Most Gold-rich Stars Formed Over 10 Billion Years Ago

By James Anderson •  Updated: 11/15/22 •  3 min read

The origin of so-called “gold-rich” stars, or stars with an abundance of heavy elements other than iron, such as gold and platinum, has been revealed by a team of researchers from the University of Notre Dame and Tohoku University.

Modern telescopes all over the world have found hundreds of stars that are rich in gold. Where, how, and when these stars were created during the Milky Way’s, our galaxy’s, history remained a mystery.

The team discovered that the majority of gold-rich stars originated in relatively small Milky Way progenitor galaxies more than 10 billion years ago, providing new insight into the stars’ early history.

R-process Enhanced Stars

Astronomers have mostly studied the presence of heavy elements by looking at the spectra of stars and figuring out how much of each chemical there is. While many elements in stars are created during the core-collapse supernova of a massive star or the thermonuclear supernova of a white dwarf, some elements appear to be created in even more unusual ways.

S-process enhanced and r-process enhanced stars, which stand for slow and rapid neutron capture, respectively, are examples of stars that contain these kinds of metals.

It is not known how r-process enhanced (RPE) stars are formed. The researchers examined a cosmological simulation of a Milky Way-like galaxy with r-process enrichment in this study.

High-Resolution Simulation

The team used a numerical simulation to follow the Milky Way’s formation from the Big Bang to the present in order to come to this conclusion. This simulation has the highest time resolution ever achieved, allowing it to precisely resolve the material cycle formed by stars in the Milky Way.

The simulation was created over several months using the ATERUI II supercomputer at the National Astronomical Observatory of Japan’s Center for Computational Science.

The first-ever analysis of the formation of gold-rich stars in the Milky Way was made possible by the simulation. According to the accepted cosmology it employed, the Milky Way expands through the merger and accretion of small progenitor galaxies.

Gold-rich Star Nucleosynthesis

The simulation results showed that some progenitor galaxies, which were formed more than 10 billion years ago, contained significant concentrations of the heaviest elements.

The abundance of the heaviest elements increased with each neutron star merger event in these dwarf galaxies, which are known locations for heavy element nucleosynthesis. The predicted abundances of the gold-rich stars that formed in these galaxies and the observed abundances of the stars today can be compared.

Today’s gold-rich stars tell us about the Milky Way’s history. The majority of gold-rich stars formed in dwarf galaxies over 10 billion years ago, according to researchers.

“These ancient galaxies are the building blocks of the Milky Way. Our findings mean many of the gold-rich stars we see today are the fossil records of the Milky Way’s formation over 10 billion years ago. Comparison with simulations and observations in the Milky Way opens a new avenue for extracting the fossil records of stars,”

said Yutaka Hirai, of Tohoku University.

Reference: Yutaka Hirai, Timothy C Beers, Masashi Chiba, Wako Aoki, Derek Shank, Takayuki R Saitoh, Takashi Okamoto, Junichiro Makino. Origin of highly r-process-enhanced stars in a cosmological zoom-in simulation of a Milky Way-like galaxy. Monthly Notices of the Royal Astronomical Society, Volume 517, Issue 4, December 2022, Pages 4856–4874

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