“Do We Know What the Sun is Made of? The Puzzle of the Solar Composition” with Sarbani Basu (Yale University, USA)

All stars, including the Sun, are predominantly made of hydrogen and helium. However, the very small amount of the remaining elements, or “metals” as they are called, has a profound effect on the structure and evolution of a star. This is mainly because metals impede radiation – the higher the metallicity, the more opaque stellar material is. Given the proximity and ease of observing the Sun, solar metallicity is used as the reference for the metallicity of other stars. However, for the last two decades, there is no consensus about what the solar metallicity is.

In the early 2000s, the solar composition was revised downwards by about 30%, a result of analyses using 3D atmospheric models and non-LTE effects. While under most circumstances, this result would have been hailed as a triumph of using the best physics to analyze spectra, it created a problem – solar models constructed with the older metallicities matched the structure of the Sun, the models with the newer metallicities are extremely discrepant.

The structure of the Sun can be determined in a model-independent manner by analyzing the frequencies with which the Sun oscillates. All such analyses have shown that the lower abundances produce models that are discrepant. Although the metallicity estimates have increased since the original low ones, they are not high enough to result in solar models whose structures agree with that of the Sun.

In this talk, the speaker will review the problem and discuss how helioseismology, i.e., the study of solar oscillations, can be used to examine solar metallicity. She will also discuss attempts that have been made to change, as well as test, inputs to solar models in an attempt to reconcile the low abundances with helioseismic results. The speaker will end by discussing how solar neutrinos might provide the answer to the question of what the Sun is made of.

Sarbani Basu is the William K. Lanman Jr. Professor and Chair of the Department of Astronomy, Yale University. She is a global authority in helioseismology, or the study of the structure and dynamics of the sun using solar oscillations, her research also uses the sun as a laboratory to study the physics within it. She received the George Ellery Hale Prize of the American Astronomical Society in 2018 for her contributions to the understanding of the internal structure and dynamics of the Sun and stars. She served as the Chair for the panel on Starts, the Sun and Stellar Populations for the Astro2020 decadal survey; the survey will determine the direction of US astrophysics for the next decade.

This webinar was recorded on June 17, 2021