Line formation in solar granulation: V. Missing UV-opacity and the photospheric Be abundance

TL;DR

This study investigates missing UV-opacity in the solar atmosphere, quantifies it using a 3D model, and refines the solar beryllium abundance, aligning it with meteoritic values and explaining the opacity with Fe I photo-ionization.

Contribution

It provides a detailed quantification of missing UV-opacity in the solar atmosphere using a realistic 3D model and revises the solar Be abundance accordingly.

Findings

Approximately 50% extra UV-opacity is identified.

The missing opacity can be explained by Fe I photo-ionization.

Revised Be abundance aligns with meteoritic values.

Abstract

The possibility of unaccounted for opacity sources in the UV for late-type stars has often been invoked to explain discrepancies between predicted and observed flux distributions and spectral line strengths. Such missing UV-opacity could among other things have a significant impact on abundance determination for elements whose only relevant spectral features are accessible in this wavelength region, such as Be. Here, the study by Balachandran & Bell (1998) is re-visited in the light of a realistic 3D hydrodynamical solar model atmosphere and the recently significantly downward revised solar O abundance obtained with the same model atmosphere. The amount of missing UV-opacity, if any, is quantified by enforcing that the OH A-X electronic lines around 313 nm produce the same O abundance as the other available diagnostics: OH vibration-rotation and pure rotation lines in the IR, the forbidden [OI] 630.0 and 636.3 nm lines and high-excitation, permitted OI lines. This additional opacity is then applied for the synthesis of the BeII line at 313.0nm to derive a solar photospheric Be abundance in excellent agreement with the meteoritic value, thus re-enforcing the conclusions of Balachandran & Bell. The about 50% extra opacity over accounted for opacity sources can be well explained by recent calculations by the Iron Project for photo-ionization of FeI.