Researchers discover new type of stellar collision

October 09, 2018

For three and a half centuries, astronomers have pondered a mystery: What did the French monk and astronomer Père Dom Anthelme see when he described a star that burst into view in June 1670, just below the head of the constellation Cygnus, the swan?

It was long thought to be a nova--a star that periodically brightens as it blows off mass. But now, an international team of astrophysicists, including two professors at the University of Minnesota, have cracked the 348-year-old conundrum. The monk witnessed the explosive merger of white and brown dwarf stars--the first ever identified.

The work, led by astrophysicists at Keele University (England), is published in the Monthly Notices of the Royal Astronomical Society.

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. 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.

"It was as if you put salsa fixings into a blender and forgot to put the lid on," said Charles Woodward, a physics and astronomy professor in the College of Science and Engineering at the University of Minnesota. "The white dwarf was like the blades at the bottom and the brown dwarf was the edibles. It was shredded, and its remains spun out in two jets--like a jet of goop shooting from the top of your blender as you searched frantically for the lid."

Woodward and fellow University of Minnesota physics and astronomy professor Robert Gehrz were members of the team that proposed studying the object and assisted in designing the program of observations, which were done at the Atacama Large Millimeter/submillimeter Array (ALMA) of telescopes in Chile.

Beneath the swan, an odd duck

The unusual star has been dubbed CK Vulpeculae, as it resides in the constellation Vulpecula (the little fox). 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.

The white dwarf and brown dwarf started out fairly ordinary--orbiting each other in a binary system, as astrophysicists 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 forms (isotopes) of chemical elements. It also caused the brightening that caught the eye of the monk Anthelme.

Rounding up the unusual suspects

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, the researchers 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 one percent the mass of the sun.

"That's too high for a classical nova outburst and too low for mergers of more massive stars, as had been proposed earlier," said Sumner Starrfield, a professor at Arizona State University who was involved in the study.

That evidence, plus isotope data, led to the conclusion that the collision was between a white dwarf and brown dwarf. And the remnant star is still blowing off material.

"Collisions like this could contribute to the chemical evolution of our galaxy and universe," noted Minnesota's Gehrz. "The ejected material travels out into space, where it gets incorporated into new generations of stars."
Other institutions involved were the University of Manchester and University of South Wales, United Kingdom; Ohio State University; the University of Warmia and Mazury, Poland; and the South African Astronomical Observatory.

The research supporters include NASA, the United States Air Force, the National Science Foundation, National Research Council (Canada), Ministry of Science and Higher Education (Poland), Comisión Nacional de Investigación Científica y Tecnológica (Chile), Ministerio de Ciencia, Tecnolog´?a e Innovaci´on Productiva (Argentina), Minist´erio da Ci?encia, Tecnologia e Inova¸c?ao (Brazil), National Institutes of Natural Sciences (Japan), Ministry of Science and Technology (Taiwan), Academia Sinica Institute of Astronomy and Astrophysics (Taiwan), and Korea Astronomy and Space Science Institute (Republic of Korea).

To read the full research paper, entitled "ALMA reveals the aftermath of a white dwarf--brown dwarf merger in CK Vulpeculae," visit the Monthly Notices of the Royal Astronomical Society website.

University of Minnesota

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