Ag I model atom and the 3D non-LTE solar silver abundance

Silver is an important light neutron-capture element whose stellar abundances can help constrain the origin of the weak r -process. The Sun is an important reference point for such studies; moreover, being a moderately volatile element in CI chondrites, the solar silver abundance is interesting as a diagnostic for the debated Sun-CI abundance versus condensation temperature trend. These studies require accurate silver abundances that go beyond the commonly used assumptions of one-dimensional (1D) atmospheres and local thermodynamic equilibrium (LTE); however, no consistent 3D non-LTE analysis of silver has been available to date. We present a new Ag I model atom built from carefully curated radiative and collisional data, including newly computed oscillator strengths using an ab initio multi-configurational Hartree-Fock method and inelastic hydrogen collision rates based on a combined asymptotic and free-electron-model approach. We assessed modelling uncertainties via targeted sensitivity tests, finding the results to be most sensitive to the hydrogen-collision data. Applying the model to the solar Ag I 328 and 338 nm resonance lines, we find severe positive-abundance corrections from coupled 3D and non-LTE effects. Using revised equivalent-width measurements, we derive a recommended solar 3D non-LTE silver abundance of log ε Ag = 1.15 ± 0.08. This is an increase of 0.19dex relative to the current reference value. Our ab initio model significantly reduces the discrepancy with the meteoritic value from 0.25 to 0.06 dex; moreover, this residual offset is consistent with what was recently reported for other moderately volatile elements. The Sun provides the benchmark test for the first silver non-LTE model atom presented here. In subsequent work, this model will be applied to determine 3D non-LTE silver abundances in metal-poor dwarfs and giants, enabling improved constraints on Galactic chemical evolution and weak r -process nucleosynthesis.

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