Massive explosion explains star's mysterious metal elements

An international team of astronomers, led by the ARC Centre of Excellence in All Sky Astrophysics in 3 Dimensions (ASTRO 3D) at the Australian National University (ANU), have potentially discovered the first evidence of the destruction of a collapsed rapidly spinning star — a phenomenon they describe as a “magneto-rotational hypernova”.

The previously unknown type of cataclysm — 10 times more energetic than a supernova and occurring barely a billion years after the Big Bang — is the most likely explanation for the presence of unusually high amounts of some elements detected in another extremely ancient and ‘primitive’ Milky Way star (known as SMSS J200322.54-114203.3), which contains larger amounts of metal elements, including zinc, uranium, europium and possibly gold, than others of the same age. Neutron star mergers — the accepted sources of the material needed to forge them — are not enough to explain their presence.

The astronomers calculate that only the violent collapse of a very early star — amplified by rapid rotation and the presence of a strong magnetic field — can account for the additional neutrons required. Their research, published in the journal Nature, could thus be the answer to a 13-billion-year-old mystery.

“The star we’re looking at has an iron-to-hydrogen ratio about 3000 times lower than the Sun — which means it is very rare: what we call an extremely metal-poor star,” said study co-leader Dr David Yong, who is based at ANU.

“However, the fact that it contains much larger than expected amounts of some heavier elements means that it is even rarer — a real needle in a haystack.”

The first stars in the universe were made almost entirely of hydrogen and helium. At length, they collapsed and exploded, turning into neutron stars or black holes, producing heavier elements which became incorporated in tiny amounts into the next generation of stars — the oldest still in existence.

Rates and energies of these star deaths have become well known in recent years, so the amount of heavy elements they produce is well calculated. And, for SMSS J200322.54-114203.3, the sums just don’t add up.

“The extra amounts of these elements had to come from somewhere,” said study co-leader Associate Professor Chiaki Kobayashi, from the University of Hertfordshire in the UK.

“We now find the observational evidence for the first time directly indicating that there was a different kind of hypernova producing all stable elements in the periodic table at once — a core-collapse explosion of a fast-spinning strongly magnetised massive star. It is the only thing that explains the results.”

Hypernovae have been known since the late 1990s. However, this is the first time one combining both rapid rotation and strong magnetism has been detected.

“It’s an explosive death for the star,” Dr Yong said. “We calculate that 13 billion years ago J200322.54-114203.3 formed out of a chemical soup that contained the remains of this type of hypernova. No-one’s ever found this phenomenon before.”

J200322.54-114203.3 lies 7500 light-years from the Sun and orbits in the halo of the Milky Way.

Another co-author, Nobel Laureate and ANU Vice-Chancellor Professor Brian Schmidt, added, “The high zinc abundance is a definite marker of a hypernova, a very energetic supernova.”

Prof Gary Da Costa Da Costa, study co-leader and Head of the First Stars team at ASTRO 3D, explained that the star was first identified by a project called the SkyMapper survey of the Southern Sky.

“The star was first identified as extremely metal-poor using SkyMapper and the ANU 2.3 m telescope at Siding Spring Observatory in western NSW,” he said. “Detailed observations were then obtained with the European Southern Observatory 8 m Very Large Telescope in Chile.”

ASTRO 3D Director Professor Lisa Kewley concluded, “This is an extremely important discovery that reveals a new pathway for the formation of heavy elements in the infant universe.”

Image caption: The star SMSS J200322.54-114203.3 (centre, with crosshairs) in the south-eastern corner of the constellation Aquila (the Eagle) close to the border with Capricornus and Sagittarius. Image credit: Da Costa/SkyMapper.

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