Thesis Supervisor: Thomas Dall
Abstract
All stars cooler than about late-F have chromospheres and coronae. These hot outer atmospheres radiate primarily in high energy transitions and a few optical emission lines, the strengths of which are closely related to the magnetic field strength. Magnetic fields are dynamo-generated and generally obey a period-activity relation; i.e. the faster the rotation, the stronger the magnetic field. However, in the very fastest rotators and in the close short-period binaries the optical emission from the chromosphere seems to deviate from this relation for unknown reasons.
Most stellar modeling today is still not taking into account the effects of rotation, and certainly not fast rotation which cause significant deviation from the spherical shape and large temperature gradients between pole and equator. Also, no current stellar models properly link the photosphere, chromosphere and corona. In addition there are severe observational problems, since observing the spectra of very fast rotating stars is difficult due to the rotational smearing of spectral features.
This project attacks the problem from two complimentary directions using new models of rotating stars and their chromospheres and coronae - models that we are developing. We shall: (1) Compare with observed spectra of suspected fast rotating stars that are seen close to pole-on so that they appear as slow rotators with unusually high magnetic activity level. Many spectra have already been secured with Subaru/HDS and CES. (2) Deconvolve severely broadened spectra, using the newly developed models. The testing of these complete models on real spectra at both extremes of apparent rotation is crucial and represents a new approach to the modeling and understanding of stellar magnetic activity. You will be expected to take part in the data analysis using the models, and also in the model development according to skills and interests. The project is a collaboration between researchers from ESO, MPA-Garching, and Nagoya University.
Research in magnetic fields is gaining considerable momentum worldwide as it is becoming clear that neither magnetic fields nor rotation can continue to be neglected in models of stellar structure and evolution. The impact of new knowledge of magnetic fields will also have a huge impact, not only on all scales of astrophysical phenomena, but also on practical applications of plasma physics.