Understanding the origin of palladium in metal-poor stars based on the non-local thermodynamic equilibrium analysis of Pd

Context. Palladium is a poorly observed neutron-capture element. Abundance determinations for stellar samples covering a broad metallicity range are needed for a better understanding of the mechanisms of Pd synthesis during the Galaxy evolution. Aims. We obtain accurate abundances of Pd for the Sun and a sample of metal-poor stars based on the non-local thermodynamic equilibrium (NLTE) line formation for Pd <ns0:sc>I</ns0:sc>. Methods. We present a new comprehensive model atom of Pd<ns0:sc>I</ns0:sc>. We derived the abundances of Pd, Sr, Ba, and Eu for 48 stars from the NLTE analyses of high-resolution spectra with a high signal-to-noise ratio provided by the ESO archives. The results we obtained are based on the synthetic spectrum method with one-dimensional (1D MARCS) model atmospheres. Results. NLTE leads to weakened Pd <ns0:sc>I</ns0:sc> lines and positive NLTE abundance corrections growing from 0.2 dex for the solar lines up to 0.8 dex for the lines in the most luminous star of the sample. Depending on the treatment of inelastic collisions with hydrogen atoms, the solar NLTE abundance is log ε⊙,Pd = 1.61±0.02 to 1.70±0.02 and agrees within the error bars with the meteoritic abundance log εmet,Pd = 1.65. NLTE largely removes the discrepancies in the LTE abundances between giant and dwarf stars of similar metallicities. Palladium is tightly correlated with Eu in the −1.71 ≤ [Fe/H] ≤ −0.56 range, which indicates r- and s-process contributions to the Pd synthesis of approximately 70% and 30%, respectively. Palladium is of pure r-process origin in our two r-II stars, and a dominant contribution of the r-process to the Pd abundances is found for another two very metal-poor (VMP; [Fe/H] < −2) stars. The two VMP stars are strongly enhanced with Sr relative to Ba and Eu and also have a higher Pd. We propose that the source of the additional Sr and Pd in these stars are VMP fast-rotating massive stars. Conclusions. NLTE is essential for obtaining the observational constraints to future models of the Galactic Pd evolution.