Palladium and silver abundances in stars with [Fe/H] > –2.6^⋆

¹ Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, A20, Datun Road, Chaoyang District, 100012, Beijing, PR China
e-mail: xswu@nao.cas.cn; gzhao@nao.cas.cn
² University of the Chinese Academy of Sciences, 19A, Yuquan Road, Shijingshan District, 100049 Beijing, PR China
³ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany

Received: 16 January 2015
Accepted: 27 April 2015

Abstract

Palladium (Pd) and silver (Ag) are the key elements for probing the weak component in the rapid neutron-capture process (r-process) of stellar nucleosynthesis. We performed a detailed analysis of the high-resolution and high signal-to-noise ratio near-UV spectra from the archive of HIRES on the Keck Telescope, UVES on the VLT, and HDS on the Subaru Telescope, to determine the Pd and Ag abundances of 95 stars. This sample covers a wide metallicity range with −2.6 ≲ [Fe/H] ≲ +0.1, and most of them are dwarfs. The plane-parallel local thermodynamic equilibrium MAFAGS-OS model atmosphere was adopted, and the spectral synthesis method was used to derive the Pd and Ag abundances from Pd iλ 3404 Å and Ag iλ 3280/3382 Å lines. We found that both elements are enhanced in metal-poor stars, and their ratios to iron show flat trends at −0.6 < [Fe/H] < +0.1. The abundance ratios of [Ag/H] and [Pd/H] are well correlated over the whole abundance range. This implies that Pd and Ag have similar formation mechanisms during the Galactic evolution.