Revisiting inelastic Cu + H collisions and the non-LTE Galactic evolution of copper

¹ Theoretical Astrophysics, Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden
² Department of Astronomy, Stockholm University, AlbaNova University Centre, Stockholm, Sweden
³ Dipartimento di Fisica e Astronomia, Universitá degli Studi di Firenze, Largo E. Fermi 1, 50125 Firenze, Italy

^★ Corresponding author: sema.caliskan@physics.uu.se

Received: 24 February 2025
Accepted: 28 March 2025

Abstract

The Galactic evolution of copper remains poorly understood, partly due to the strong departures from local thermodynamic equilibrium (LTE) affecting Cu I lines. A key source of uncertainty in non-LTE modelling is the treatment of inelastic Cu + H collisions. We present new rate coefficients based on a combined asymptotic LCAO (linear combination of atomic orbitals) and free electron model approach, which show significant differences from previous calculations. Applying these updated rates to non-LTE stellar modelling, we find reduced line-to-line scatter and improved consistency between metal-poor dwarfs and giants. Our non-LTE analysis reveals a strong upturn in the [Cu/Fe] trend towards lower [Fe/H] < −1.7. We show that this may reflect the interplay between external enrichment of Cu-rich material of the Milky Way halo at low metallicities, and metallicity-dependent Cu yields from rapidly rotating massive stars. This highlights the unique diagnostic potential of accurate Cu abundances for understanding both stellar and Galactic evolution.