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Asteroseismology enables measurement of helium abundance of a solar analog binary system

Updated: May 9, 2021

Turbulent diffusion in F-type stars
Helium ionisation in the interior of a solar-type star.

The knowledge about chemical elemental abundances in the universe comes mainly through spectroscopy. The observed spectra of stars and other astronomical objects can be used to measure their chemical compositions. However, this technique becomes inapplicable to most stars (over 95%) including our own Sun when it comes to measure the abundance of the second most abundant element, viz. the helium. In fact, it was only during 1990s when the solar helium abundance could be measured reliably using the study of solar oscillations (helioseismology). The difficulty arises because helium atom is excited at higher temperatures than expected in the near surface layers of such stars. This limitation on the measurement of the stellar helium abundance poses major challenges in the study of stellar structure and evolution and Galactic chemical evolution.

Thanks to the French-led CoRoT and NASA Kepler sattelites, the recent developments in the study of stellar oscillations (asteroseismology) allow determination of the helium abundance in solar-type stars. This approach utilizes the fact that the ionization of helium within the stellar interior leaves a characteristic signature on the observed oscillation frequencies. It can be demonstrated that this signature contains information about the helium content present in the outer convection zone. We used this technique together with the Kepler asteroseismic data for a binary system, 16 Cyg A & B, to infer their surface helium abundance. This is the first measurement of the helium abundance in solar-type stars after the Sun.

This blog is based on one of my research article which was published in 2014 in the Astrophysical Journal (publicly available on arXiv). The article is titled "Asteroseismic estimate of helium abundance of a solar analog binary system".

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