A data-driven estimate of the protosolar helium mass fraction

Abstract

The protosolar helium mass-fraction is a key ingredient of solar, planetary models and enrichment laws. However, the assumed values often rely on simplified descriptions of the transport of chemicals in solar models. They are also based on the inferred helium mass fraction in the solar convective envelope, which is itself sensitive to uncertainties in the solar equation of state. We update the reference protosolar helium abundance by including the effects of macroscopic mixing at the base of the convective zone and more recent determinations of the helium mass fraction in the convective envelope. We combine results from our inversions to spectroscopic abundances, as well as literature values to provide a robust interval of the current helium mass fraction in the convective zone. We combine this measurement to models including light element depletion to provide an udpated protosolar helium abundance. We show that macroscopic mixing at the base of the envelope is key to infer protosolar helium. We find a revised interval of primordial helium mass fraction of 0.27575 +- 0.00315 slightly lower than previous estimates when combining our latest estimate of surface helium mass fraction and spectroscopic abundances. We find that the effects of macroscopic mixing are partially compensated by an increase in the inferred solar helium mass fraction in recent studies. We also derive more precise estimates based on various reference works in the litterature. Using the usual surface helium mass fraction, the primordial helium mass fraction drops to 0.2669 +- 0.00415 due to the inclusion of macroscopic mixing. The dominant source of uncertainty is found the surface helium abundance inferred from helioseismic constraints and more specifically, the impact on the equation of state of the solar material on this inference result.

Figures (10)

• Figure 1: Diffusion velocity of helium for as a function of normalized radius in the radiative zone of solar models (a standard solar model in light blue and models including macroscopic mixing and/or overshooting at the BCZ in red, green and purple).
• Figure 2: Evolution of the surface metallicity of calibrated solar models, $(Z/X)_{S}$, as a function of time (a standard solar model in light blue and models including macroscopic mixing and/or overshooting at the BCZ in red, green and purple). The assumed final metallicity at the solar age is that of Asplund2021.
• Figure 3: Evolution of the helium mass fraction in the convective envelope as a function of time for the models of Table \ref{['tabModelsDTurb']}. The "Observed" value (red cross) is taken as that of BasuY2004.
• Figure 4: Left panel: Evolution of surface Lithium abundance as a function of age (in log scale) for the models of Table \ref{['tabModelsDTurb']}. The "Observed" value is taken from Wang2021. Right panel: Evolution of the surface Beryllium abundance as a function of age (in log scale) for the models of Table \ref{['tabModelsDTurb']}. The "Observed" value is taken from Amarsi2024.
• Figure 5: Left panel: Evolution of the helium mass fraction in the convective envelope as a function of time for the models of Table \ref{['tabModelsOpOvDTurb']}. The "Observed" value (red cross) is taken as that of BasuY2004. Right panel:Evolution of the helium mass fraction in the convective envelope as a function of time for the models of Table \ref{['tabModelsOvDTurb']}.