Standard solar models: Perspectives from updated solar neutrino fluxes and gravity-mode period spacing

Abstract

Context. Thanks to the vast and exquisite set of observations that have been made available for the Sun, our star is by far an ideal target for testing stellar models with a unique precision. A recent issue under consideration in the field is related to the progress in the solar surface abundances derivation that has led to a decrease of the solar metallicity. While the former high-metallicity models were in fair agreement with other observational indicators from helioseismology and solar neutrino fluxes, it is no longer the case for low-metallicity models. This issue has become known as ‘the solar problem’. Recent data are, however, promising to shed a new light on it. For instance, in 2020, the Borexino Collaboration released the first-ever complete estimate of neutrinos emitted in the CNO cycle, which has reaffirmed the role of the neutrino constraints in the solar modelling process and their potential in exploring related issues. In parallel, a newly claimed detection of solar gravity modes of oscillation offers another opportunity for probing the stratification in the Sun’s central layers. Aims. We propose combining the diagnoses from neutrinos and helioseismology, both from pressure and gravity modes, in assessing the predictions of solar models. We compare in detail the different physical prescriptions currently at our disposal with regard to stellar model computations. Methods. We build a series of solar standard models based on a variation of the different physical ingredients directly affecting the core structure: opacity, chemical mixture, nuclear reactions rates. We compare the predictions of these models to their observational counterparts for the neutrinos fluxes, gravity-mode period spacing, and low-degree pressure mode frequency ratios. Results. The CNO neutrino flux confirms previous findings, exhibiting a preference for high-metallicity models. Nevertheless, we find that mild modification of the nuclear screening factors can re-match low-metallicity model predictions to observed fluxes, although it does not restore the agreement with the helioseismic frequency ratios. Neither the high-metallicity or low-metallicity models are able to reproduce the gravity-mode period spacing. The disagreement is huge, more than 100σ to the observed value. Reversely, the family of standard models narrows the expected range of the Sun’s period spacing: between ∼2150 and ∼2190 s. Moreover, we show this indicator can constrain the chemical mixture, opacity, and – to a lower extent – nuclear reactions in solar models.