On the role of doubly-diffusive mixing in low-mass red giant branch stars

The purpose of this thesis is to investigate mixing in red-giant stars. Thanks particularly to the observations of globular clusters, it is clear that that standard stellar evolution fails to reproduce measured stellar abundances as a function of (red giant branch) luminosity. The empirical data has demonstrated that theorists are missing vital physics in their codes during this phase of evolution. Solutions to this problem have been postulated for many decades now, with mechanisms attributed (but not limited) to rotation, magnetic fields or internal gravity waves. A recently suggested instability, namely thermohaline mixing, serves as the focus of this investigation. It is caused by the competition between heat and chemical diffusion and is only possible due to the unique conditions following dredge-up events. Using observations of lithium and carbon we determine whether this mechanism and/or its current parameterisation can influence the surface composition of red giant branch stars. The quality of the data collected from the Very Large Telescope in Chile, some of which is gathered from our own observing programs, allows us to further constrain the mixing process(es)involved and improve the stellar models.