The solar photospheric abundance of hafnium and thorium. Results from CO5BOLD 3D hydrodynamic model atmospheres

TL;DR

This study uses 3D hydrodynamic models to accurately determine the solar photospheric abundances of hafnium and thorium, highlighting the importance of convective line asymmetries for precise measurements and assessing 3D effects across different stellar types.

Contribution

It provides the first detailed 3D-based solar abundance of hafnium and thorium, emphasizing the significance of convective asymmetries in abundance determinations and evaluating 3D effects in various stellar atmospheres.

Findings

Hf abundance agrees with previous 1D analyses

Th abundance is lower than previous estimates when accounting for line asymmetry

3D effects are small for Hf in G- to mid F-type dwarfs

Abstract

Context: The stable element hafnium (Hf) and the radioactive element thorium (Th) were recently suggested as a suitable pair for radioactive dating of stars. The applicability of this elemental pair needs to be established for stellar spectroscopy. Aims: We aim at a spectroscopic determination of the abundance of Hf and Th in the solar photosphere based on a \cobold 3D hydrodynamical model atmosphere. We put this into a wider context by investigating 3D abundance corrections for a set of G- and F-type dwarfs. Method: High-resolution, high signal-to-noise solar spectra were compared to line synthesis calculations performed on a solar CO5BOLD model. For the other atmospheres, we compared synthetic spectra of CO5BOLD 3D and associated 1D models. Results: For Hf we find a photospheric abundance A(Hf)=0.87+-0.04, in good agreement with a previous analysis, based on 1D model atmospheres. The weak Th ii 401.9 nm line constitutes the only Th abundance indicator available in the solar spectrum. It lies in the red wing of an Ni-Fe blend exhibiting a non-negligible convective asymmetry. Accounting for the asymmetry-related additional absorption, we obtain A(Th)=0.09+-0.03, consistent with the meteoritic abundance, and about 0.1 dex lower than obtained in previous photospheric abundance determinations. Conclusions: Only for the second time, to our knowledge, has am non-negligible effect of convective line asymmetries on an abundance derivation been highlighted. Three-dimensional hydrodynamical simulations should be employed to measure Th abundances in dwarfs if similar blending is present, as in the solar case. In contrast, 3D effects on Hf abundances are small in G- to mid F-type dwarfs and sub-giants, and 1D model atmospheres can be conveniently used.