Distribution of stars and gas in a galaxy simulation. Yellow dots represent stars and light blue dots represent gas. (Credit: Yutaka Hirai)
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Recently, hundreds of gold-rich stars have been detected by state-of-the-art telescopes around the world. New simulations of galaxy formation, with the highest resolution in both time and mass, show that these gold-rich stars formed in progenitor galaxies, small galaxies that merged to create the Milky Way.
Stars are thermonuclear chemical factories that produced most of the elements incorporated into the planets. Most elements heavier than iron, including precious metals such as gold and platinum, arose from the fast process of neutron capture (r-process). When stars die, they release the heavy elements they have created into interstellar space where the elements can be reincorporated into the next generation of objects. Like planets, new stars also incorporate the 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. But until now, a theoretical framework to explain the observed chemical diversity has been lacking.
Now, an international research team, led by Yutaka Hirai of Tohoku University and the University of Notre Dame, has tracked the formation of a virtual Milky Way-like galaxy from the Big Bang to the present day with a numerical simulation. This simulation has the highest time and mass resolutions to date, allowing the team to investigate the cycle of new materials released by old stars and absorbed into new ones.
Using the ATERUI II supercomputer at the Japan National Astronomical Observatory’s Center for Computational Astrophysics, the team successfully ran the simulation over several months, making it possible for the first time to analyze the formation of gold-rich stars in the Milky Way
According to the research, most of the gold-rich stars formed more than 10 billion years ago in small galaxies, known as progenitor galaxies. Some, but not all, progenitor galaxies undergo neutron star fusion, where large amounts of r-process heavy elements are produced and released, enriching that particular small galaxy. The predicted abundance of gold-enriched stars in the final Milky Way-sized galaxy matches what is actually observed.
“The study’s findings open a new avenue for mining the fossil record of stars,” says Hirai. “Gold-Rich Stars Today Tell Us the Story of the Milky Way.”
Looking ahead, Hirai and his team plan to simulate the formation of the Milky Way and clarify the origins of individual stars with the help of the new Fugaku supercomputer.
These findings were published as Hirai et al. “Heavily enhanced star origin by the r process in a cosmological expansion simulation of a Milky Way-like galaxy” in Monthly Notices of the Royal Astronomical Society 14 November 2022.
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