Non-local thermodynamic equilibrium (NLTE) abundances of europium (Eu) for a sample of metal-poor stars in the galactic halo and metal-poor disk with 1D and 〈3D〉 models

¹ Yunnan observatories, Chinese Academy of Sciences, PO Box 110, Kunming 650011, China
² Max-Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
³ International Centre of Supernovae, Yunnan Key Laboratory, Kunming 650216, China
⁴ Heidelberg University, Grabengasse 1, 69117 Heidelberg, Germany
⁵ South-Western Institute for Astronomy Research, Yunnan University, Kunming, Yunnan 650091, PR China
⁶ CAS Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijin, 100101, PR China
⁷ Department of Astronomy, University of Florida, Bryant Space Science Center, Gainesville, FL 32611, USA
⁸ Joint Institute for Nuclear Astrophysics – Center for Evolution of the Elements, USA

^★ Corresponding author; guoyanjun@ynao.ac.cn

Received: 16 July 2024
Accepted: 4 December 2024

Abstract

Context. As a key to chemical evolutionary studies, the distribution of elements in galactic provides a wealth of information to understand the individual star formation histories of galaxies. The r-process is a complex nucleosynthesis process, and the origin of r-process elements is heavily debated. Europium (Eu) is viewed as an almost pure r-process element. Accurate measurements of europium abundances in cool stars are essential for an enhanced understanding of the r-process mechanisms.

Aims. We measure the abundance of Eu in solar spectra and a sample of metal-poor stars in the Galactic halo and metal-poor disk, with the metallicities ranging from −2.4 to −0.5 dex, using non-local thermodynamic equilibrium (NLTE) line formation. We compare these measurements with Galactic Chemical Evolution (GCE) models to explore the impact of the NLTE corrections on the contribution of r-process site in Galactic chemical evolution.

Methods. In this work, we used NLTE line formation, as well as one-dimensional (1D) hydrostatic and spatial averages of three-dimensional hydrodynamical (<3D>) model atmospheres to measure the abundance of Eu based on both the Eu II 4129 Å and Eu II 6645 Å lines for solar spectra and metal-poor stars.

Results. We find that for Eu II 4129 Å line the NLTE modeling leads to higher (0.04 dex) solar Eu abundance in 1D and higher (0.07 dex) in <3D> NLTE while NLTE modeling leads to higher (0.01 dex) solar Eu abundance in 1D and lower (0.03 dex) in <3D> NLTE for Eu II 6645 Å line. Although the NLTE corrections for the Eu II λ 4129 Å and Eu II λ 6645 Å lines are opposite, the discrepancy between the abundances derived from these individual lines reduces after applying NLTE corrections, highlighting the critical role of NLTE abundance determinations. By comparing these measurements with Galactic chemical evolution (GCE) models, we find that the amount of NLTE correction does not require significant change of the parameters for Eu production in GCE models.