In the last five years, hundreds of gold-rich stars have been discovered by advanced telescopes around the world. New galaxy formation simulations with the highest temporal and mass resolution show that these stars formed more than 10 billion years ago in small galaxies that converged to form the Milky Way.
Stars are thermonuclear forges that produce most of the elements that make up planets. Most elements heavier than iron, including precious metals such as gold and platinum, are captured by the fast neutron capture process (the process right). When stars die, they release the heavy elements they create into interstellar space, which can then be incorporated into the next generation of objects.
Like planets, new stars contain heavy elements released by previous generations. By studying the chemical composition of stars, we should be able to deduce the type of environment in which the stars formed. The mystery was when, where and how these stars formed in the history of the Milky Way. So far, there has been no theoretical framework to explain the observed chemical diversity.
Reconstructing the history of the Milky Way with a supercomputer
An international team of researchers led by Yutaka Hirai from Tōhoku University (Japan) and the University of Notre Dame (Indiana, USA) has traced the formation of a virtual galaxy similar to the Milky Way from the Big Bang to the present day using a numerical simulation. This simulation has the highest time and mass resolutions yet, allowing the team to study the cycling of new materials released from old stars and absorbed by new ones. These results (free access on arXiv) were published on November 14th in the Monthly Bulletins of the Royal Astronomical Society.
Using the Aterui II supercomputer at the Center for Computational Astrophysics of the National Astronomical Observatory of Japan, the team successfully ran the simulation for several months, enabling the first analysis of gold-rich star formation in the Milky Way.
Stellar gems produced in very old galaxies
The standard cosmology used by the researchers predicts that the Milky Way grew through the accretion and merger of small galaxies called progenitor galaxies. Simulation data showed that some, but not all, progenitor galaxies contained large amounts of the heaviest elements. Each neutron star merger event – a confirmed site of heavy element nucleosynthesis – has increased the abundance of the heavier elements in these smaller galaxies. According to research, most of the gold-rich stars formed in these progenitor galaxies more than 10 billion years ago. The predicted abundance of gold-enriched stars in the final Milky Way-sized galaxy is consistent with what is actually observed in our galaxy today.
Looking to the future, Hirai and his team want to use the new Fugaku supercomputer to simulate the formation of the Milky Way and clarify the origin of individual stars.