New observations of heavy elements in old metal-poor stars

Camilla J. Hansen (Goethe University in Frankfurt)

August 11, 2025

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Abstract: Immense progress followed the detection of the blue kilonova (AT2017gfo) in 2017. However, a large number of open questions related to the nature and detailed physics of the r-process remain. Exactly how and where does the r-process form the heavy (Z>30) elements? Numerous stellar observations and various Galactic chemical evolution models indicate a need for more r-processes or sites – or show that the r-process is not as universal and robust as earlier believed. This picture is present both in the more metal-rich, younger disk as well as in the Milky Way’s old halo. Observations of old, metal-poor stars show a large abundance spread in the heavy elements. This spread indicates that different processes could take place and contribute different amounts of the heavy elements in each explosion event. Here, observations of poorly studied heavy elements like Hf, Os, Ir and Pt recently provided new insight into the physics and nature of the r-process.