Lithium-7 problem in solar-like stars / Wan Aishah Wan Harun

Wan Aishah , Wan Harun (2022) Lithium-7 problem in solar-like stars / Wan Aishah Wan Harun. Masters thesis, Universiti Malaya.

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Abstract

Although 7Li is a stable isotope, the element is fragile to an extreme temperature higher than 106 K. Therefore, 7Li content in a star can only survive in the region of the convection zone, where it can be observed by using high-resolution spectral line analysis. However, many theoretical predictions have shown that lithium abundance in the Sun should be higher by 2 dex than the observed photospheric value. Other similar missing lithium is found in some stars, which contributes to the lithium problem in general. Therefore, this thesis aims to analyze the mechanisms that may lead to the higher mixing of lithium burning. We first calibrate the solar model by using MESA code to implement past solutions suggested from the literature. We verified that the existing solutions are indeed failed to explain the discrepancy. Hence, we enhanced the convection mixing process to induce more lithium burning of the surface. The computation showed that the higher convection parameter U = 2.4 could effectively deplete another one dex of lithium. While the increase by a factor three of the gravitational settling, can only enhance the lithium by 0.1 dex. Interestingly, we successfully reproduced the observed lithium content when we had adopted a higher rotational velocity kick-off of 46 km/s. However, the result is ambiguous as we can not explain how the initially applied rotation can be spun down to the present solar rate. We found that lithium mixing below the convection base is highly dependent on the opacity of the solar mixture. Finally, we had predetermined the initial lithium abundance at ZAMS to be 1.42 dex in which can represent today’s photospheric lithium value.

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