Postdoctoral positions
A 2+1 year postdoctoral position in either hydrodynamical modelling of stellar mass transfer, or heavy element nucleosynthesis will be available from the 1st of October 2013.Ph.D. projects
One Ph.D. position is currently available from the 1st of October 2013. A second position will become available from October 2014.Mixing processes in post-accretion systems
It is common for mass to be transfered between stars in binary systems. Often this means that material that has undergone nuclear burning in one star is dumped onto pristine matter on its companion. The surface abundances that we are able to observe depend on what happens to this material. Many processes, like convection, are able to mix this accreted material deep into the receiving star. This also opens up alternative routes to stellar nucleosynthesis.
In this project, the student will compute detailed stellar evolution models for accreting stellar systems. In particular, they will look at the effects of rotation and radiative levitation. The models will be compared to the detailed abundances available in some of the oldest stars in the Universe, the so-called carbon-enhanced metal-poor stars.
Masters projects
Simulating He accretion onto CO White Dwarfs
Type Ia supernovae are a hot topic in modern astronomy: they are one of the best probes of the expansion history of the Universe and play a pivotal role in the chemical evolution of galaxies. However, the physical origin of these dramatic events is current unknown. The best models for Type Ia's involve the thermonuclear explosion of a massive carbon-oxygen white dwarf stars in interacting binary systems. A key element of many of the models is the need to grow the white dwarf's mass by accreting material from its companion star. For some leading models, phases in which accretion of He-rich material is vital.
Despite its importance, He accretion by white dwarfs is not well-understood. In this project, the student would carry out stellar evolutionary calculations for CO white dwarfs accreting He-rich material to determine the fate of the white dwarf: when does it genuinely grow in mass and when is the accreted material ultimately expelled again?
Mixing and nucleosynthesis in low-mass stars
The final phase of the life of a low-mass star is an important time for the formation of many different elements. The unusual structure of these stars opens up many possible pathways for nucleosynthesis as stellar material can be exposed to proton-, neutron- and alpha-captures, via a complex sequence of convective motions. Predictions of state-of-the-art calculations fail to reproduce the abundance patterns observed in many stars (as well as the more precise determinations made in pre-solar grains obtained from meteorites), with some of the lightest elements being a particular problem. This suggests that stellar theory has missed something important: something other than convection is at work in these objects. In this project, the student will compute detailed nucleosynthesis models to determine the characteristics of this missing process. They will investigate how deep material needs to be transport in order to activate the necessary nuclear reactions, and how fast the transport process needs to be. Ultimately, these characteristics will be compared to known non-convective processes with the aim of determining the physical cause of the mixing.
Heavy element nucleosynthesis
The production of some of the heavy elements beyond iron occurs by what is called the slow neutron-capture process (the s-process). For the s-process to occur the neutron density needs to be low enough that when a nucleus captures a neutron -- which most likely forms an unstable nucleus -- the new isotope has time to decay back to stability before the next neutron is absorbed. In this way, heavy elements from iron to lead can be produced. The s-process is observed to happen in the final phase of the life of low-mass stars, with alpha-captures on to carbon-13 nuclei providing the source of neutrons. However, for this to work one must first produce a pocket of carbon-13 in the star and at present we do not know how to do this. In this project, the student will use a nucleosynthesis code to investigate the nature of the carbon-13 pocket needed to produce the s-process. They will determine how the size and shape of the pocket affects the production of the s-process nuclei. This will then allow us to examine what physical processes might be responsible for the formation of the carbon-13 pocket.
Bachelors projects
Mixing and nucleosynthesis in low-mass stars
The final phase of the life of a low-mass star is an important time for the formation of many different elements. The unusual structure of these stars opens up many possible pathways for nucleosynthesis as stellar material can be exposed to proton-, neutron- and alpha-captures, via a complex sequence of convective motions. Predictions of state-of-the-art calculations fail to reproduce the abundance patterns observed in many stars (as well as the more precise determinations made in pre-solar grains obtained from meteorites), with some of the lightest elements being a particular problem. This suggests that stellar theory has missed something important: something other than convection is at work in these objects. In this project, the student will compute detailed nucleosynthesis models to determine the characteristics of this missing process. They will investigate how deep material needs to be transport in order to activate the necessary nuclear reactions, and how fast the transport process needs to be. Ultimately, these characteristics will be compared to known non-convective processes with the aim of determining the physical cause of the mixing.
Heavy element nucleosynthesis
The production of some of the heavy elements beyond iron occurs by what is called the slow neutron-capture process (the s-process). For the s-process to occur the neutron density needs to be low enough that when a nucleus captures a neutron -- which most likely forms an unstable nucleus -- the new isotope has time to decay back to stability before the next neutron is absorbed. In this way, heavy elements from iron to lead can be produced. The s-process is observed to happen in the final phase of the life of low-mass stars, with alpha-captures on to carbon-13 nuclei providing the source of neutrons. However, for this to work one must first produce a pocket of carbon-13 in the star and at present we do not know how to do this. In this project, the student will use a nucleosynthesis code to investigate the nature of the carbon-13 pocket needed to produce the s-process. They will determine how the size and shape of the pocket affects the production of the s-process nuclei. This will then allow us to examine what physical processes might be responsible for the formation of the carbon-13 pocket.