This is an image of Reticulum II obtained by the Dark Energy Survey, using the Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory. The nine stars described in the paper are circled in red. The insets show the very strong presence of barium, one of the main neutron capture elements the team observed, in three stars. Background image is courtesy of Dark Energy Survey/Fermilab. Foreground image is courtesy of Alexander Ji, Anna Frebel, Anirudh Chiti, and Josh Simon. Credit: Background image is courtesy of Dark Energy Survey/Fermilab. Foreground image is courtesy of Alexander Ji, Anna Frebel, Anirudh Chiti, and Josh Simon.
The lightest few elements in the periodic table formed minutes after the Big Bang. Heavier chemical elements are created by stars, either from nuclear fusion in their interiors or in catastrophic explosions. However, scientists have disagreed for nearly 60 years about how the heaviest elements, such as gold and lead, are manufactured. New observations of a tiny galaxy discovered last year show that these heavy elements are likely left over from rare collisions between 2 neutron stars.
The new galaxy, called Reticulum II because of its location in the southern constellation Reticulum, commonly known as The Net, is one of the smallest and closest to us known. “Reticulum II has more stars bright enough for chemical studies than any other ultra-faint dwarf galaxy found so far,” Josh Simon of Carnegie, explained.
A generic illustration of the neutron capture process on an unspecified element, courtesy of Carnegie Institution for Science.
Such ultra-faint galaxies are relics from the era when the universe’s first stars were born. They orbit our own Milky Way galaxy and their chemical simplicity can help astronomers understand the history of stellar processes dating back to the ancient universe, including element formation.
Many elements are formed by nuclear fusion, in which 2 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, during which an existing element acquires additional neutrons one at a time that then “decay” into protons, changing the makeup of the atom into a new element. Neutrons can be captured slowly, over long periods of time inside the star, or in a matter of seconds, when a catastrophic event causes a burst of neutrons to bombard an area. Different types of elements are created by each method.
Surprisingly, 7 of Reticulum II’s 9 brightest stars contained far more elements produced by rapid neutron captures than have been detected in any other dwarf galaxy. “These stars have up to a 1000X more neutron capture elements than any other stars observed in similar galaxies,” said Alexander Ji, MIT.
Finding so many more heavy elements in one dwarf galaxy proves the source of Reticulum II’s neutron capture elements must have been a rare event -much less common than an ordinary supernova. What’s more, the sheer amount of these neutron capture elements in Reticulum II far exceeds what most supernovae can even make. “Producing rapid neutron capture elements in a neutron star merger explains these observations beautifully,” said Anna Frebel, also of MIT.
Old stars in the Milky Way show a pattern of neutron capture elements similar to that found in Reticulum II. This indicates the process of making neutron capture elements in larger galaxies is likely the same as it is in dwarf galaxies, suggesting that even the heavy elements on Earth originated in neutron star mergers. “Because this galaxy is so small, it preserves evidence of ancient rare events incredibly cleanly,” said Simon. “We’re lucky to have found such an important galaxy so close to us.”
https://carnegiescience.edu/node/2014
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