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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Modeling and measurement of torqued procession in radio pulsars
- Authors: Tiplady, Adrian John
- Date: 2005
- Subjects: Pulsars , Radio telescopes , Radio astronomy , Precession , Hartebeeshoek Radio Astronomy Observatory (HartRAO)
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5475 , http://hdl.handle.net/10962/d1005260
- Description: The long term isolated pulsar monitoring program, which commenced in 1984 using the 26 m radio telescope at the Hartebeeshoek Radio Astronomy Observatory (HartRAO), has produced high resolution timing residual data over long timespans. This has enabled the analysis of observed spin down behaviour for 27 braking pulsars, most of which have dataspans longer than 14 years. The phenomenology of observed timing residuals of certain pulsars can be explained by pseudo periodic effects such as precession. Analytic and numerical models are developed to study the kinematic and dynamic behaviour of isolated but torqued precessing pulsars. The predicted timing residual behaviour of the models is characterised, and confronted with timing data from selected pulsars. Cyclic variations in the observed timing residuals of PSR B1642-03, PSR B1323-58 and PSR B1557-50 are fitted with a torqued precession model. The phenomenology of the observed timing behaviour of these pulsars can be explained by the precession models, but precise model fitting was not possible. This is not surprising given that the complexity of the pulsar systems is not completely described by the model. The extension of the pulsar monitoring program at HartRAO is used as motivation for the design and development of a new low cost, multi-purpose digital pulsar receiver. The instrument is implemented using a hybrid filterbank architecture, consisting of an analogue frontend and digital backend, to perform incoherent dedispersion. The design of a polyphase filtering system, which will consolidate multiple processing units into a single filtering solution, is discussed for future implementation.
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Radio observation of the Gum Nebula Region
- Authors: Woermann, Beate
- Date: 1997
- Subjects: Radio sources (Astronomy) Nebulae -- Observations Radio astronomy
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5485 , http://hdl.handle.net/10962/d1005271
- Description: This thesis describes the results of an investigation of the physical properties of the Gum Nebula. For this investigation a radio continuum map of the region was made at 2326 MHz and resolution ⅓° with the HartRAO antenna. This map was used to generate spectral index images and an infrared to radio flux density ratio (IRR) image. The latter image shows that the IRR of the nebula is in the range 20 to 250, identifying it as an old SNR. Several spectral index images of this region were generated using two different methods, one based on the isolation of the nebula from its background radiation, the other based on TT-plots (Turtle et al., 1962). The two methods yield similar results, which show that the nebula has a thermal shell with a non-thermal region in its interior. Below the galactic plane the thermal region dominates and above the plane the nonthermal region. These results suggest a model of an old SNR with an H II region shell. Spectral line observations of hydrogen recombination lines and hydroxyl (OH) were made with the HartRAO and the Mopra telescopes. The detection of hydrogen recombination lines at four positions in the thermal regions of the nebula give electron temperatures and emission measures in the ranges 4000 to 6000 K and 220 to 460 pc.cm⁻⁶ respectively. The turbulent velocities are of the order of 20 km/s. A search for shocked OH lines at 1667 MHz and 1720 MHz in the Gum Nebula gave results that were negative, but numerous unshocked 1667 MHz OH lines were detected. The latter were used in a test for an expansion of the nebula. The most plausible fit to the data gives an expansion centre at l = 260.5°, b = -2.5° and at a distance of 0.7 kpc from us. The front face angular radius and expansion velocity are 10.5° and 16 km/s respectively. The back face angular radius and expansion velocity are 8.50 and 7 km/s respectively.
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