From the Sun to solar-like stars: how does the solar modelling problem affect our studies of solar-like oscillators?

Abstract: Since the first observations of solar oscillations in 1962, helioseismology has probably been one of the most successful fields of astrophysics. Besides the improvement of observational data, solar seismologists developed sophisticated techniques to infer the internal structure of the Sun. Back in 1990s these comparisons showed a very high agreement between solar models and the Sun. However, the downward revision of the CNO surface abundances in the Sun in 2005, confirmed in 2009, induced a drastic reduction of this agreement leading to the so-called solar modelling problem. More than ten years later, in the era of the space-based photometry missions which have established asteroseismology of solar-like stars as a standard approach to obtain their masses, radii and ages, the solar modelling problem still awaits a solution. We will briefly present the results of new helioseismic inversions, discuss the current uncertainties of solar models and possible solutions to the solar modelling problem. We will also discuss how the solar problem can have significant implications for asteroseismology as a whole by discussing the modelling of the exoplanet-host star Kepler-444, thus impacting the fields requiring a precise and accurate knowledge of stellar masses, radii and ages, such as Galactic archaeology and exoplanetology.

• The paper investigates how the persistent 'solar modelling problem,' caused by revised solar abundances, impacts the study of solar-like oscillating stars using helioseismic inversions.
• Applying the analysis to Kepler-444, the study shows that discrepancies in solar models directly influence asteroseismic characterization, revealing the significance of unrecognized mixing processes like core overshooting.
• Findings underscore the critical need for incorporating additional physics into stellar models to accurately characterize stellar interiors, impacting fields like Galactic archaeology and exoplanetology.

Understanding Solar Modelling and Its Implications for Solar-Like Stars

The paper, "From the Sun to solar-like stars: how does the solar modelling problem affect our studies of solar-like oscillators?" authored by G. Buldgen and colleagues, explores a persistent issue in astrophysics: the solar modelling problem. This problem pertains to the inconsistency between solar models and helioseismic data following the revision of solar abundances, specifically the abundances of carbon, nitrogen, and oxygen (CNO). This research revisits these foundational issues using novel helioseismic inversions and extends its implications to the study of solar-like stars, as exemplified by the exoplanet-host star Kepler-444.

The Solar Modelling Problem

Historically, helioseismology has been a robust tool for validating theoretical solar models until the downward revision of solar metallicity introduced discrepancies between models and observations. The revision in CNO abundances has induced significant debate and research, focusing on resolving these discrepancies via modifications in modelling components, such as opacities, equations of state, and additional mixing processes in the solar interior.

This paper explores the impacts of various physical parameters and modelling uncertainties to understand the solar problem better. Modifications in opacity tables, chemical abundance parameters, and assumptions about mixing processes are proposed solutions, highlighting the complex interplay of multiple factors contributing to the solar modelling problem. It is emphasized that even small variations can significantly impact the inference of solar and stellar properties, underlining the intricacies in stellar modelling.

Asteroseismic Modelling of Kepler-444

The paper further applies these insights to model Kepler-444, a solar-like oscillator, demonstrating that the discrepancies in solar models have direct implications for asteroseismology. The study's seismic modelling strategy uses a combination of statistical tools and seismic inversions for accurate stellar characterization. This involves iterating through various models with different physical inputs to gauge how differences in stellar modelling parameters affect derived properties such as mass, radius, and age.

One interesting outcome of the study is the detection that unrecognized mixing processes—like core overshooting in Kepler-444—significantly impact these properties. Such processes contribute to maintaining out-of-equilibrium burning phases, leaving observable signatures in the star's oscillation frequencies. This emphasizes the importance of accounting for non-standard stellar model components when deducing fundamental parameters from seismic data.

Implications and Future Directions

The implications of these findings are profound, impacting not only the accuracy of characterizing stellar interiors but also affecting broader fields such as Galactic archaeology and exoplanetology. As space-based photometry missions continue to yield high precision data, the need to refine stellar models by incorporating additional physics becomes increasingly important. The inadequacies exposed in current models by the solar problem necessitate a reevaluation of our approaches to stellar modelling and point to the potential for more comprehensive models that include dynamic processes such as mixing and overshooting.

The paper culminates with a call for further research to explore these modelling disparities and a caution against the overinterpretation of seismic data without considering model limitations. As observational capabilities increase, the opportunity to refine our models of stellar interiors will grow, potentially leading to more robust and universally applicable models of stellar evolution and dynamics.