Pasadena, California— A team of astronomers led by Ian Roederer of the University of Michigan and including Erika Holmbeck of Carnegie have identified the widest range of elements ever observed in a star beyond our own Sun. Their findings are published in The Astrophysical Journal Supplement Series.
Researchers have identified 65 elements in the star, called HD 222925. Of these, 42 are from the bottom of the periodic table. Identifying them will help astronomers understand one of the primary methods by which the heavy elements of the universe were created: the process of rapid neutron capture.
“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,” said Roederer, a former Carnegie postdoc. “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.”
Many elements are formed by nuclear fusion, in which two atomic nuclei fuse together and release energy, creating a different, heavier atom. But elements heavier than zinc are made by a process called neutron capture, in which an existing element acquires additional neutrons one by one which then “decays” into protons, changing the composition of the atom into a new one. element.
Neutrons can be captured slowly, over long periods inside the star, or within seconds, when a catastrophic event causes a burst of neutrons to bombard an area. Different types of items are created by each method. But it is the second that interests this research.
“Astronomers have debated for many years what phenomena could trigger such a bombardment and in 2017 it was confirmed that heavy elements could be created by the collision of two neutron stars – a discovery in which Carnegie astronomers played a crucial role,” said Holmbeck. “Certain types of supernovae could also produce these heavy elements, but this remains to be confirmed by observation.”
Holmbeck is working on a follow-up paper to determine if the chemical abundances seen in HD 222925 were formed by a neutron star merger or a supernova.
“The additional elements identified in this study provide a new baseline that can be compared
to simulations, which we can use to reveal the origin story of the heavy elements seen in the unique chemical signature of HD 222925.”
The material the team identified in HD 222925 was synthesized very early in the universe’s youth. It was ejected and sent back into space, where it then reformed into the star they were studying. This means that HD 222925 can be used as an indicator of what a neutron star merger or supernova would have produced.
Crucially, astronomers used an instrument on the Hubble Space Telescope that can collect ultraviolet spectra. This 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. Astronomers also used one of the observatory’s Magellan telescopes Carnegie’s Las Campanas in Chile. to collect light from HD 222925 in the optical portion 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. .
“We now know the detailed element-by-element output of an r-process event that occurred early in the universe,” said co-author Anna Frebel of MIT. “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.
This work was supported by NASA through grants from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under contract with NASA; the US National Science Foundation; the NASA Astrophysics Data Analysis Program; the US Department of Energy and NOIRLab, which is operated by AURA under a cooperative agreement with the NSF.