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Type Ia supernova caused by merging white dwarfs
Posted: 18 February 2010

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The supernova explosions in faraway galaxies that are used to measure the expansion of the Universe may be caused by the mergers of two white dwarfs, rather than single white dwarfs exploding, say new observations from NASA’s Chandra X-ray Observatory. But don’t panic – the concept of dark energy is still safe.

An animation dramatically showing the merger and destruction of two white dwarfs.

White dwarfs are the burnt-out husks of dead stars that were once like our Sun, and they are involved in a special type of stellar explosion called a Type Ia supernova. These are special because they all supposedly have the same intrinsic luminosity when they explode, so by directly comparing them we can tell which are near and which are far. Astronomers call them ‘standard candles’ and they are crucial for measuring how fast dark energy is carrying galaxies away from us.

For decades the best theory for why white dwarfs explode was that they strip gas from the surface of a companion star, which flows into a glowing disc of hot gas around the white dwarf, gradually accumulating on its surface. This gas can reach millions of degrees Celsius, emitting X-rays. Once the white dwarf grows in size to greater than 1.4 times the mass of the Sun (a level called the Chandrasekhar limit), runaway nuclear fusion reactions ignite on the surface, quickly converting the entire white dwarf into an immense thermonuclear explosion. Because they all explode with the same 1.4 solar masses they should then all have the same luminosity.

Merging white dwarfs appear to cause Type Ia supernovae in elliptical galaxies. Sun-like stars eventually evolve into white dwarfs; one can imagine a binary star system with two Sun-like stars one day leaving behind a pair of white dwarfs that over millions of years slowly spiral towards one another before merging and exploding. Image: NASA/CXC/M Weiss.

The X-rays being emitted from these discs of hot gas should leave a background glow of radiation in each galaxy. However, when astronomers Marat Gilfanov and Akos Bogdan from the Max Planck Institute for Astrophysics in Germany searched a number of elliptical galaxies for this X-ray glow, they found that it was up to 50 times fainter than expected. This means that there must be fewer accreting white dwarfs than had been thought, but this doesn’t match up with the supernova rate. Something else must be to blame for the majority of Type Ia supernovae.

Gilfanov and Bogdan turned to an alternative theory that suggests most Type Ia supernovae are caused by two white dwarfs merging. With no accretion discs involved, there would be no X-ray output. “To many astrophysicists, the merger scenario seemed less likely because too few double-white-dwarf systems appeared to exist,” says Gilfanov. “Now this path to supernovae will have to be investigated in more detail.”

Accretion from a normal star onto a white dwarf does not appear to be the cause of most Type Ia supernovae in elliptical galaxies, judging by the lack of X-rays that would be coming from their accretion discs. Image: NASA/CXC/M Weiss.

That’s not the only caveat. Gilfanov and Bogdan chose elliptical galaxies for their study because they have less neutral gas and dust to absorb the X-rays than spiral galaxies have. It is possible that in spiral galaxies most type Ia supernovae are created by accreting white dwarfs, and this will be tested in a follow-up study. On the other hand, perhaps the accreting white dwarfs are there in the ellipticals, but their X-rays are being blocked somehow, or maybe in reality accreting white dwarfs produce fewer X-rays than theory predicts. Despite these uncertainties, Professor Mario Livio of the Space Telescope Science Institute in the United States is impressed with the results.

“They [Gilfanov and Bogdan] have taken a fresh approach and looked at what happens before the explosion occurs,” he says. “They looked at global properties of galaxies and concluded that mergers were more likely in ellipticals. There is no question that these findings place very real constraints on Type Ia supernovae.”

Even if the merger scenario is accepted, problems still exist. To date there is still no convincing model as to how exactly a white dwarf merger would create a supernova; in fact, some recent theoretical models show that they would collapse into a neutron star rather than explode. Assuming they do explode, estimates suggest that their brightness would not be too different to supernovae generated by single white dwarfs, but the masses of the white dwarfs merging could be different, meaning that no two Type Ia supernovae would be precisely alike. Whilst the differences in luminosity and how they fade from peak brightness would not be sufficient to discredit the existence of dark energy, they would have an impact on attempts to measure the strength of dark energy and the speed of the expansion of the Universe throughout cosmic history. This is crucial for determining the fate of the evolution of the Universe, and whether it is destined to expand forever.