The Cosmic Dawn ended the cosmic ‘dark ages’ after the Big Bang with the first stars. But the masses of the first stars remains a cosmic mystery at the forefront of current research.
Large efforts in Australia (e.g., the Square Kilometre Array, SKA, or SkyMapper) and internationally (e.g., the James Web Space Telescope, JWST) try to find signatures of the first stars.
The early stars had up to several hundred solar masses. The earliest stars of 140–260 solar masses became Pair-Instability Supernovae (PISNe). PISNe would have left a unique chemical signature in the atmosphere of next-generation stars that is quite unlike that of Type II and Type Ia supernovae.
But there was no sign of this signature – until now. An international study published today in Nature outlines the first definitive association of a Galactic halo star with an abundance pattern originating from a PISN.
The study, involving Monash astrophysicist Professor Alexander Heger, shows that the chemically peculiar star (LAMOST J1010+2358) in the Galactic halo is clear evidence of PISNe from very massive first stars in the early Universe.
The study was led by Professor ZHAO Gang from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) and is a collaboration involving NAOC, Monash University, Yunnan Observatories of CAS, and the National Astronomical Observatory of Japan.
The study found that the most likely progenitor of the star is a 260-solar-mass PISN.
“This study provides an essential clue to constraining the initial mass function of stars in the early universe,” said Professor Heger.
“Before this study, there was no evidence of PISN in the first generation of stars,” he said.
Using the Subaru telescope, researchers conducted a follow-up high-resolution spectroscopic observation of J1010+2358. From this, they found the amounts of more than ten elements. The Subaru telescope was used to observe J1010+2358 again and calculate abundances for over ten elements.
This star has low sodium and cobalt abundances. Its sodium-to-iron ratio is below 1/100 that of the sun. This star also has a big difference in the abundance of elements with odd and even charge numbers, like sodium, magnesium, cobalt, and nickel.
“Sodium, magnesium, cobalt, and nickel abundances show a pattern unique to PISNe,” Professor Heger said.
“The peculiar odd-even variance, along with deficiencies of sodium and α-elements in this star, are consistent with the predicted chemical fingerprint of primordial PISN from first- generation stars with 260 solar masses.”
J1010+2358 proves the existence of PISNe. This supernova type is due to a hydrodynamical instability caused by electron–positron pair formation at the end of a very massive star’s life.
PISNe disrupt the entire star, leaving no remnant. A PISN explosion can be from a few up to a hundred times more powerful than a “normal” supernova. The explosion that made J1010+2358 was among the most energetic PISNe.
The iron abundance of LAMOST J1010+2358 ([Fe/H] = -2.42) is substantially greater than the most metal-poor stars in the Galactic halo, suggesting that second-generation stars created in the gas dominated by PISN ashes can be quite metal-rich.
“J1010+2358 may be the oldest star we know,” Professor Heger said. “The stars that make PISN have the shortest lifetimes, and the metal-rich gas they make can form the next generation of stars – those we observe – more swiftly than the metal-poorer gas that makes the stars known before. No star of the first generation has ever been found.”
“The identification of such a massive primordial star suggests that the first stars were more massive than stars forming in the present universe,” Professor Heger said. “This confirms why we never found long-lived low-mass primordial stars.”
Pair Instability Supernovae were first hypothesised more than 80 years ago. “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 corner stone in our understanding of how massive stars explode.”
PISNe play a critical role in our understanding of the birth masses of black holes. They determine what gravitational wave signals we can observe from black hole mergers.
“J1010+2358, for the first time, confirms the existence of primordial stars of several hundred solar masses,” Professor Heger said. “Stars in this mass range can end their lives as massive back holes that swallow almost the entire mass of the star instead of exploding,” he said.
"These may be the first stellar black holes in the universe.” The implied mass predictions give gravitational wave observers a guidance for what to look in the future.
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