Using the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers found evidence that a white dwarf and a brown dwarf collided in a ‘blaze of glory’ that was witnessed on Earth in 1670 as a ‘new star,’ or nova.
CK Vulpeculae resides in the constellation Vulpecula. It is just below the star Albireo, the head of Cygnus, the swan. That location is inside the Summer Triangle of bright stars, which is now high in the south after nightfall. The star is approximately 2,200 light-years from Earth. Image credit: ALMA / ESO / NAOJ / NRAO / Eyres et al.
On 20 June 1670, French monk and astronomer Père Dom Anthelme spotted a nova just below the head of the constellation Cygnus.
When it first appeared it was easily visible with the naked eye; over the subsequent two years the flare varied in brightness and disappeared and reappeared twice, before finally vanishing from view for good.
Modern astronomers studying the remains of this cosmic event initially thought it was triggered by the merging of two main-sequence stars — stars on the same evolutionary path as our Sun.
This ‘new star’ was long referred to as Nova Vulpeculae 1670, and later became known as CK Vulpeculae.
However, we now know that CK Vulpeculae was not what we would today describe as a nova, but is in fact the merger of two stars — a white dwarf and a brown dwarf.
“White dwarfs are the remnants of stars like the Sun at the end of its life, while brown dwarfs are ‘failed stars’ that have 15-75 times the mass of Jupiter, but not enough to ignite the thermonuclear fusion reactions that power the Sun and other stars,” explained University of South Wales astronomer Stewart Eyres and co-authors.
“The two stars orbited each other until they got too close and merged, spewing out debris whose chemical composition gave away the secret of the mystery object’s origin. The brown dwarf got the raw end of the deal.”
The white dwarf and brown dwarf started out fairly ordinary — orbiting each other in a binary system, as astronomers believe most stars are born. The white dwarf had an estimated 10 times the brown dwarf’s mass.
As they merged, the brown dwarf was torn apart and its remains dumped on the surface of the white dwarf.
That star’s crushing gravity heated the brown dwarf material and caused thermonuclear ‘burning’ that spilled out a cocktail of molecules and unusual isotopes of chemical elements.
CK Vulpecula isn’t visible to the naked eye, but through the telescope, the debris ejected during the merger appears as two bright rings of dust and gas that form a glowing hourglass structure around a compact central object.
Studying the light from two background stars that had passed through the system, Dr. Eyres and colleagues noted the presence of lithium, a light element that can’t exist in the interiors of stars, where nuclear fusion occurs.
They also found organic molecules like formaldehyde and methyl alcohol, which also would perish in stellar interiors. Thus, these molecules must have been produced in the debris from the collision.
The amount of dust in the debris was about 1% the mass of the Sun.
“The material in the hourglass contains the element lithium, normally easily destroyed in stellar interiors,” Dr. Eyres said.
“The presence of lithium, together with unusual isotopic ratios of the elements C, N, O, indicate that an (astronomically!) small amount of material, in the form of a brown dwarf star, crashed onto the surface of a white dwarf in 1670, leading to thermonuclear ‘burning,’ an eruption that led to the brightening seen by the Carthusian monk Anthelme and the astronomer Hevelius, and in the hourglass we see today.”
“Stellar collisions are the most violent events in the Universe. Most attention is given to collisions between neutrons stars, between two white dwarfs — which can give a supernova — and star-planet collisions,” said co-author Professor Albert Zijlstra, from the University of Manchester.
“But it is very rare to actually see a collision, and where we believe one occurred, it is difficult to know what kind of stars collided. The type we believe that happened here is a new one, not previously considered or ever seen before. This is an extremely exciting discovery.”
“Collisions like this could contribute to the chemical evolution of our Galaxy and Universe,” said co-author Professor Robert Gehrz, from the University of Minnesota.
“The ejected material travels out into space, where it gets incorporated into new generations of stars.”
The research was published in the Monthly Notices of the Royal Astronomical Society.
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S.P.S. Eyres et al. ALMA reveals the aftermath of a white dwarf-brown dwarf merger in CK Vulpeculae. MNRAS, published online September 26, 2018; doi: 10.1093/mnras/sty2554
