Astronomers have identified a relatively bright, old star, HD 222925 in the southern constellation of Tucana. Astronomers have identified 65 elements in the star, with 42 of the elements being the heavier ones listed along the bottom of the periodic table. This makes HD 222925 the star with the widest known range of elements identified, outside our own Solar System. Investigation of this metal poor star allows researchers to understand how all the elements in the universe are created. The calcium in our bones, the iron in our bloods and the nitrogen in our brains were all forged in the cores of long dead stars. Identifying the range of elements in a single star allows scientists to understand what is known as the rapid neutron capture process.
The cores of stars are fusion furnaces that cook up the elements from helium to iron. Iron is the 26th element on the periodic table which lists over 100 elements, so the heavier elements such as gold, lead and uranium have to emerge from a process other than nuclear fusion. Scientists have suspected for a long time that the heavier elements are created using a process known as neutron capture, where neutrons are added to an element to make it unstable, which then decays radioactively, increasing its atomic number by one.
70 years ago, astronomers confirmed one type of neutron capture, known as the slow process, or the s-process. However, another type, the rapid process, known as the r-process was not confirmed till the LIGO/VIRGO collaboration detected a nuclear star merger in 2017. One of the authors of the new paper, James Lawler says, “We were able to determine a complete r-process abundance pattern for what we think is probably one event that happened early in the beginning of the universe. So that r-process template now can be used to screen various models of the nuclear physics that produce the r-process and see if the models for all sites are physically correct.”
Lead author of the study Ian Roederer says, “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. 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.”
The r-process begins with a neutrons being added to lighter elements such as iron, within a second, creating elements such as selenium, silver, gold, platinum and thorium, the kind found in HD 222925. These elements are rarely detected in stars. Roederer says, “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. There aren’t very many environments in which that can happen—two, maybe.”
The merger of two neutron stars is one of these environments. Neutron stars are the remnant cores of collapsed stars, and are among the smallest and densest celestial objects known. Another place the r-process can take place is the explosive death of a star itself. Roederer says, “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? That’s where our study comes in.”
The elements identified in HD 222925 were produced by either supernovae explosions or neutron star mergers very early in the universe. The explosion or the collision flung the material across space, into the cloud of dust and gas, which clumped together under the influence of gravity to form HD 222925. Any model in the future that demonstrates how the r-process gives rise to the heavier elements in nature will have to have the same signature as HD 222925. Co-author of the paper Anna Frebel says, “We now know the detailed element-by-element output of some r-process event that happened early in the universe. Any model that tries to understand what’s going on with the r-process has to be able to reproduce that.”
A paper describing the findings has been accepted for publication in Astrophysical Journal Supplement Series, with a preprint version being hosted on Arxiv.