Subgiant Branch: A Quantitative Investigation of Initial Helium Abundance and Stellar Evolutionary Models
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Date
2025-05
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The Ohio State University
Abstract
In this thesis, we inferred birth helium abundances for stars and tested the linear enrichment relations between star-produced helium and metal. We used stellar models to predict the luminosities of stars predicted by standard chemical evolution scenarios. We then calculated a map of luminosity-helium derivatives (δlogL/δY ) across different stellar parameters. This procedure is necessary because stellar helium is not directly observed, and we isolated helium fraction as an independent degree of freedom in stellar models. Based on the offsets between model-predicted luminosity and observed luminosities, we calculated the helium correction required to reproduce the data: ΔY , and the consequent Y +ΔY as our indirectly-inferred true helium fraction. Our calculations serve as empiricallyinformed examination of the model-assumed chemical composition.
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Keywords
Stellar Chemical Evolution, Stellar Evolutionary Models, Helium Abundance, Astronomy and Astrophysics, Stellar Structure