Star really is made of stardust

Artist’s impression of LS IV-14 116.

An artist’s impression of the star LS IV-14 116. The white clouds are rich in zirconium and sit above the blue surface of the star. Artwork by Natalie Behara.

  • Star 2,000 light-years away is rich in zirconium
  • Zirconium is used on Earth to make fake diamonds
  • The star has enough to make 4 billion diamonds

USING DATA OBTAINED with the Anglo-Australian Telescope at the Siding Spring Observatory in New South Wales, a team of astronomers has found what at first sight appears to be the most zirconium-rich star ever discovered.

Zirconium, the material used by jewellers to make false diamonds, glitters in clouds above the star’s surface.

The team— led by graduate student Naslim Neelanamkodan and her supervisor Dr Simon Jeffery from Armagh Observatory in Northern Ireland—made the discovery while looking for chemical clues that explain why a small group of stars reaching the end of their lives, known as helium-rich hot subdwarfs, have much less hydrogen on their surfaces than other similar stars.

The team looked at a star called LS IV-14 116, located 2,000 light-years from Earth.

The scientists used the telescope’s instruments to split the light of the star into a spectrum. Different elements and molecules give rise to characteristic patterns in such spectra, enabling scientists to determine identify which ones are present. Each one produces its own spectral “line”.

As expected, the spectrum of LS IV-14 116 had the usual lines arising from more common elements, but other strong lines were less easy to identify.

A careful study showed four of these lines were due to a form of zirconium that only exists at temperatures above 20,000 degrees Celsius and that had never previously been found in the spectrum of an astronomical body.

Team member Professor Alan Hibbert, from Queen’s University Belfast, computed what the zirconium atom line strengths would be like. And from that, the team calculated the zirconium abundance in LS IV-14 116 to be 10,000 times higher than in the Sun (meaning that one atom in every 200,000 is zirconium rather than one in 2 billion).

Anglo-Australia Telescope

The research was conducted using data gathered with the Anglo-Australian Telescope in New South Wales. Photo by Barnaby Norris.

Further work showed the remaining unidentified lines to come from strontium, germanium and yttrium, and these elements are found to be between 1,000 and 10,000 times more abundant than normal.

Four billion tonnes of fake diamonds

The team thinks that the unusual abundances in LS IV-14 116 are caused by the formation of cloud layers in the star’s atmosphere—the only part of a star that can be seen directly.

High concentrations of certain elements, mainly metals heavier than calcium, build up in these clouds, but the same elements are scarce in layers above and below, meaning that their overall abundance is near normal.

The team also suggests that the star is shrinking from being a bright, cool giant star to a faint, hot subdwarf. As the star shrinks, different elements sink down or float up in the atmosphere to a region where they become highly visible, making the apparent composition very sensitive to the star’s recent history.

Most stars like the Sun have about ten zirconium atoms for every million silicon atoms. LS IV-14 116 has two million zirconium atoms for every one million silicon atoms.

It is estimated that the zirconium layer seen in LS IV-14 116 would weigh about 4 billion tons, or 4,000 times the world’s annual production of zirconium.

“It was very exciting to discover these completely new chemical signatures in our data,” said Dr Simon Jeffery. “The peculiar abundances measured in this star, and hopefully in others, offer a new tool to explore a stage of stellar evolution which is extremely difficult to observe directly.”

“The huge excess of zirconium was a complete surprise,” said Naslim. “We had no reason to think this star was more peculiar than any other faint blue star discovered so far.”

These stars are all freaks,” adds Dr Simon O’Toole of the Australian Astronomical Observatory in Sydney, and an expert on hot subdwarfs.

“In these hot subdwarfs the metals actually float to the surface of the star, pushed out by radiation pressure,” Dr O’Toole explained. “You can get nickel-crusted stars, lead-crusted stars, tin-crusted stars.”

But he said that LS IV-14 116 was the first one he’d heard of with an outer layer of zirconium.

The scientists have published their results in the journal Monthly Notices of the Royal Astronomical Society.

Adapted from information issued by RAS / AAO.

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