Palladium and Silver Abundances in Extremely Metal-poor Stars

Abstract Based on the high-resolution and high signal-to-noise ratio near-ultraviolet spectra from the ESO VLT 8.2 m Kueyen telescope, we investigate the abundances of palladium (Pd) and silver (Ag) in a sample of 27 extreme metal-poor stars. The metallicity range of our sample stars is from ∼−2.2 to ∼−3.66 dex, and they are crucial for understanding the formation and evolution of weak r -process elements in our early Galaxy. The stellar atmospheric parameters, including effective temperature ( T eff ), surface gravity (log g ), metallicity ([Fe/H]), and microturbulence ( ξ t ) are derived using the spectroscopic method, and the nonlocal thermodynamic equilibrium effects for both Fe i and Fe ii lines have been considered when determining the iron abundance. Our results show that both [Pd/Fe] and [Ag/Fe] exhibit flat trends with metallicities between −0.7 and −3.0 dex, with an increasing scatter at lower metallicities. This study also finds a clear correlation between [Ag/H] and [Pd/H], indicating that both elements likely share a similar nucleosynthetic origin.