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A ‘gold standard’ star with 42 heavy elements discovered in Milky Way

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Astronomers have discovered a bright ‘Gold Standard’ star in the Milky Way Galaxy near our sun.

A relatively bright star in the Milky Way Galaxy’s neighborhood of our sun has allowed astronomers to detect the widest range of elements in a star beyond our solar system yet.

The star HD 222925 has 65 elements, according to the research performed by University of Michigan astronomer Ian Roederer. Heavy elements, which are listed at the bottom of the periodic table, account for 42 of the elements discovered.

The “rapid neutron capture process,” or one of the primary mechanisms by which heavy elements in the universe were generated, can be better understood by identifying these elements in a single star. Their findings have been accepted for publication in the Astrophysical Journal Supplement Series and are available on arXiv.

“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,” write the authors. “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.”

HD 222925 is a ninth-magnitude star in the Tucana constellation in the southern sky.

The “r-process,” as it’s often known, starts with the presence of lighter components like iron. The nuclei of the lighter elements are then rapidly bombarded by neutrons, on the order of a second. Heavier elements like selenium, silver, tellurium, platinum, gold, and thorium, like the ones seen in HD 222925, are produced, according to the astronomers, and are rarely observed in stars.

“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,” add the authors. “There aren’t very many environments in which that can happen — two, maybe.”

The merging of neutron stars is one of these situations that has been confirmed. Neutron stars are the tiniest and densest celestial objects known. They are the collapsed cores of supergiant stars. Gravitational waves are produced when neutron star couples collide, and astronomers first observed gravitational waves from merging neutron stars in 2017. Another possibility is that the r-process occurs after big stars explode.

“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?.” According to the researchers, “that’s where our study comes in.”

The components found in HD 222925 were created by large supernovae or neutron star mergers very early in the universe, according to the team. The material was flung into space and rebuilt into the star Roederer is studying today.

This star can then be used as a proxy for the outcome of one of those occurrences. Any future model that shows how the r-process or nature produces elements in the lowest two-thirds of the periodic table must have the same signature as HD 222925, according to the authors.

The astronomers took advantage of a Hubble Space Telescope sensor that can capture ultraviolet spectra. This apparatus was crucial in allowing astronomers to catch light from a cold star like HD 222925 that was in the ultraviolet section of the light spectrum.

The astronomers also collected light from HD 222925 in the optical part of the spectrum using one of the Magellan telescopes at Las Campanas Observatory in Chile, a cooperative in which U-M is a partner.

These spectra record the “chemical fingerprint” of elements within stars, and interpreting them allows astronomers to determine not only which elements are present in the star, but also how much of each element is present.

Anna Frebel, one of the study’s co-authors, contributed to the general interpretation of the HD 222925’s element abundance pattern and how it affects our knowledge of the formation of the elements in the universe.

“We now know the detailed element-by-element output of some r-process event that happened early in the universe,” she adds. “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’s co-authors are members of the R-Process Alliance, a group of astrophysicists dedicated to answering the key questions of the r-process. One of the team’s main objectives is to identify which elements, and in what amounts, were created in the r-process in unprecedented detail.

Image Credit: The STScI Digitized Sky Survey

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