At the heart of the unusual planetary nebula Henize 2-428, a group of astronomers led by Dr Miguel Santander-Garcia from the Observatorio Astronomico Nacional and the Instituto de Ciencia de Materiales de Madrid has discovered two massive white dwarf stars that are expected to merge and trigger a Type Ia supernova in approximately 700 million years.
Artist’s impression of the planetary nebula Henize 2-428; the size of the central stars has been exaggerated to better illustrate their appearance. Image credit: Gabriel Perez / SMM / IAC.
The pair of white dwarfs has a total mass of 1.76 times that of the Sun – the most massive such pair yet found.
As the two stars orbit each other, they are expected to slowly get closer and closer, and when they merge, about 700 million years from now, they will contain enough material to ignite a Type Ia supernova.
This type of supernovae occurs when a white dwarf star acquires extra mass — either by accretion from a stellar companion or by merging with another white dwarf.
Once the mass exceeds the Chandrasekhar limit the star loses its ability to support itself and starts to contract. This increases the temperature and a runaway nuclear reaction occurs and blows the star to pieces.
Dr Santander-Garcia and his colleagues actually set out to try to solve a different problem. They wanted to find out how some stars produce such strangely shaped and asymmetric nebulae late in their lives.
“We wanted to answer the, still unresolved, question of how a bipolar nebula can form from an essentially spherical star,” said Dr Santander-Garcia, who is a co-author of a paper published in the journal Nature.
One of the objects the scientists studied was a mysterious nebula called Henize 2-428.
This image of Henize 2-428 was obtained using the Very Large Telescope at the Paranal Observatory in Chile; in the heart of this colorful nebula lies a unique object consisting of two white dwarf stars, each with a mass a little less than that of the Sun; these stars are expected to slowly draw closer to each other and merge in around 700 million years. Image credit: ESO.
“When we looked at this object’s central star with ESO’s Very Large Telescope, we found not just one but a pair of stars at the heart of this strangely lopsided glowing cloud,” said co-author Dr Henri Boffin of the European Southern Observatory.
“This supports the theory that double central stars may explain the odd shapes of some of these nebulae, but an even more interesting result was to come.”
The astronomers found that each of the stars has a mass slightly less than that of our Sun and that they orbit each other every 4 hours.
The two stars are sufficiently close to one another that, according to the Einstein’s theory of general relativity, they will grow closer and closer, spiraling in due to the emission of gravitational waves, before eventually merging into a single star within the next 700 million years.
The resulting star will be so massive that nothing can then prevent it from collapsing in on itself and subsequently exploding as a supernova.
“Until now, the formation of supernovae Type Ia by the merging of two white dwarfs was purely theoretical. The pair of stars in Henize 2-428 is the real thing,” said co-author Dr David Jones from the Instituto de Astrofisica de Canarias and the Universidad de La Laguna, Tenerife, Spain.
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M. Santander-Garcia et al. The double-degenerate, super-Chandrasekhar nucleus of the planetary nebula Henize 2-428. Nature, published online February 09, 2015; doi: 10.1038/nature14124
