Helioseismology and solar abundances

Abstract

Helioseismology has allowed us to study the structure of the Sun in unprecedented detail. One of the triumphs of the theory of stellar evolution was that helioseismic studies had shown that the structure of solar models is very similar to that of the Sun. However, this agreement has been spoiled by recent revisions of the solar heavy-element abundances. Heavy element abundances determine the opacity of the stellar material and hence, are an important input to stellar model calculations. The models with the new, low abundances do not satisfy helioseismic constraints. We review here how heavy-element abundances affect solar models, how these models are tested with helioseismology, and the impact of the new abundances on standard solar models. We also discuss the attempts made to improve the agreement of the low-abundance models with the Sun and discuss how helioseismology is being used to determine the solar heavy-element abundance. A review of current literature shows that attempts to improve agreement between solar models with low heavy-element abundances and seismic inference have been unsuccessful so far. The low-metallicity models that have the least disagreement with seismic data require changing all input physics to stellar models beyond their acceptable ranges. Seismic determinations of the solar heavy-element abundance yield results that are consistent with the older, higher values of the solar abundance, and hence, no major changes to the inputs to solar models are required to make higher-metallicity solar models consistent with helioseismic data.

Solar System Abundances of the Elements

“…A decrease in heavy element abundances led to solar model results that no longer stood the test from helioseismology. A detailed review of this problem is given by Basu & Anita (2008). The recommended N, O, and Ne abundances here are larger than previously recommended in L03 and A05, and it will be interesting to see if these abundances can bring solar models again in closer agreement with helioseismological constraints.…”

“…Settling or diffusion of heavy elements from the photosphere to the interior boundary layer of the convection zone and beyond lowered the elemental abundances (relative to H) from protosolar values 4.56 Ga ago (see Basu & Anita 2008). Over the Sun's lifetime, diffusion decreased abundances of elements heavier than He by ~13% from original protosolar values, whereas that of He dropped a little more by about ~15%; modeling these depletions also dependent on opacities, hence abundances.…”

“…Lower abundances for some elements derived from these models also cause problems for standard solar models that describe the evolution of the sun to its current radius and luminosity (see Basu & Anita 2008). Another problem is that some of the 3D abundances compare worse to meteoritic data than before.…”

“…The mass fraction Z, which combines all other heavy elements is also important as it governs opacities and thus the density structure of the Sun's outer convection zone. The depth of the solar convection zone derived from helioseismic data poses constraints on the permissible fraction of heavy elements (a detailed review on helioseismology and the He abundance problem is given by Anita & Basu 2008).…”

Detailed Opacity Calculations for Astrophysical Applications

“…The OP Rosseland mean at these conditions is much lower and equal to 854 cm 2 /g. Solar models typically find a location of the boundary between these zones that differs significantly from the measured one (by more than 13 standard deviations [61]). Several hypotheses can explain this difference:…”