The Daily Galaxy

Unraveling the mysteries of the oldest stars in the Milky Way

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“The R-Process Alliance aims to answer the big unanswered questions related to decoding the mysteries of the oldest stars in the Milky Way – by bringing together an interdisciplinary group of observers, theorists and experimenters,” said Ia, an astronomer from the University of Michigan.n Röderer wrote in an e-mail to The Daily Galaxyabout the discovery of a relatively bright star HD 222925. The ancient object is a rare ninth-magnitude star located toward the southern constellation of Tucana, where astronomers have been able to identify the widest range of elements in its photosphere, more than in any star beyond our solar system.

“These questions,” Roederer wrote in his email, “include: How many sites can produce r-process items? What are the detailed physical characteristics of these sites? What is the production rate of r-process items? in cosmic time Where did the first r-process enrichment events occur, and is this different from where most r-process enrichment occurs today? does it occur more often in some environments than in others What elements have been produced by the r-process, and in what quantities?

The R process

Physically, the process of fast neutron (“r”) capture is a primary pathway for creating the heaviest elements we find naturally on Earth, such as gold, silver, and uranium, according to the R Process Alliance. Identifying the astrophysical site with the right conditions that allow the r-process to occur presents a challenge both observationally and theoretically. One way to study the r-process is to use the principle of galactic archeology of ancient stars which, although lacking in elements like iron, are unexpectedly enhanced with even heavier elements made by the r-process. , such as europium. These “r-enhanced” stars are the stellar progeny of explosive r-process events.

The study, led by Roederer, identified 65 elements in the star, HD 222925. Forty-two of the elements identified are heavy elements which are listed at the bottom of the periodic table of elements. Roederer uses ancient stars from the Milky Way to study the origins of the heaviest elements found on Earth. Every star, he writes in his biological, retains a chemical memory of the time and place where he was born. By studying the abundance patterns of common elements (like carbon, magnesium, or iron) and obscure elements (like arsenic, tellurium, europium, platinum, or lead), Roederer can probe the physics which produced these elements in ancient supernovae.

Identifying these elements in a single star will help astronomers understand what’s called the “fast neutron capture process,” or one of the main ways the heavy elements of the universe were created. Their results are published on arXiv and have been accepted for publication in The Astrophysical Journal Supplement series.

“To the best of my knowledge, this is a record for any object beyond our solar system. And what makes this star so unique is that it has a very high relative proportion of elements listed in the lower two-thirds of the periodic table. We even detected gold,” Roederer said. “These elements were made by the fast neutron capture process. That’s really the thing we’re trying to study: physics to understand how, where and when these things were made.

Physics of the R process

The process, also called “r-process”, begins with the presence of lighter elements such as iron. Then, rapidly, on the order of a second, neutrons are added to the nuclei of the lighter elements. This creates heavier elements such as selenium, silver, tellurium, platinum, gold and thorium, of the kind found in HD 222925, all of which are rarely detected in stars, astronomers say.

While some r-process elements include expensive precious metals like silver and gold, other r-process elements are needed in trace amounts for human bodily functions. Iodine regulates the thyroid glands and hormones essential for human growth and development. Selenium improves cognition, immune system function and fertility. Perhaps r-process elements are needed for other complex extraterrestrial lifeforms scattered throughout our Milky Way.

Two environments of the R process

“You need lots of free neutrons and a set of very high energy conditions to release them and add them to the nuclei of atoms,” Roederer said. “There aren’t many environments in which it can happen – two, maybe.”

One such environment has been confirmed: neutron star mergers. Neutron stars are the collapsed cores of supergiant stars and are the smallest and densest known celestial objects. Colliding pairs of neutron stars cause gravitational waves and in 2017 astronomers first detected gravitational waves from merging neutron stars. Another way the r-process could occur is after the explosive death of massive stars.

“This is an important step forward: recognizing where the r-process can occur. But it’s a much bigger step to say, ‘What did this event actually do? What was produced there?’” Roederer said. our study comes into play.”

The elements Roederer and his team identified in HD 222925 were produced either in a neutron star merger or in a massive supernova early in the Universe. The material was ejected and sent back into space, where it later reformed into HD 222925.

This star can then be used as an indicator of what one of these events would have produced. Any model developed in the future that demonstrates how the r-process or nature produces elements on the lower two-thirds of the periodic table must have the same signature as HD 222925, Roederer says.

Ultraviolet spectra

Crucially, astronomers used an instrument on the Hubble Space Telescope that can collect ultraviolet spectra. This instrument was essential to allow astronomers to collect light in the ultraviolet part of the light spectrum – faint light from a cool star such as HD 222925.

The astronomers also used one of the Magellan Telescopes – a consortium of which UM is a partner – at the Las Campanas Observatory in Chile to collect light from HD 222925 in the optical part of the light spectrum.

The “chemical fingerprint”

These spectra encode the “chemical fingerprint” of elements in stars, and reading these spectra allows astronomers to not only identify the elements contained in the star, but also the amount of an element contained in the star. .

Anna Frebel is a co-author of the study and a professor of physics at the Massachusetts Institute of Technology. She helped with the overall interpretation of the element abundance pattern of HD 222925 and how it informs our understanding of the origin of elements in the cosmos.

“We now know the detailed element-by-element output of an r-process event that occurred early in the universe,” Frebel said. “Any model that tries to understand what is happening with the r-process must be able to reproduce it.”

Many of the study’s co-authors are part of a group called R-Process Alliance, a group of astrophysicists dedicated to solving the big questions of the r process. This project marks one of the main objectives of the team: to identify which elements, and in which quantities, have been produced in the r process with an unprecedented level of detail.

The Last Word – Ian Roederer

“It’s this last question: what elements were produced by the process r, and in what quantities? Roederer concludes in his email to the Daily Galaxy, “that this particular study was intended to answer. With the much larger sample sizes generated by other R-Process Alliance activities, we will be in a much better position to answer the other questions.

More information: Ian U. Roederer et al, The R-Process Alliance: a nearly complete R-process abundance model derived from ultraviolet spectroscopy of the metal-poor star enhanced by R-Process HD 222925. arXiv:2205.03426v1 [astro-ph.SR]

Image Credit: Astronomers recently discovered one of the first stars to form in the Milky Way, J0815+4729, pictured at the top of the page in this artist’s concept. This low-mass star is one of the most iron-poor and carbon-rich stars discovered to date, suggesting that it formed shortly after the Big Bang. Gabriel Perez/SMM/IAC

Maxwell Moeastrophysicist, NASA Einstein Fellow, University of Arizona via Ian Roederer And University of Michigan

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