Magnetic fields and X-ray emission in pre-main sequence stars
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In this thesis, I use numerical models of stellar coronae to investigate coronal magnetic fields, X-ray emission, and accretion geometries of classical T Tauri stars. This is based on recently published Zeeman-Doppler Imaging (ZDI) magnetograms. I also investigate the effects of time-variable eclipsing of stellar flares on their observed lightcurves. I investigate how our ability to model stellar magnetic processes is affected by missing magnetic flux in observed ZDI magnetograms and find that the loss of unresolved small-scale field regions has a significant effect on our ability to model magnetically confined X-ray emitting coronae. However, it has little effect on predicted large scale field structures. I survey the sample of classical T Tauri stars with existing ZDI magnetograms and find that the field complexity is correlated with field strength and stellar rotation rate, such that rapidly rotating stars have weak complex fields, and slowly rotating stars have strong simple fields. It is not clear whether this is a result of the finite resolution of the ZDI technique, magnetic star-disc interactions, or the evolution of pre-main sequence stars. Using observed X-ray emission measures and temperatures for each of these stars, I model the closed X-ray emitting coronae and find that they typically extend several stellar radii from the stellar surface. The coronal extent is primarily determined by the complexity of the magnetic field, with simple fields extending a large distance from the stellar surface, and more complex fields being truncated closer to the stellar surface. Using observed mass accretion rates, I predict circumstellar disc truncation radii for these stars and find that they are typically several stellar radii from the stellar surface, with the locations of accretion footpoints being a strong function of the field strengths and complexities. In several cases, the disc is truncated significantly outside the maximum radius at which the corona can extend. This result is significant as studies into magnetospheric accretion generally assume that the magnetic field has a closed geometry at the inner edge of the disc. The lightcurve of a typical stellar flare consists of a single impulsive rise phase followed by a slower exponential decay. However, a large number of the observed flares do not possess typical morphologies, and instead show multiple rise phases, or large dips in their lightcurves. Using the largest flares observed by the Chandra Orion Ultradeep Project, I show that these atypical lightcurves could have been caused by the time variable eclipsing of typical flares due to the rotation of the host stars. However, this interpretation is unable to account for the large number of atypical lightcurves in the COUP flare sample, and so other physical mechanisms must be involved. Significantly, I find that most flares that are eclipsed by their host stars still retain typical lightcurves showing no obvious signs of eclipsing.
Thesis, PhD Doctor of Philosophy
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