A comparative polarimetric study of the 43 GHz and 86 GHz SiO masers toward the supergiant star VY CMa
- Authors: Richter, Laura
- Date: 2012
- Subjects: Masers Supergiant stars Polarization (Light) Very long baseline interferometry
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5454 , http://hdl.handle.net/10962/d1005239
- Description: 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 reqwre very long baseline interferometric imaging with very accurate polarimetric 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 reduction 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= l and v=2 J= 1-0 SiO masers to the east and northeast of the assumed stellar position. The v=l 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 of the overlap is generally consistent with the predictions of hydrodynamical circumstellar SiO maser simulations. Where the v=l J = 1-0 and J =2-1 features overlap, the v=l 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 intercomparisons of linear polarisation in the v=l J=1-0 and J=2-1lines, linear polarisation versus saturation level, linear polarisation versus distance from the star, circular polarisation in the v= l J = 1-0 and J=2-1 lines, circular versus linear polarisation and modeling of ~ 900 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.
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- Date Issued: 2012
An analysis of pacing and sequencing of reading instruction in three grade 1 classrooms where isiXhosa is the language of learning and teaching
- Authors: Masola, Athambile
- Date: 2012
- Subjects: Reading (Elementary) -- Study and teaching -- Foreign speakers -- Research -- South Africa Literacy -- Education (Primary) -- Research -- South Africa English language -- Study and teaching -- South Africa -- Foreign speakers
- Language: English
- Type: Thesis , Masters , MEd
- Identifier: vital:1420 , http://hdl.handle.net/10962/d1003301
- Description: Reading development in the Foundation Phase (FP) is central to children’s success as they progress through school. However, evaluations in South Africa such as the Annual National Assessments (ANAs) and the Progress in International Reading Literacy Study (PIRLS) (2006) show that reading achievement in the Foundation Phase is low, especially for children learning with African languages as their home language. This thesis examines pacing and sequencing in three Grade 1 literacy classes where isiXhosa is the language of learning and teaching (LOLT). The research took the form of a case study, focusing on the teachers’ classroom practice and how they understand their practice. Documents related to reading development in the Foundation Phase are analysed with regard to pacing and sequencing, examining how the curriculum frames literacy for teachers in the Foundation Phase. The teachers’ understanding in relation to their social context is also an important part of this research. In the thesis the way in which the teachers sequence and pace learning is analysed, drawing on Bernstein’s notions of internal and external framing, and Bourdieu’s notion of the habitus. The intended curriculum and the implemented curriculum are then compared. The results confirm that in low socio‐economic status (SES) schools, the level of poverty has an impact on teaching practice. The results also show that pacing and sequencing are reliant on the degree of planning amongst teachers. The level of development amongst learners also plays a role in how teachers organise their practice in order to differentiate the teaching of reading for all learners. The understanding that teachers have about their teaching practice are layered and influenced by their experiences, history and memories as teachers. The thesis concludes that there is a need to develop teachers’ understanding of planning and organising the teaching of reading, especially when the learners are from poor communities and do not get support in the home. In order for teachers to improve their practice, it is important for them to have the opportunity to reflect on and understand their practice.
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- Date Issued: 2012