A centuries-old astronomical mystery is solved


What was thought to be the formation of a star was actually a cataclysmic collision. Ben Lewis reports.


The hourgalss shape of CK Vulpeculae.

The hourgalss shape of CK Vulpeculae.

European Southern Observatory

Nearly 350 years ago, the French monk and astronomer Voituret Anthelme joined others around the world watch a new star wink into existence. Over the next two years, skywatchers observed the bright pinprick of light fade and reappear twice, before disappearing entirely from view.

Centuries later, astronomers from Arizona State University in the US and collaborators around the world have discovered that what at the time was thought to be the birth of the new star, called a nova, was actually the collision between a white dwarf and a brown dwarf star.

Using the huge Atacama Large Millimetre/submillimetre Array (ALMA) radio telescope in Chile, the team explored the debris left over after the encounter to establish the story of what Anthelme and others saw all those years ago. That investigation has been published in the journal Monthly Notices of the Royal Astronomical Society.

What has become known as CK Vulpeculae was not a collision and merger between two main sequence stars – the type which includes the sun – but a massive crunch between a dying star – the white dwarf – and a “failed” star without sufficient mass to sustain thermonuclear fusion – the brown dwarf – which produced two huge rings of debris.

According to the researchers, led by Stewart Eyres, the white dwarf would have been about 10 times more massive than the brown dwarf, though much smaller in size.

Orbiting each other as a binary system, the brown dwarf spiralled inwards, drawn by its denser partner, the intense forces of which ripped it apart.

When the objects collided, they spilled out a cocktail of molecules and unusual isotopes, which let the researchers gain new insights into the nature of the phenomenon.

“This is the first time such an event has been conclusively identified,” says Sumner Starrfield, an astronomer from Arizona State University.

The debris is still visible, and forms two rings of dust and gas joined by a compact central object and looks remarkably like an hourglass. It contains the tell-tale signatures of lithium, isotopes of carbon, nitrogen and oxygen, and organic molecules such as formaldehyde and methanamide.

The chemicals were spotted thanks to light from more distant stars shining through the dust clouds.

“These organic molecules, which we could not only detect with ALMA, but also measure how they were expanding into the surrounding environment, provide compelling evidence of the true origin of this blast,” says Starrfield.

Eyres adds: “The presence of lithium, together with unusual isotopic ratios of the elements carbon, nitrogen, and oxygen point to material from a brown dwarf star being dumped on the surface of a white dwarf.

“The thermonuclear ‘burning’ and an eruption of this material resulted in the hourglass we see today.”

Interestingly, the formaldehyde and methanamide compounds would not survive in an environment undergoing nuclear fusion. The researchers suggest they must have been produced in the debris from the explosion.

However, this opens up a new understanding not only of the history of astronomy, but also of the formation of planetary systems. With organic molecules being present in the debris of star collisions, it’s feasible that they could already exist before planets begin coalescing.

“Such collisions are probably not rare and this material will eventually become part of a new planetary system, implying that they may already contain the building-blocks of organic molecules as they are forming,” says Starrfield.

  1. https://en.wikipedia.org/wiki/Anthelme_Voituret
  2. http://dx.doi.org/10.1093/mnras/sty2554
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