced when gas hitting the star's surface undergoes nuclear fusion. A typical galaxy should contain hundreds of such "super-soft" X-ray sources. Instead we see only a handful. As a result, a recent paper suggested that the alternative, merger scenario was the source of Type Ia supernovae, at least in many galaxies.
That conclusion relies on the assumption that accreting white dwarfs will appear as super-soft X-ray sources when the incoming matter experiences nuclear fusion. Di Stefano and her colleagues have argued that the data do
not support this hypothesis.
Di Stefano points out that a merger-induced supernova would also be preceded by an epoch during which a white dwarf accretes matter that should undergo nuclear fusion. White dwarfs are produced when stars age, and different stars age at different rates. Any close double white-dwarf system will pass through a phase in which the first-formed white dwarf gains and burns matter from its slower-aging companion. If these white dwarfs produce X-rays, then we should find roughly a hundred times as many super-soft X-ray sources as we do.
Since both scenarios - an accretion-driven explosion and a merger-driven explosion - involve accretion and fusion at some point, the lack of super-soft X-ray sources would seem to rule out both types of progenitor. The alternative proposed by Di Stefano is that the white dwarfs are not luminous at X-ray wavelengths for long stretches of time. Perhaps material surrounding a white dwarf can absorb X-rays, or accreting white dwarfs might
emit most of their energy at other wavelengths.
If this is the correct explanation, says Di Stefano, "we must devise new methods to search for the elusive progenitors of Type Ia supernovae."
Images available online at http://www.cfa.harvard.edu/news/2010/pr201009_images.html
Image credit: Christiian Joore