Decoding the Mysteries of the Oldest Stars in the Milky Way

 

Old Star

 

“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 experimentalists,” University of Michigan astronomer Ian Roederer wrote in an email to The Daily Galaxy, about  the discovery of a relatively bright star HD 222925. The ancient object is a rare, ninth-magnitude star located toward the southern constellation 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 elements? What are the detailed physical characteristics of those sites? What is the rate of r-process element production across cosmic time? Where did the earliest r-process enrichment events occur, and is that different from where most r-process enrichment occurs today? Does r-process enrichment occur more often in some environments than others? Which elements were produced by the r-process, and in what amounts?

The R Process

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

The study, led by Roederer, has identified 65 elements in the star, HD 222925. Forty-two of the elements identified are heavy elements that are listed along the bottom of the periodic table of elements. Roederer uses ancient stars in the Milky Way to study the origins of the heaviest elements found on Earth. Every star, he writes in his bio, retains a chemical memory of the time and place where it 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 that produced these elements in ancient supernovae.

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

“To the best of my knowledge, that’s 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 the elements listed along the bottom two-thirds of the periodic table. We even detected gold,” Roederer said. “These elements were made by the rapid neutron capture process. That’s really the thing we’re trying to study: the physics in understanding how, where and when those elements were made.”

Physics of the R Process

The process, also called the “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, the kind found in HD 222925, and all of which are rarely detected in stars, according to the astronomers.

While some r-process elements include expensive precious metals like silver and gold, other r-process elements are required 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 necessary for other complex alien lifeforms sprinkled throughout our Milky Way. 

Two Environments of the R Process

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

One of these environments has been confirmed: the merging of neutron stars. Neutron stars are the collapsed cores of supergiant stars, and are the smallest and densest known celestial objects. The collision of neutron star pairs causes gravitational waves and in 2017, astronomers first detected gravitational waves from merging neutron stars. Another way the r-process might occur is after the explosive death of massive stars.

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

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

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

Ultraviolet Spectra

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

The astronomers also used one of the Magellan telescopes—a consortium of which U-M is a partner—at 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 within stars, and reading these spectra allows the astronomers not only to identify the elements contained in the star, but also how much of an element the star contains.

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

“We now know the detailed element-by-element output of some r-process event that happened early in the universe,” Frebel said. “Any model that tries to understand what’s going on with the r-process has to be able to reproduce that.”

Many of the study co-authors are part of a group called the R-Process Alliance, a group of astrophysicists dedicated to solving the big questions of the r-process. This project marks one of the team’s key goals: identifying which elements, and in what amounts, were produced in the r-process in an unprecedented level of detail.

The Last Word –Ian Roederer

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

More information: Ian U. Roederer et al, The R-Process Alliance: A Nearly Complete R-Process Abundance Template Derived from Ultraviolet Spectroscopy of the R-Process-Enhanced Metal-Poor Star HD 222925. arXiv:2205.03426v1 [astro-ph.SR], arxiv.org/abs/2205.03426

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

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

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