Evidence for very massive stars in the early universe

Astronomers report in the journal Nature that they have discovered a star on the outskirts of the Milky Way with a chemical composition unlike anything they have seen before.

It matches the theoretical expectations of the chemical fingerprint left by very early and very massive stars. This is the clearest evidence yet that the first stars included very massive stars. The first stars were born from clouds of gas containing only hydrogen and helium, and nuclear fusion inside stars and supernova explosions created new elements, the first steps in the formation of a diverse world of matter. Theories predict that the first stars may have included many very massive stars rarely seen in the Universe today. Stars greater than 140 times the mass of the Sun may have changed the environment of the Universe with intense ultraviolet radiation and may have had a significant impact on the formation of the next generation of stars by highly energetic supernovae.

However, there is a lack of clear observational evidence for the existence of such supernovae caused by very massive stars. Great efforts have been made to observe very old stars in the Milky Way, along with observations of distant galaxies and intergalactic matter. Some of the old stars were born from gas clouds that captured elements ejected by the first stars, and their chemical compositions record the material produced by the first supernovae. Since PISNe caused by very massive stars produces chemical compositions that are very different from those of ordinary core-collapse supernovae, we can hope to identify the signature of very massive stars among the old stars. A team of astronomers from the National Astronomical Observatory of Japan (NAOJ), the National Astronomical Observatories of China (NAOC) and other institutes have conducted studies using the Chinese survey telescope LAMOST to identify early generation stars in the Milky Way and measured their Detailed chemical compositions using the Subaru Telescope.

Among them, they have discovered the star J1010+2358 with characteristic chemical compositions produced by a pair instability supernova. This is the clearest trace of such supernovae found to date, and strongly supports the theory that stars with masses more than 140 times the mass of the Sun did indeed form in the early Universe.

The discovery of J1010+2358 is direct evidence for hydrodynamic instability due to the production of electron-positron pairs in the theory of the evolution of very massive stars. The creation of electron-positron pairs reduces the thermal pressure inside the core of a very massive star and leads to a partial collapse. “It provides an essential clue to constrain the initial mass function in the early universe,” says Professor ZHAO Gang, corresponding author of the study. “Prior to this study, no evidence of supernovae from stars this massive had been found in metal-poor stars.”