Understanding Stellar Convection

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All elements heavier than hydrogen and helium are made in stars. The most common elements in the universe (carbon, oxygen) are made in the cores of stars, while heavier elements are made during the final phases of a stars death, and the details of this process are very sensitive to the size of the stellar core at the end of the main sequence. Convection, especially convective overshoot, can change the amount of material available to stellar cores, but the amount of convective overshoot is largely unknown. We will use stellar pulsation to constrain the amount of convective overshoot in a sample of six stars (Objective 1). Measuring the convective overshoot for individual stars is difficult, and has been done for only a handful of main sequence stars so far. This sample of six stars will significantly increase the number of stars with known convective overshoot parameters. As this measurement is made in more stars, we will be better able to characterize how the convective overshoot varies with the mass and age of a star. We will also investigate techniques that will allow measurements of convective overshoot in large numbers of stars (Objective 2). I have recently developed a technique that can in principle be used to constrain the convective overshoot in a sample of one type of variable star, the delta Scuti stars. We will investigate whether this technique holds when the composition of a star is allowed to vary, and also whether this technique can be extended to other classes of variable stars. Again, this will significantly increase the number of stars with known convective overshoots, allowing us to characterize the behaviour of the convective overshoot parameter with mass and age. Finally, we will investigate the effects of convection in more evolved stars. The Cepheid variables are important distance indicators in astronomy, and are expected to have significant convective envelopes. While most Cepheids are observed to have a single pulsation period, a significant number are multi-periodic. The origin of the multiple periods is unknown, but it has been proposed that it is related to the interaction between convection and pulsation in the stellar envelope. We will perform 2 and 3 dimensional simulations of Cepheid variables to investigate this interaction and explain the origin of the multi-periodic Cepheids. This will have important implications for the accuracy of the Cepheid distance scale, and hence for our understanding of the scale and history of the universe.