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Birthplace of gold-rich stars finally revealed

The simulated Milky Way-like galaxy at present taken from the simulation produced in the study. (Image simulation by Takayuki Saitoh; visualization by Takaaki Takeda, courtesy of the Royal Astronomical Society).

Researchers at the University of Notre Dame and Tohoku University have revealed the birthplace of so-called “gold-rich” stars—with an abundance of heavy elements beyond iron, including gold and platinum.

In a paper published in the Monthly Notices of the Royal Astronomical Society, the scientists explain that most gold-rich stars formed in small progenitor galaxies of the Milky Way over 10 billion years ago. These findings shed light on the stars’ past for the first time.

To reach this conclusion, the team tracked the Milky Way’s formation from the Big Bang to the present with a numerical simulation. This simulation has the highest time resolution yet achieved or, in other words, it can precisely resolve the cycle of materials formed by stars in the Milky Way.

Using the ATERUI II supercomputer in the Center for Computational Science at the National Astronomical Observatory of Japan, the researchers were able to analyze the standard cosmology it uses and predict that the Milky Way grows by the accretion and merging of small progenitor galaxies.

The simulation data revealed that some of the progenitor galaxies contained large amounts of the heaviest elements. Each event of neutron star merger—a confirmed site of heavy element nucleosynthesis—increased the abundance of the heaviest elements in these small galaxies. The gold-rich stars formed in these galaxies and their predicted abundances can be compared with the observations of the stars today.

“The gold-rich stars today tell us the history of the Milky Way—we found most gold-rich stars are formed in dwarf galaxies over 10 billion years ago,” Yutaka Hirai, one of the study authors at Tohoku University, said in a media statement.

“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.”

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