|
| |
|
I am available to supervise projects on a wide variety of subjects, ranging from stellar population studies to detailed stellar evolution. In particular, if you are interested in the following subjects please contact me: - Chemical abundances in single- and binary-stars, particularly at low metallicity
- Mass-transfer and orbital evolution in binary stars
- Quantiative studies of large numbers of massive stars: comparing models to the new Tarantula VLT FLAMES survey
- Explaining spin rates in massive stars
- Mixing in stars: rotation, magnetic fields, chemical gradients
- Globular clusters and the multiple-population problem
- Galactic chemical evolution
- Stellar explosion progenitor studies, in particular the progenitors of gamma-ray bursts and type Ib/c supernovae
| | Masters Project: Blue Stragglers in the Period Gap | | Blue Straggler stars are bluer (hotter) and more luminous than they should be for their age. There are two main channels for making these stars: 1) mergers, which might be common in dense stellar environments, e.g. globular clusters; 2) Mass transfer in binary stars. Stars that evolve through this second channel should remain as binary stars during their blue straggler phase of evolution, and indeed many blue stragglers are seen in binaries. We can model the mass transfer by means of computer simulations, e.g. Geller, Hurley and Mathieu (2013, AJ 145,8), but the predicted post-mass transfer period distribution disagrees with that observed. While blue stragglers are observed to have periods in the range 100 to 1000 days, none are predicted in the models. One might say these are just a few stars, but the same discrepancy is seen in e.g. the Barium stars, see Izzard, Dermine and Church 2010. Something is clearly wrong with our mass transfer algorithm! However, the latest results of Abate et al. 2011 - which use hybrid wind-Roche lobe overflow mass transfer - may explain these intermediate period stars. This project will use our binary-star population synthesis code binary_c/nucsyn to test whether Wind-RLOF can solve either the Barium-star intermediate period problem, or the similar problem seen in blue straggler systems. |  | | Masters Project: "Off-grid" AGB stars |  | Asymptotic giant branch stars make most of the carbon, nitrogen and much of the heavy metal content of the Universe. Stars with masses between 1.5 and 3.0 solar masses make primarily carbon, while those between 3.0 and 8.0 solar masses are hot enough to burn carbon, via the CNO cycle, into nitrogen.
AGB stars are too big to survive in close binary systems. They are so large that they will often overflow material onto a companion star, with which they may merge. If the companion itself is evolved, e.g. it is a white dwarf, then the white dwarf can merge with the AGB star to form a new star with an anomalously large core mass - an "off grid" AGB star. The aim of this project is to determine whether these "off grid" AGB stars exhibit the same nucleosynthesis as normal AGB stars: how much carbon do they dredge up? What range of core and envelope masses lead to dredge up and CNO cycling of the envelope? To do this, a detailed stellar evolutionary code (with the help of Richard Stancliffe) will be used to modify single AGB stars such that they have increased core masses. They will then be evolved to determine surface abundances and ejected yields. The results will be implemented in our binary-star population synthesis code binary_c/nucsyn, which should lead to a publication. | | Bachleor Project: Stability of Mass Transfer Algorithms in Binary Star Simulations | | Most stars more massive than our Sun exist not alone but with a companion star in a close orbit. As they age, these stars grow larger and may interact by transferring mass between each other. Many important stellar astrophysics phenomena only or mostly occur in such systems, for example X-ray binaries, type Ia supernovae, gamma-ray bursts, cataclysmic variable stars, thermonuclear novae, to name just a few. Our models of mass transfer are mostly based on an explicit solution method which is often numerically unstable. An implicit method is often not practical, because it requires too much computer code to be changed, so this project would build on the explicit method and try to make it more numerically stable so that the rate of mass transfer -- and evolution of the stars -- is calculated more accurately. First it will be shown why and when the explicit method is unstable, then the mass transfer algorithm will be modified to improve stability while retaining accuracy and computational speed (in both the explicit and implicit cases). If successful, the results will be implemented in our binary-star population synthesis code binary_c/nucsyn. Suggested reading : Buening and Ritter 2005 (astro-ph/0510126). | | Bachleor Project: models for Hot Bottom Burning |  | Stars in the mass range 1 < M/M⨀ > 8 evolve through a red giant phase during which carbon is mixed from their core into their envelope. These are often seen as carbon stars. However, in stars with masses above about 4M⨀ the base of the stellar envelope is so hot that nuclear burning occurs in a convection zone which is connected to the stellar surface, a process called Hot Bottom Burning. Carbon and oxygen are converted into nitrogen and, if the burning is hot enough, the NeNa and MgAl chains can be activated (see e.g. this paper). In my binary_c/nucsyn code I simulate this process in a very simple - but fast - way using a single burning zone model. However, this has its limitations. This project will improve the hot-bottom burning model by implementing existing burning algorithms on a polytropic envelope structure. This should better match fast model results to detailed (slow) models. If successful, the results will be implemented in our binary-star population synthesis code binary_c/nucsyn. |
|
|
| |
• Telephone 
• Fax 
