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Deep turbulent mixing can explain asteroseismic inference of the helium abundance in F-type stars

Turbulent diffusion in F-type stars
Turbulent mixing at the bottom of convection zone.

Chemical elements tend to segregate over time in the interior of stars including the Sun. The helium and other heavier elements (called metals in astronomy) slowly sink to the interior, while the hydrogen rises to the surface. This phenomenon is known as atomic diffusion. The models of atomic diffusion are well understood in the context of the Sun and more generally for the so-called G-type stars. However, the same models fail catastrophically while explaining the spectroscopic measurements of the metal abundances as well as the asteroseismic inference of the helium abundance for more massive and hot F-type stars. The models predict complete depletion of the surface helium and metals which is in contrast to the observation, indicating missing physics in our understanding of such stars.

Stars produce energy in their core by nuclear fusion, which is transported to the surface and eventually gets radiated out. In the outmost layers of F-type stars, the energy is transported from interior to the surface via convection, similar to the energy transport from bottom to the top in a pan of boiling water. We demonstrated in the article that mixing caused by turbulence at the bottom of the convection zone can reduce the segregation of chemical elements arbitrarily. A much deeper mixing was necessary to explain both the observed helium and metallicity of three analysed F-type stars observed by the Kepler satellite compared to those obtained in previous studies based solely on the measurement of the metal abundances.

This blog is based on one of my research article which was published in 2019 in Monthly Notices of the Royal Astronomical Society (publicly available on arXiv). The article is titled "Helium settling in F stars: constraining turbulent mixing using observed helium glitch signature".

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