Investigating the impact of Solar Fusion III reaction rates on helioseismic constraints and solar neutrino fluxes

Technical Summary: Investigating the impact of Solar Fusion III reaction rates on helioseismic constraints and solar neutrino fluxes

Problem Statement
Nuclear reaction rates are fundamental inputs for solar and stellar evolution models, dictating stellar lifetimes and energy production. While the Sun serves as a primary testbed for these physical ingredients due to precise constraints from helioseismology (e.g., convective envelope base, helium mass fraction) and neutrino flux measurements, there remains a tension between model predictions and observations. This study investigates the impact of the recently published Solar Fusion III (SFIII) reaction rates (Acharya et al. 2025) on solar models. Specifically, it examines whether these updated rates improve the agreement with helioseismic inversions and neutrino flux measurements, or if they exacerbate existing discrepancies, particularly regarding the boron ($\phi_B$), beryllium ($\phi_{Be}$), and CNO ($\phi_{CNO}$) neutrino fluxes.

Methodology
The authors computed a comprehensive set of Standard Solar Models (SSMs) and Non-Standard Solar Models (NSSMs) using the Liège Stellar Evolution Code (CLES).

• Model Variations: Models were calibrated using initial hydrogen ($X_0$) and heavy element ($Z_0$) mass fractions and the mixing-length parameter ($\alpha_{MLT}$) to match the solar radius, luminosity, and metallicity ($Z/X$). Two abundance sets were tested: Asplund et al. (2021, AAG21) and Magg et al. (2022, MB22).
• Reaction Rates: The primary focus was on models utilizing the SFIII rates. For comparison, models were also computed using previous compilations (NACRE II and Solar Fusion II).
• Physical Ingredients: The study incorporated microscopic diffusion, various opacity tables (OPAL and OPLIB), and macroscopic turbulent diffusion at the base of the convective zone (BCZ) to reproduce solar lithium and beryllium depletion.
• Screening Tests: Models were run with modified electronic screening coefficients (deactivated to emulate dynamical screening effects) and with reaction rates perturbed within their quoted uncertainties (specifically $^7$Be(p,$\gamma$)$^8$B and $^{14}$N(p,$\gamma$)$^{15}$O).
• Constraints: Models were evaluated against helioseismic inversions of the squared adiabatic sound speed ($c^2$) and entropy proxy ($S$) profiles, as well as predicted neutrino fluxes compared to Borexino and other experimental data.

Key Results

1. Helioseismic Constraints: The adoption of SFIII rates leads to a slight improvement in the agreement with helioseismic constraints in the solar core (reducing discrepancies in sound speed by $<0.2\%$). However, this improvement is accompanied by a degradation in agreement at the radiative side of the BCZ. The overall effect is complex, reminiscent of the transition between OPAL and OPLIB opacity tables, where gains in one region are offset by losses in another.
2. Neutrino Fluxes: In contrast to the mixed helioseismic results, the use of SFIII rates induces a significant reduction in predicted neutrino fluxes for $^7$Be, $^8$B, and CNO cycles.
   • For models using AAG21 abundances, the agreement with observed fluxes worsens significantly.
   • For MB22 abundances, while $^7$Be and $^8$B fluxes remain within $1\sigma$ of some measurements, the $^8$B flux shows disagreement with Borexino values.
   • Combining SFIII rates with OPLIB opacities or macroscopic mixing (NSSMs) further degrades the agreement with neutrino measurements.
3. Uncertainty Analysis: The authors tested whether the current quoted uncertainties in key reactions ($^7$Be(p,$\gamma$)$^8$B and $^{14}$N(p,$\gamma$)$^{15}$O) could reconcile the models with observations. They found that even extreme variations within these uncertainties (e.g., a 10% increase in $^7$Be rate or a 30% increase in $^{14}$N rate) are insufficient to restore agreement with observed neutrino fluxes, particularly for models incorporating macroscopic mixing or OPLIB opacities.
4. Screening Effects: Deactivating electronic screening for all reactions (pp chain and CNO) leads to a significant decrease in neutrino fluxes, creating strong discrepancies with measurements. This contrasts with some previous studies where screening was only modified for specific pp-chain reactions.

Significance and Claims
The paper concludes that the introduction of Solar Fusion III reaction rates does not systematically resolve the tensions between solar models and observational data. While there is a marginal improvement in helioseismic sound speed profiles in the core, the rates cause a notable degradation in the agreement with neutrino flux measurements.

The authors argue that the origin of these discrepancies lies in the altered thermodynamic conditions of the solar core (lower central temperature and density) required to calibrate the models with the new rates, combined with a slightly reduced efficiency of the CNO cycle at fixed conditions. The study emphasizes that current uncertainties in the key nuclear reaction rates are too small to bridge the gap between theory and observation. Consequently, the authors suggest that other physical processes, such as planetary formation effects on surface metallicity or mass loss, may need to be considered to reconcile solar models with the observed CNO neutrino flux. The work underscores the extreme sensitivity of neutrino fluxes to reaction rates and the complexity of the solar modeling problem, where improvements in one physical ingredient often degrade agreement with other constraints.