An extremely important discovery reveals a new pathway for the formation of heavy elements in the infant universe.
A ‘magneto-rotational hypernova’ soon after the Big Bang fuelled high levels of uranium and zinc in an ancient red giant –a stellar oddity. The massive explosion from a previously unknown source – 10 times more energetic than a supernova – could be the answer to a 13-billion-year-old Milky Way mystery.
First Evidence of a “Magneto-Rotational Hypernova”
Astronomers led by David Yong, Gary Da Costa and Chiaki Kobayashi from Australia’s ARC Centre of Excellence in All Sky Astrophysics in 3 Dimensions (ASTRO 3D) based 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 – which occurred 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.
One the Oldest Surviving Stars in the Universe
That star, a red giant known as SMSS J200322.54-114203.3, contains larger amounts of metal elements, including zinc, uranium, europium and possibly gold, than others of the same age. Pristine stars like SMSS J200322.54-114203.3 are among the oldest surviving stars in the Universe found near the center of the Milky Way, surviving members of the fundamental building blocks that assembled our galaxy’s inner bulge. They contain only trace amounts of heavy elements produced by nucleosynthesis and explosions of the first generation of massive stars.
Some of the oldest stars in the Milky Way shown below (ESO / Digitized Sky Survey 2 / Davide De Martin)
Sky Mapper on the Case
Using a specially-built, 1.3-meter telescope at Siding Spring Observatory, the SkyMapper Southern Sky Survey is creating a deep, multi-epoch, multi-colour digital survey of the entire southern sky, including discovering the oldest stars in the Galaxy like SMSS J200322.54-114203.3.
Unique Chemical Composition
“Recent simulations predict that the oldest surviving stars may be preferentially found in the inner bulge,” David Yong told The Daily Galaxy. “However,” he explained, “these stars are incredibly rare. The SkyMapper survey includes some 600,000,000 objects. The team has selected 26,000 candidate metal-poor stars. The most promising 2,600 stars have been observed at low spectral resolution. The best 150 stars were observed at high spectral resolution. Only SMSS J200322.54-114203.3 shows the high-nitrogen, high-zinc, high heavy element signature. Recent simulations predict that the oldest surviving stars may be preferentially found in the inner bulge.”
Mergers of binary neutron stars – which produce the majority of heavy elements like gold in the universe – are still not enough to explain the unique chemical composition of SMSS J200322.54-114203.3. 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 observed chemical abundances.
“The star we’re looking at has an iron-to-hydrogen ratio about 3000 times lower than the Sun – which means it is a very rare: what we call an extremely metal-poor star,” said Dr Yong, who is based at the 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.
The Sums Just Don’t Add Up
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 Associate Professor Chiaki Kobayashi from the University of Hertfordshire, 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-magnetized massive star. It is the only thing that explains the results.”
A Different Type of Hypernova
Hypernovae have been known since the late 1990s. However, this is the first time one combining both rapid rotation and strong magnetism has been hypothesized.
“It’s an explosive death for the star,” said Yong. “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.
“The high zinc abundance is a definite marker of a hypernova, a very energetic supernova,” adds co-author, Nobel Laureate and ANU Vice-Chancellor Professor Brian Schmidt.
Head of the First Stars team in ASTRO 3D, Gary Da Costa from ANU, explained that the star was first identified by the SkyMapper survey of the southern sky. “The star was first identified as extremely metal-poor using SkyMapper and the ANU 2.3m telescope at Siding Spring Observatory in western NSW,” he said. “Detailed observations were then obtained with the European Southern Observatory 8m Very Large Telescope in Chile.”
In an email to The Daily Galaxy, Da Costa noted that “SkyMapper struggles a bit to identify extremely metal-poor (EMP) star candidates in the inner Bulge because of the high reddening and image crowding (Siding Spring Observatory does not have the excellent seeing that sites in Chile or Hawaii have), plus it is only a 1.3m telescope.”
Image credit top of page: Hypernova, drawing by NASA/GSFC/Dana Berry