Astronomers discover a ‘gold standard’ star in the Milky Way

The star HD 222925 is a ninth magnitude star located towards the southern constellation of Tucana. Credit: STScI Digitized Sky Survey

In the Milky Way’s solar neighborhood is a relatively bright star, and astronomers have been able to identify the widest range of elements in a star beyond our solar system to date.

The study, led by University of Michigan astronomer Ian 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. .

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 posted 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 the elements listed in the lower two-thirds of the periodic table. We even detected gold,” Roederer said. “These elements were made by the process of fast neutron capture. That’s really the thing we’re trying to study: the physics to understand how, where and when these elements were made.”

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.

“You need a lot 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.

“It’s an important step forward: to recognize where the r-process may be happening. But it’s a much more important step to say, ‘What did this event actually do? What happened there? “, Roederer said. “This is where our study comes in.”

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

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.

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 coming 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.

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 the R-Process Alliance, a group of astrophysicists dedicated to solving big r-process questions. 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.

First identification of a heavy element born from a collision of neutron stars

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]

Provided by the University of Michigan

Quote: Astronomers find ‘gold standard’ star in Milky Way (May 11, 2022) retrieved May 11, 2022 from

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