Richter, Laura (2012) A comparative polarimetric study of the 43 GHz and 86 GHz SiO masers toward the supergiant star VY CMa. PhD thesis, Rhodes University.
The aim of this thesis is to perform observational tests of SiO maser polarisation and excitation models, using component-level comparisons of multiple SiO maser transitions in the 43 GHz and 86 GHz bands at milliarcsecond resolution. These observations require very long baseline interferometric imaging with very accurate plarimetric calibration. The supergiant star VY CMa was chosen as the object of this study due to its high SiO maser luminosity, many detected SiO maser lines, and intrinsic scientific interest. Two epochs of full-polarisation VLBA observations of VY CMa were performed. The Epoch 2 observations were reduced using several new data reducation methods developed as part of this work, and designed specifically to improve the accuracy of circular polarisation calibration of spectral-line VLBI observations at millimetre wavelengths. The accuracy is estimated to be better than 1% using these methods. The Epoch 2 images show a concentration of v=1 and v=2 J=1-0 SiO masers to the east and northeast of the assumed stellar position. The v=1 J=2-1 masers were more evenly distributed around the star, with a notable lack of emission in the northeast. There is appreciable spatial overlap between these three lines. The nature ofthe overlap is generally consistent with the predictions of hydrodynamical circumstellar SiO maser simulations. Where the v=1 J=1-0 and J=2-1 features overlap, the v=1 J=2-1 emission is usually considerably weaker. This is not predicted by current hydrodynamical models, but can be explained in the context of collisional pumping in a low density environment. six observational tests of weak-splitting maser polarisation models were performed, including inter-comparisons of linear polarisation in the v=1 J=1-0 and J=2-1 lines, linear polarisation versus saturation level, linear polarisation versus distance from the star, circular polarisation in the v=1 J=1-0 and J=2-1 lines, circular versus linear polarisation and modeling of ~ 90° electric-vector position angle rotations. The polarisation model tests generally do not support non-Zeeman circular polarisation mechanisms. For the linear polarisation tests, the results are more consistent with models that predict similar linear polarisation across transitions. The scientific importance of these tests is described in detail and avenues for future work are described.
|Item Type:||Thesis (PhD)|
|Uncontrolled Keywords:||Masers, Supergiant stars, VY CMa, Polarization, Very long baseline interferometry|
|Subjects:||Q Science > QB Astronomy|
Q Science > QC Physics
|Divisions:||Faculty > Faculty of Science > Physics & Electronics|
|Supervisors:||Jonas, Justin Leonard|
|Deposited By:||Ms Chantel Clack|
|Deposited On:||22 Aug 2012 07:15|
|Last Modified:||22 Aug 2012 07:15|
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