Quantifying the Distribution of Europium in the Milky Way Disk: Its Connection to Planetary Habitability and the Source of the r-process

Abstract

The astrophysical site of the r-process remains one of the most pressing questions in stellar nuclear synthesis. Although multiple theoretical sites have been proposed, with some observational counterparts available, the current Galactic distribution cannot be reproduced from a single consistent injection site. To disentangle the prospective sites of r-process production, the distribution of r-process elements in the Galaxy today must be scrutinized. In this study, we find that the intrinsic star-to-star distribution of the r-process element europium (Eu) at a fixed metallicity and temperature has a small intrinsic [Eu/H] scatter of 0.025 dex. In addition to a small dispersion, we demonstrate an anticorrelation between [Eu/α] and [α/H] consistent with r-process production being metallicity-dependent in core-collapse supernova and/or being produced in double neutron star mergers with a delay time distribution greater than t−1. Furthermore, using Eu as a proxy for the radioactive r-process elements U and Th, and assuming that rocky planetary abundances reflect their parent star’s composition, we show how these elements play a key role in the evolution of the magnetic dynamo on Earth-mass planets. Specifically, we find that only above [α/H] > −0.25 do most stars’ planetary systems meet the threshold abundance of [Eu/α] < +0.06 to support a persistent magnetic dynamo, supporting the notion of a “habitable metallicity range” in the Galactic disk.