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Artist’s impression of two white dwarf stars destined to merge and create a Type Ia supernova in 700-million years time. ESO/L. Calçada, CC BY-SA

White dwarf merger is set to prove supernova theory

Two white dwarfs found orbiting each other at the centre of a planetary nebula are now known to have enough mass that they will eventually trigger a special kind of supernova, according to research published today in Nature.

Finding a stellar pair in a planetary nebula is not too surprising. In fact the study, led by Miguel Santander-Garcia from the Observatorio Astronomico Nacional, Spain, was aimed at investigating asymmetric planetary nebulae to test whether their odd-shapes could be caused by the presence of two central stars.

Taking just four hours to orbit each other, the stars are close enough that within the next 700 million years the two are set to merge and combine their mass into a single star.

But what sets this discovery apart is that the combined mass of the stars is at least 1.5 solar masses, and more likely even higher at 1.76 solar masses. This value exceeds the maximum mass that can be contained in a white dwarf and remain stable.

Hence, when these two stars merge, the new star must collapse, triggering what is categorised by astronomers as a Type Ia supernova.

Burnt out suns

The stellar pair was found in the planetary nebula Henize 2-428 from observations taken with the European Southern Observatory’s Very Large Telescope at the Paranal Observatory in Chile.

They are the remains of stars like our sun. As old age approaches, an average-sized star will puff off its outer layers sending out intricate shells of gas.

What is left behind is called a white dwarf. It lights up the gas surrounding it to create the beautiful planetary nebula.

The strangely shaped planetary nebula Henize 2-428. The light from the two white dwarfs is combined to form the bright central star. ESO

A white dwarf is highly compressed and very dense. It has shrunk down to about the size of the Earth (for comparison, the sun’s diameter is currently more than 100 times that of the Earth).

The star is prevented from collapsing any further by pressure produced by what is known as electron degeneracy. In other words, the star is so compressed that the electrons themselves cannot find the space to move. This pressure balances the star’s inward pull of gravity.

Tipping point

But there is a limit to how much mass a white dwarf can contain and remain supported by electron degeneracy.

That limit is 1.4 solar masses, and it was determined by Indian-born American astrophysicist Subrahmanyan Chandrasekhar in 1930, and is now known as the Chandrasekhar-limit.

It was met with some controversy at the time. For it meant that if a star tipped over this limit then there was no way to prevent the star from completely collapsing in on itself.

This made it inevitable that bizarre objects such as neutron stars and black holes could actually exist.

Stellar explosions

Today, we have no such qualms imagining black holes and the events that trigger them. The most common is when a massive star (more than eight times the mass of our sun) can no longer sustain thermonuclear reactions, and hence it runs out of energy and is crushed by gravity.

The resulting supernova explosion is known as a core-collapse supernova. But a Type Ia supernova occurs when a white dwarf is pushed over the Chrandrasekhar-limit.

In the simplest scenario the white dwarf accretes gas from a companion star. When the limit is reached, the white dwarf explodes as a supernova, and the companion is left behind.

But a number of Type Ia supernovae do not fit this scenario. For example, the supernova remnant SNR 0509-67.5, contains no sign of a secondary star. It prompted the scenario that perhaps a supernova could be triggered by two white dwarfs colliding, which would destroy any evidence of their existence.

Smoking gun

Astronomers have so far identified five other white dwarf pairs. Follow-up observations of the stars in Henize 2-428 with the Mercator telescope in the Canary Islands, Spain, determined that these are the most massive pair of white dwarfs currently known.

What’s more, each star is massive, containing around 0.9 solar masses. This is backed up by their similar luminosities and temperatures.

Has the mystery of SNR 0509-67.5 in the Large Magellanic Cloud now been solved? NASA, ESA, CXC, SAO, B. Schaefer and A. Pagnotta, Hubble Heritage, J. Hughes

These observations give us confidence that white dwarf mergers should now be considered more than just theoretical.

Brightest supernova explores dark energy

Type Ia supernovae are the brightest of all supernovae. Such a supernova can outshine its entire galaxy and is easily seen across vast distances.

Furthermore, because they rely on the demise of a white dwarf, the flash of the explosion produces a burst of light that can be carefully determined.

It is these characteristics that make Type Ia supernovae so important for measuring cosmic distances and they have been crucial in uncovering dark energy.

Type Ia supernovae like SN 1994D can be seen across vast distances. High-Z Supernova Search Team, HST, NASA

This interesting discovery announced today not only helps improve our knowledge of how planetary nebulae obtain their structure, it could also lead to a better determination of dark energy, the mysterious force that is causing the expansion of the universe to accelerate.

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