A data-driven estimate of the protosolar helium mass fraction

¹ STAR Institute, Université de Liège, Liège, Belgium
² Department of Physics, Kurume University, 67 Asahimachi, Kurume, Fukuoka 830-0011, Japan
³ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Bd de l’Observatoire, CS 34229, 06304 Nice cedex 4, France
⁴ Centre Spatial de Liège, Université de Liège, Angleur-Liège, Belgium

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 15 January 2026
Accepted: 2 March 2026

Abstract

Context. The protosolar helium mass fraction is a fundamental ingredient of solar, planetary models and enrichment laws used to model stellar populations. However, the assumed values often rely on simplifying descriptions of the transport of chemicals in solar models. Furthermore, they are based on the inferred helium mass fraction in the solar convective envelope, which is itself sensitive to uncertainties in the equation of state of the solar material.

Aims. We aim to update the reference protosolar helium abundance by taking into account the effects of macroscopic mixing at the base of the convective zone and using more recent determinations of the helium mass fraction in the convective envelope.

Methods. We combined results from our own inversions of the composition of the solar envelope with spectroscopic abundances, as well as values in the literature, to provide a robust interval of the current helium mass fraction in the convective zone. We combined this measurement with solar models taking into account light element depletion to provide an updated protosolar helium abundance.

Results. We show that macroscopic mixing at the base of the convective envelope of the Sun cannot be neglected to infer the protosolar helium abundance. We demonstrate that as soon as this effect is included, the protosolar helium abundance is significantly reduced and that lithium and beryllium depletion can be used to calibrate this effect over the solar evolution. We find a revised interval of a primordial helium mass fraction of 0.27575 ± 0.00315 slightly lower than previous estimates when combining our latest estimate of the 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, but also derive more precise estimates based on various reference works in the literature. If the usual surface helium mass fraction is used, the primordial helium mass fraction drops to 0.2669 ± 0.00415 as a result of the inclusion of macroscopic mixing. The dominant source of uncertainty on this value is found to be 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.