University of Tasmania
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S˜ìvÑRALS : outhern hemisphere parallax interferometric radio astrometry legacy survey

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posted on 2023-05-28, 00:11 authored by Lucas HylandLucas Hyland
The exact structure of the spiral arms of our home Galaxy, the Milky Way, is an unknown that can be resolved by a higher sampling of parallaxes to high mass star-forming regions from both the Northern and Southern Hemispheres. The Northern Hemisphere has been very well sampled by surveys like BeSSeL and the VERA key project, yet they are unable to observe sources in the southern sky and complete the picture of the Milky Way. A large maser astrometry project - the Southern Hemisphere Parallax Interferometric Radio Astrometry Legacy Survey or S˜ìvÑRALS, commenced mid-2020 and aims to determine the distances to Southern Hemisphere high mass star formation regions. Using three 12m and one 30m radio telescopes spread over Australia, S˜ìvÑRALS will measure parallaxes for dozens of methanol masers in the 3rd and 4th Galactic Quadrant and thereby determine spiral arm properties and Galactic kinematics inaccessible to Northern Hemisphere instruments. However, S˜ìvÑRALShardware is different and less suited to the task than from the previous large astrometry surveys and effort is required to develop new observing methods and calibration techniques to account for the differences. The aims of this thesis are as follows: Firstly, to analyse BeSSeL VLBA data and measure parallaxes for three 22 GHz water masers and one 6.7 GHz methanol maser located in the First Galactic Quadrant. This increases the understanding of Galactic structure and establishes a benchmark for VLBI astrometry for S˜ìvÑRALSto aspire to. I have been able to successfully measure the parallax and proper motion of the methanol maser and 2 of the water masers, and measure proper motions for the last water maser. I then use these results to determine the locations of these all masers in the Galaxy and find all four masers are likely to be in the Perseus spiral arm. Secondly, to determine a target list for S˜ìvÑRALS by conducting a targeted survey of known Southern Hemisphere 6.7 GHz methanol masers. Signiffcant effort is required to measure parallax and therefore identification of the best targets for each Galactic region is important for time and data quality. I find that there are 53 suitable first targets for S˜ìvÑRALSand a further 29 likely appropriate for future VLBI astrometry. I can determine the compactness of 103 methanol masers, equivalent to a 55% of the surveyed maser where the remaining 45% are too weak or diffuse. Thirdly, to develop and test inverse MultiView, a phase calibration technique initially conceived for ionospheric calibration. I find that inverse MultiView can be used to model and subtract residual delay errors due to additional effects like residual troposphere and interferometer baseline offsets. I also find that inverse MultiView can outperform traditional techniques and enable target-calibrator separations of at least 8¬¨‚àû at 8:4 GHz. With inverse MultiView I can achieve microsecond astrometry on a relatively new interferometric array which will be used for S˜ìvÑRALS, thereby paving the way for future high accuracy Southern Hemisphere maser parallaxes.


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