(ORDO NEWS) — In the vicinity of our sun, the Milky Way galaxy hosts a relatively bright star, and in it astronomers have been able to determine the widest range of elements in a star outside our solar system.
A study led by University of Michigan astronomer Jan Roederer identified 65 elements in the star HD 222925. Forty-two of the identified elements are heavy elements, which are at the bottom of the periodic table of elements.
Identifying these elements in a single star will help astronomers understand the so-called “rapid neutron capture process,” or one of the main ways in which heavy elements are created in the universe. The results of the study are published on the arXiv website and accepted for publication in The Astrophysical Journal Supplement.
“To my knowledge, this is a record for any object outside our solar system. What makes this star so unique is the very high relative proportion of elements listed in the bottom two-thirds of the periodic table.
We even found gold,” Roederer said. “These elements were produced by a fast neutron capture process. That’s what we’re trying to study: physics, to understand how, where and when these elements were produced.”
The process, also called the “r-process”, starts with the presence of light elements such as iron. Then, quickly – on the order of a second – neutrons are added to the nuclei of light elements.
The result is heavier elements such as selenium, silver, tellurium, platinum, gold and thorium – the kind found on HD 222925, all of which astronomers say are rarely found in stars.
“In order to release neutrons and add them to the nuclei of atoms, you need a lot of free neutrons and very high energy,” said Roderer. “There aren’t many environments where this can happen.”
One such medium has been confirmed: the merger of neutron stars. Neutron stars are the collapsed cores of supergiant stars, which are the smallest and densest known celestial objects.
The collision of pairs of neutron stars produces gravitational waves, and in 2017, astronomers first detected gravitational waves from merging neutron stars. Another way in which the r-process can occur is through the explosive death of massive stars.
“This is an important step forward: recognizing where the r-process can take place. But a much larger step is to say: “What actually happened as a result of this event? What was produced there?” Roederer said. “That’s the crux of our research.”
The elements that Roederer and his team identified in HD 222925 were either formed in a massive supernova or a neutron star merger early in the universe.
This star can be used as a proxy for what might happen as a result of one of these events. Any model developed in the future that demonstrates how the r-process or nature produces elements in the lower two-thirds of the periodic table should have the same signature as HD 222925, Roederer says.
Significantly, astronomers used the Hubble Space Telescope’s instrument, which can collect ultraviolet spectra. This instrument played a key role in allowing astronomers to gather ultraviolet light, the faint light coming from a cool star like HD 222925.
Astronomers also used one of the Magellan telescopes, a consortium of which UM is a partner, at the Las Vegas Observatory. Campanas in Chile to collect light from HD 222925 in the optical part of the light spectrum.
These spectra encode the “chemical fingerprints” of elements within stars, and reading these spectra allows astronomers not only to determine the elements contained in a star, but also the amount of an element in a star.
Anna Froebel is a co-author of the study and a professor of physics at the Massachusetts Institute of Technology. She helped with the general interpretation of the picture of the elements in HD 222925 and how it affects our understanding of the origin of the elements in space.
“Now we know the detailed element-by-element outcome of some r-process event that occurred in the early stages of the universe,” Fröbel said. “Any model that tries to understand what’s going on with the r-process should 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 tackling big questions about the r-process. This project marks one of the group’s key goals: to determine which elements and in what quantities were obtained in the r-process with an unprecedented level of detail.
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