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Two white dwarf stars dance to a violent death

Amy Tyndall, News reporter
Feb 21, 2015, 1:11 UTC

Sen—Astronomers have discovered two white dwarf stars at the heart of a planetary nebula that, despite the serene appearance now, are destined to end their lives in a violent supernova explosion.

The two massive stars will gravitationally attract as they orbit each other in a celestial dance to the death, and will merge in around 700 million years time to make sure they go out with a bang.

The planetary nebula is known as Henize 2-428. Such nebulae, which have nothing to do with planets, represent the final stages of a star's life as it transitions from the red giant phase to the white dwarf phase.

As the star evolves, it loses the ability to hold on to its outer layers of dust and gas and throws them off into the surrounding space. The star's temperature increases causing this stellar material to be heated up by emitted UV radiation, creating the colourful nebula clouds we see in the images.

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Image of Henize 2-428 taken with ESO’s Very Large Telescope at the Paranal Observatory in Chile. The two white dwarf stars lie in the core of this planetary nebula, formed in the latter part of their lives as they expel their outer layers into the surrounding space. Image credit: ESO

The expectation is that a single star goes on to produce a perfectly spherical planetary nebula. However, this is obviously not the case as seen with beautiful examples such as the Ring Nebula—they are in fact incredibly complex with rings, jets, and knots of gas and dust.

The team who made the discovery about Henize 2-428 are part of a larger task force setting out to show that these non-spherical, or "asymmetric" planetary nebulae can only be formed from a "binary system", consisting of two stars orbiting each other. The exact mechanics of how they go on to produce these intricate structures is still a fairly open question.

Dr Miguel Santander-Garcia is lead author of the study on Henize 2-428 now published in the science journal Nature. He told Sen: "It is unusual but not rare to have a binary system with two white dwarfs. So far we have found half a dozen white dwarf binaries, and estimates indicate they could make up 25 per cent of the total planetary nebulae with binary central stars."

The team first detected the binary system within Henize 2-428 using ESO's Very Large Telescope at the Paranal Observatory in Chile. Dr Henri Boffin, one of the co-authors of the study, oversaw the observations.

"We used the FORS2 spectrograph to take spectra of the central star of the planetary nebula. We did so on several occasions, and looked if there were any changes," Boffin told Sen. "We noted that there was clearly a signature of two stars, and not just one. Moreover, we also saw indications that the two stars were moving on short timescale. We had caught a close binary!"

The short orbital period of just 4 hours means they are very close to each other and it is therefore predicted that they will merge in around 700 million years time.

The total mass of the two combined is approximately 1.8 times that of the Sun (1.8 solar masses), making it the most massive of its type to date. This value is greater than the 1.4 solar mass Chandrasekhar Limit, which is the heaviest a white dwarf can be and still have all of its forces in balance.

As a result, it will undergo a "thermonuclear runaway" situation, whereby an increasing temperture due to the merger will lead to a change in the stellar conditions that causes a further increase in temperature, etc., creating an unstable system. The merged star will collapse in on itself before rebounding outwards in a violent Type Ia supernova explosion.

"That was a theoretical path to produce supernovae until now," said Santander-Garcia.

"The formation of Type Ia supernova is still a great mystery in astronomy," Boffin told us. "There are several theories proposed, but as always in science, theory needs to be backed-up by observations. Our observations show for the first time that such systems exist!"

An artist’s impression video showing the core of Henize 2-428 with its white dwarf binary system. As tht two stars merge, the system becomes very unstable and the result is a destructive Type Ia supernova. Credit: ESO/L. Calçada

Santander-Garcia is aware that the case is far from closed on Henize 2-428 and already knows how he would like to extend the project.

"First, we'd like to measure the mass with more accuracy to make sure it's over the Chandrasekhar Limit and confirm that it is indeed a supernova-producing merger," he told Sen. "If so, Einstein's General Relativity predicts that the orbit will progressively shrink because of the energy loss in the form of gravitational wave emission.

"I'd love to be able to measure a decrease in orbital period, but it's so slow that, even with current cutting-edge technology, it will take decades to detect even the slightest change."