| 其他摘要 | As increasing spectroscopic survey data are released, the anomalous lithium enrichment behaviour in giants is gradually becoming clearer. Currently, about one percent of giants are lithium-rich giants, with lithium abundances mainly distributed in the range of 1.5 dex to 4.0 dex. Although they are only a small fraction of red giant stars, their appearance challenges standard theories of stellar evolution. The physical attribution of their appearance can help refine theories of stellar structure and evolution and deepen the understanding of stars. Currently, there is no universally recognised cause for their formation. Therefore, we will investigate the physical mechanism of their anomalous lithium abundances. As for red giants with lithium abundances less than 1.5 dex, although they are the vast majority, the standard stellar models also show clear limitations in explaining these stars. Therefore, we will also seek more appropriate physical processes for the lithium abundance distribution and evolutionary behaviour of these stars. Our study focuses on the internal mixing processes in stars. This is because lithium is only effectively present in the surface convection zone and lithium is derived from the decay of beryllium. The transport of beryllium produced by hydrogen burning in the stellar interior will then be the most critical physical process to achieve lithium enrichment on the stellar surface.With the help of the stellar evolutionary programme MESA, the first step is to construct the simplest stellar models, i.e. in which convective mixing is the only mixing process. On this basis, we will introduce some extra mixing processes that may drive the transport of beryllium inside star.The distribution of lithium abundances in red giant branch stars can be explained by thermohaline mixing. However, for red clump stars, the lithium abundances predicted by the thermohaline mixing model is completely out of the observed range. First, we construct convective mixing, convective overshooting, and thermohaline mixing models. The results show a large deviation between the lithium abundance predicted by the three models in the mass and metallicity grids and the observed lithium abundances of red clump stars. We then consider internal gravity waves in the red giant branch phase, excited by perturbations generated by turbulence within the convective envelope at the convective boundary. The internal gravity waves will propagate within the radiation region and reflect off the surface of the helium core, forming a resonant cavity between the hydrogen-burning shell and the convective envelope. We treat this as a diffusion process, which will then play a role in the path of beryllium transport. The results of the internal gravity waves model show a behaviour of first enrichment and then decay of surface lithium. The internal gravity waves model first increases the lithium abundance in the tip of red giant branch to near 1.5 dex, and then decreases the lithium abundance to near 0.5 dex as the red clump star evolves, bringing the surface lithium into the interior of the star to participate in the reaction. The model-predicted lithium abundance distribution coincides with most of the observations of red clump stars determined by asteroseismic methods, and in addition, the internal gravity waves model predicts that the lithium abundances of red clump stars will show an evolutionary behaviour of decreasing with age.For lithium-rich red giant branch stars and red clump stars, we start with a convective model. During the first dredge-up, the surface convective zone of the star erodes into the stellar interior, and its convective boundary reaches regions of inhomogeneous chemical composition left by the main sequence hydrogen burning phase. When the convective boundary is in such a physical environment, the convective boundary should be re-selected. We recalibrate the convective zone with the Ledoux criterion and, in addition, use different convective boundaries at different evolutionary stages. The results show that this convective model is able to suppress the dilution of lithium by the surface convective zone during the first dredge-up phase, thus exhibiting lithium enrichment features. The results of modelling for the parameter grid of population I show that most red giant branch stars can naturally evolve into lithium-rich red giants with the help of convective mixing, and that lithium-rich red clump stars with lower masses and metallicities also be explained by this model. However, the model cannot explain stars with unusually high lithium abundances. In addition, lithium-rich giants are rare, so we propose to introduce some extra lithium depletion processes to match the observations.Other physical processes should also be considered in explaining the anomalous lithium behaviour of red giants. In the internal gravity waves model, we suggest introducing rotation-driven mixing. The differential rotation between the helium core and the convective envelope may change the length of the mixing zone driven by internal gravity waves, and it is expected that the slight difference of length between the bottom of the mixing zone and the hydrogen-burning shell will greatly affect the beryllium transport efficiency. Furthermore, the role of some external processes on stellar lithium enrichment cannot be ignored, and we propose to introduce a matter accretion process. This may have two effects, one is the weakening of stellar mass growth on lithium depletion during the first dredge-up, and the other is the possible lithium contamination effect on the stellar surface. Both of these aspects are directly or indirectly realised by lithium enrichment. |
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