Study finds peculiar chemical footprint from one of the first stars in the universe

The team used data from telescopes in China and the US to uncover chemical evidence from one of the earliest stars in the universe. 

The life cycle of a star 

The life cycle of a star is determined by its mass. Low-mass stars, like the Sun, eventually cool down and end up as white dwarfs. On the other hand, heavy-mass stars go out in a supernovae explosion and end up as neutron stars or black holes. 

But for the earliest stars in the universe, we don't have any information about how their life cycle ended. According to numerical simulations, the mass of the early stars is estimated to have reached up to several hundred times the mass of our Sun. 

These first-generation stars, also known as primordial stars, were believed to be massive, ranging from 140 to an astonishing 260 times the mass of our Sun. They were composed solely of hydrogen and helium, lacking the heavier elements synthesized by later stars.

Their life ended in an explosion very different from the supernova explosion we know today, called the pair-instability supernovae (PISNe). 

According to astronomers, PISNes are very different from ordinary supernovae and leave a unique chemical signature in the atmosphere of next-generation stars. Until now, these signatures have not been found.

LAMOST J1010+2358

The researchers analyzed data from two high-resolution telescopes, the Subaru Telescope in Hawaii and the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) near Beijing. 

The team examined the chemical footprints of these early stars in the halo of our Milky Way galaxy. In particular, they focused on a star named LAMOST J1010+2358. The star exhibited a peculiar chemical footprint. 

According to the team's data, J1010+2358 has extremely low sodium and cobalt abundances, with the sodium to iron ratio over 100 times lower than the Sun. Additionally, the star showcased significant variations in abundance between odd-charge-number and even-charge-number elements, such as sodium/magnesium and cobalt/nickel.

In a press release, Dr. Xing Qianfan, the first author of the study, said, "The peculiar odd-even variance, along with deficiencies of sodium and α-elements in this star, are consistent with the prediction of primordial PISN from first-generation stars with 260 solar masses."

Additionally, the iron abundance of J1010+2358 is significantly greater than that of the most metal-poor stars in the galactic halo. This indicates that the second-generation stars created in the PISN-dominated gas may be more metal-rich than anticipated.

The observation of J1010+2358's peculiar chemical composition gives strong evidence for the existence of very massive stars in the early universe. It supports the predicted chemical signatures, from numerical simulations, of PISNe resulting from stars exceeding 140 solar masses. 

The findings of this research are fundamental in astronomy, astrophysics, and cosmology. The lives and deaths of the earliest stars contribute to our knowledge of the chemical evolution of galaxies, star formation, and element formation.

The findings of the study are published in the journal Nature.

Study abstract:

The most massive and shortest-lived stars dominate the chemical evolution of the pre-galactic era. On the basis of numerical simulations, it has long been speculated that the mass of such first-generation stars was up to several hundred solar masses. The very massive first-generation stars with a mass range from 140 to 260 solar masses are predicted to enrich the early interstellar medium through pair-instability supernovae (PISNe). Decades of observational efforts, however, have not been able to uniquely identify the imprints of such very massive stars on the most metal-poor stars in the Milky Way. Here we report the chemical composition of a very metal-poor (VMP) star with extremely low sodium and cobalt abundances. The sodium with respect to iron in this star is more than two orders of magnitude lower than that of the Sun. This star exhibits very large abundance variance between the odd- and even-charge-number elements, such as sodium/magnesium and cobalt/nickel. Such peculiar odd–even effect, along with deficiencies of sodium and α elements, are consistent with the prediction of primordial pair-instability supernova (PISN) from stars more massive than 140 solar masses. This provides a clear chemical signature indicating the existence of very massive stars in the early universe.