Chemical composition of the solar surface
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J. Astrophys. Astr. (2020)41:41 https://doi.org/10.1007/s12036-020-09666-3
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Chemical composition of the solar surface CARLOS ALLENDE PRIETO1,2, 1
Instituto de Astrofı´sica de Canarias, Vı´a La´ctea S/N, 38205 La Laguna, Tenerife, Spain. Departamento de Astrofı´sica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain. E-mail: [email protected] 2
MS received 22 November 2020; accepted 23 November 2020 Abstract. The Sun provides a standard reference against which we compare the chemical abundances found anywhere else in the Universe. Nevertheless, there is not a unique ‘solar’ composition, since the chemical abundances found in the solar interior, the photosphere, the upper atmosphere, or the solar wind, are not exactly the same. The composition of the solar photosphere, usually preferred as a reference, changes with time due to diffusion, convection, and probably accretion. In addition, we do not know the solar photospheric abundances, inferred from the analysis of the solar spectrum using model atmospheres, with high accuracy, and uncertainties for many elements exceed 25%. This article gives an overview of the methods and pitfalls of spectroscopic analysis, and discusses the chemistry of the Sun in the context of the solar system. Keywords. The Sun—chemical composition—stellar atmospheres.
1. Introduction The chemical composition of the Sun, our nearest and most influencing star, is not yet known with high accuracy. The temperatures in the solar interior, as well as in the corona, can reach millions of degrees, and even in the photosphere, where they reach the lowest values, are still thousands of degrees, making it hard to retrieve samples to measure in a laboratory. Our knowledge about the solar chemical composition relies on indirect measurements, through the analysis of oscillations, particles, or light, through physical models of the solar structure. The abundances derived from optical spectroscopy, which mainly probe the solar photosphere, are usually preferred as indicative of the gas in the protosolar nebula from which our star formed. In the stellar core, hydrogen burning progressively increases the fraction of helium. In the higher atmosphere, significant chemical anomalies are found, most notably the so-
This article is part of the Topical Collection: Chemical elements in the Universe: Origin and evolution.
called FIP effect, referring to the low abundances found for elements with a high first ionization potential. If more stable in time than other regions of the Sun, the solar photosphere is not free from chemical changes. Convective mixing in the solar envelope, which covers the outer 30% of the solar radius, continuously cycles photospheric material into regions with much higher temperatures, where lithium is destroyed. At the solar age (about 4500 million years), the photospheric abundance of this element appears depleted relative to its value at birth by a factor of about 150. Models of the Sun indicate that diffusion has reduced, ove
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