First chemical signature of a Pair-Instability Supernova from massive first stars

An unusual star has provided chemical evidence of the universe’s very massive first stars.

The first stars in the universe ended the period after the Big Bang, known as the cosmic ‘dark ages.’ The emergence of the first stars is referred to as the Cosmic Dawn, and the mass of these stellar pioneers is a mystery at the forefront of current cosmological research.

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Astronomers believe that the earliest stars were up to several hundred times the mass of the sun (one of our suns = one solar mass).

Stars of 140-260 solar masses are undergo Pair-Instability Supernovae, or PISNe. These stellar explosions would have left unique chemical signatures in the atmosphere of the stars that formed out of the PISNe.

For the first time, just such a PISNe signature has been found.

An international study published in Nature outlines the first definitive evidence for a star formed out of a PISNe. The star, LAMOST J1010+2358, exists in the galactic halo of the Milky Way and may be the oldest second-generation star recorded.

It was already known from the Large sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST) that LAMOST J1010+2358 is a very poor-metal star. These rare stars have low iron abundance.

The analysis of J1010+2358 used 2015 data from the Subaru Telescope of the National Astronomical Observatory of Japan (NAOJ), located at the Mauna Kea Observatory on Hawaiʻi.

Through examination of the optical and infrared spectrum of J1010+2358, the researchers determined that the star’s elemental make up was even more strange than they’d known. Amounts of more than 10 elements were calculated.

It has low levels of sodium and cobalt. The level of sodium to iron is less than one-hundredth that of our sun. J1010+2358 also has a big difference in the abundance of ions with odd and even charge numbers, like sodium, magnesium, cobalt, and nickel.

“Sodium, magnesium, cobalt, and nickel abundances show a pattern unique to PISNe,” says co-author and Monash University professor Alexander Heger.

“The peculiar odd-even variance, along with deficiencies of sodium and α-elements [isotopes of elements where the number of protons and neutrons is a multiple of four] in this star, are consistent with the predicted chemical fingerprint of primordial PISN from first-generation stars with 260 solar masses,” says Heger.

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“J1010+2358 may be the oldest star we know.

“The stars that make PISN have the shortest lifetimes, and the metal-rich gas they make can form the next generation of stars more swiftly than the metal-poorer gas that makes the stars known before. No star of the first generation has ever been found.”

Heger also explains that PISNe, which were first hypothesised more than 80 years ago, are well understood theoretically, but a trace of this kind of stellar explosion had never been identified before.

“They are the only type of supernovae we fully understand how they work. Yet they are also the only type we have never uniquely identified before. This discovery is a cornerstone in our understanding of how massive stars explode.”