Stellar Archeology: The Nuclear Fingerprints of Massive Stars.

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May 29, 2023

May 29, 2028

All the elements in the universe heavier than helium were created inside the cores of stars by nuclear reactions. Massive stars, that is, stars made of much more matter than our Sun, are believed to have played an essential role in producing the vast number of chemical elements we find in nature today. There are still many open questions related to the origin of the elements, particularly those heavier than iron, and the astrophysical conditions in which they were made. Unprecedented insight into how the elements were created is gained through combining observations from modern astronomy, isotopic analysis of meteorite samples, and microscopic nuclear physics. However, our predictions as to how stars forged the chemical elements rely critically on our knowledge of the underlying nuclear reactions that made them. Many important reactions involve short-lived radioactive nuclei not found in nature, and so must instead be produced in the laboratory. We are now entering into a golden-era for studies of nuclear reactions on radioactive isotopes, with several new facilities now coming into operation, such as the Facility for Rare Isotope Beams (United States), the Advanced Rare Isotope Laboratory (Canada) and the HIE-ISOLDE upgrade at CERN. My research aims to use these premier laboratories to study the astrophysical reactions that are key to understanding how elements are made inside massive stars.

Many mysteries surround how elements were made in the first stars and the emergence of elements heavier than iron. The first stars are believed to have been by far larger than our Sun, and so burned their nuclear fuel quickly. This means that few are left to observe directly today, though the James Webb Space Telescope will take us much closer to achieving this goal. Instead, like archeologists, we must piece together their evolution from the fingerprints they leave behind, preserved in the different elements seen in very old stars formed after this first generation. However, relating these chemical fingerprints to the characteristics of the first stars requires us to properly understand the nuclear reactions that made them. A key focus of my research is centred on measurements of nuclear reactions that will allow us to use observations to better understand the creation of elements in the early universe, especially the poorly understood origins of the heavy elements.

Another mystery, related to origin of the heavy elements, is posed by the unknown origin of a special group of isotopes called the "p-nuclei". These 30 or so isotopes account for less than 1% of the nuclei heavier than iron and cannot be formed by the normal heavy-element production mechanisms. Instead, we believe they might be produced in core-collapse supernovae, the violent explosions ending the life of massive stars. Currently, however, our best models are unable to reproduce the amounts of p-nuclei seen in meteorite samples, which preserve the chemical fingerprints of supernovae. These predictions are affected by key reactions on unstable isotopes that haven't yet been measured, due to extreme technical challenges. Recently, I co-led an experiment which measured one of these reactions for the very first time and, through this fellowship, I will continue to lead a program of further measurements at new facilities to understand the origins of the p-nuclei.