Helioseismic determination of the solar metal mass fraction

Context. The metal mass fraction of the Sun Z is a key constraint in solarmodelling, but its value is still under debate. The standard solar chemicalcomposition of the late 2000s have the ratio of metals to hydrogen Z/X =0.0181, with a small increase to 0.0187 in 2021, as inferred from 3D non-LTEspectroscopy. However, more recent work on a horizontally and temporallyaveraged <3D> model claim Z/X = 0.0225, consistent with the high values oftwenty-five years ago based on 1D LTE spectroscopy. Aims. We aim to determine aprecise and robust value of the solar metal mass fraction from helioseismicinversions, thus providing independent constraints from spectroscopic methods.Methods. We devise a detailed seismic reconstruction technique of the solarenvelope, combining multiple inversions and equations of state to accuratelyand precisely determine the metal mass fraction value. Results. We show that alow value of the solar metal mass fraction corresponding to Z/X = 0.0187 isfavoured by helioseismic constraints and that a higher metal mass fractioncorresponding to Z/X = 0.0225 are strongly rejected by helioseismic data.Conclusions. We conclude that direct measurement of the metal mass fraction inthe solar envelope favours a low metallicity, in line with the 3D non-LTEspectroscopic determination of 2021. A high metal mass fraction as measuredusing a <3D> model in 2022 is disfavoured by helioseismology for all modernequations of state used to model the solar convective envelope.