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Gold-rich stars as Milky Way ‘fossils’

Gold-rich stars: A gold-colored spiral galaxy.
Simulated Milky Way-like galaxy at the present time, taken from the simulation produced in the study of gold-rich stars. Simulation: Takayuki Saitoh (Kobe University/Tokyo Tech ELSI). Visualization: Takaaki Takeda (VASA Entertainment Co. Ltd.). Image via Royal Astronomical Society (CC BY 4.0).

Gold born in supernovae and neutron star collisions

Where does gold come from? It comes from the ground, of course, but Earth’s gold originated in outer space. It’s produced in supernovae (exploding stars), and in neutron star collisions, in a set of nuclear reactions known to scientists as the r-process. Astronomers using state-of-the-art telescopes have discovered hundreds of gold-rich stars in our Milky Way galaxy. But they’ve wanted to understand when, where, and how these stars formed. On November 14, 2022, the Royal Astronomical Society – based in London – announced a new numerical simulation, showing that most gold-rich stars were formed in low-mass dwarf galaxies that merged with our Milky Way some 10 billion years ago.

They said this new work sheds light on these stars’ past for the first time. And they pointed out that gold-rich stars compose a “fossil record,” thus giving astronomers a new tool to use in probing our galaxy’s history.

The scientists conducting the study are from the University of Notre Dame and Tohoku University. The research appears in the peer-reviewed scientific journal Monthly Notices of the Royal Astronomical Society.

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Tracing Milky Way history

These astronomers are using the term “gold-rich” to describe stars that contain high abundances of heavy elements (those heavier than iron), including what they called “jewelry store elements,” gold and platinum.

They used the ATERUI II supercomputer in the Centre for Computational Science at the National Astronomical Observatory of Japan to produce their numerical simulation. They said it tracks the Milky Way’s formation from the Big Bang to the present, with a high level of detail with respect to time. They said:

This simulation has the highest time resolution yet achieved: it can precisely resolve the cycle of materials formed by stars in the Milky Way …

The standard cosmology it used predicts that the Milky Way grows by the accretion and merging of small progenitor galaxies [the small galaxies that merged with our Milky Way to create the large galaxy we live in today].

Gold-rich stars as ‘fossils’

The simulation data revealed that some of the progenitor galaxies – those that existed over 10 billion years ago – contained large amounts of the heaviest elements. Those heavy elements came from neutron star mergers, they said, a known source of heavy element creation. They said:

The gold-rich stars formed in these galaxies, and their predicted abundances can be compared with the observations of the stars today.

Yutaka Hirai of Tohoku University commented:

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. 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.

And he added:

Comparison with simulations and observations in the Milky Way opens a new avenue for extracting the fossil records of stars.

Bottom line: New computer simulations are showing that most gold-rich stars were formed in low-mass dwarf galaxies that merged with our Milky Way galaxy some 10 billion years ago.

Source: Neutron star mergers as the astrophysical site of the r-process in the Milky Way and its satellite galaxies

Via Royal Astronomical Society

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