My research focuses on stellar evolution and stellar nucleosynthesis. I work on how stars live their lives and how they ultimately perish. I am interested in how material is moved around in stellar interiors and how this affects which isotopes can be synthesized. A more detailed description of some of the areas of my research is given below.
I work on a range of topics regarding both single and binary stars, looking at how these stars live and die and how they contribute to the build-up of all the elements we see in the Universe today. I work with the 1D stellar evolution code STARS, which I have been using since starting my Ph.D. The code is capable of modelling the lives of both low- and high-mass stars, both as single stars and as members of binary systems. The code has an extended nuclear network for looking at stellar nucleosynthesis in detail.
I primarily work on the late phases of evolution of low-mass stars. These stars end their lives during the asymptotic giant branch (AGB), where they have an unstable double-shell burning structure. This structure leads to some very interesting and complex nucleosynthesis and it is during this phase that these stars produce large quantities of carbon and heavy elements like strontium and barium, which are then returned to the interstellar medium by strong winds. I look at how the transport of material within these stars by processes like convection and internal gravity waves affects what isotopes these stars can produce.
Metal-poor stars represent some of the oldest objects in the Universe. They are low-mass objects, thought to be around 80% of the Sun's mass and as a consequence they are relatively unevolved objects. A surprising fraction (maybe as much as 20%) of these metal poor objects are extremely rich in carbon. They also display a variety of enrichments of heavy elements, including barium, europium and lead. As these objects are unevolved, they cannot have produced these elements themselves and they must have come from elsewhere.
The most numerous class of the carbon-enhanced metal-poor stars (or CEMP stars, for short) are those that display substantial enrichments of barium. These systems are believed to be the end product of the evolution of binary star systems. The system would have originally started out with a star of around 1 to 3 solar masses being orbited by a companion star of around 0.8 solar masses with a period of thousands of days. The more massive star (the primary) evolves quickest, reaching the asymptotic giant branch (AGB) phase before the less massive star (the secondary) has time to do anything. While on the AGB, the primary makes both carbon and s-process elements (like barium) and this material is transfered to the secondary by a strong stellar wind. The primary becomes a while dwarf, disappearing from view. We are left with only the secondary: an unevolved object that is rich in both carbon and s-elements.
CEMP stars carry the signatures of the earliest generations of AGB stars. By studying their composition, we can learn about what isotopes these stars produced and how their structure and evolution is different from their higher metallicity cousins. In addition, they are good laboratories for binary star physics. They allow us to probe questions about how mass is transferred between stars and what happens to that material once it is dumped onto the secondary.
1D stellar evolution provides an excellent description of stars in most stages of their lives, but it is not without its limitations. Many of the processes that go on inside of a star are inherently three dimensional and our approximate treatments may lose important details. Fortunately, we can appeal to hydrodynamics to try to improve the situation.
I have been using the DJEHUTY hydrodynamics code, developed at Lawerence Livermore National Laboratory, to investigate convective motions in AGB stars. In particular, I have been looking at what happens when protons are drawn into the convective region that is being driven by helium burning. 1D codes suggested that the convective region should split into two zones, one driven by helium burning, and one by hydrogen burning. This is based on a diffusive approximation for the transport of hydrogen. My simulations show that the convective transport is much more rapid that predicted by the 1D codes, and that a splitting of the convective zone is unlikely to happen.
We are now in the process of analysing these runs in more detail, with the aim of producing better algorithms for use in the 1D code. In addition, we are also modelling other phases of evolution where the approximations of 1D codes may be problematic.