Standard Stars for Polarimetric Measurement and Their Statistics

Naghizadeh-Khouei, Jaber (1991) Standard Stars for Polarimetric Measurement and Their Statistics. MSc(R) thesis, University of Glasgow.

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The most important aspect of stellar polarimetry involves the studying of polarimetric standard stars. The techniques of polarimetry have advanced progressively with the associated improvement of accuracy. This has not been accompanied with establishment of better standards of reference. Two fundamental reductions must be made to the polarization data before any inferences can be drawn from observations: (A) Instrumental polarization corrections should be performed on the data by observing "zero" polarization standards, (B) Absolute position angle calibration, so that the data may be presented in a particular coordinate frame so allowing comparison of data collected on different instruments. The latter can be obtained from knowledge of the orientation of the polarimeter relative to north-South direction. Laboratory techniques should be used, where available, for absolute calibration of position angle. It is however, far more convienient to observe highly polarized stars with a well defined position angle of polarization; the equatorial coordinate frame is normally chosen for reference. These standards also provide means of testing the stability of the instrument. The aim of this study is to present a scheme that may be used to quantify some criteria for establishment of future standard polarimetric stars. There have been only few authors that have paid any attention to establishing polarimetric standards. Discussions presented in Chapter 1 deal with the present standing of polarimetric standards and the areas where further improvements are required. Since the discovery of interstellar polarization by Hall and Hiltner, there has been a significant increase in our precision of polarization measurements. It is therefore essential to investigate the statistical behaviour of errors involved with the measurements of polarization and position angle; the former has been quantitatively well studied by previous workers, but the latter has lacked the detail it deserves. We have constructed accurate confidence intervals of position angle of polarization and have provided statistical formulation of the distribution. It is demonstrated in Chapter 2 that confidence values of position angle at low levels of signal-to-noise ratio are significantly different from the Gaussian distribution assumed in the literature. Also a data simulation method was performed in order to determine the distribution of differences in two values of position angle at low signal-to-noise, since analytical solutions would be too complex to handle for this situation. The literature shows that some of the well established standards are challenged either because of their imprecise tabulated values or because of suspected polarimetric variability. Proper statistical techniques should be used however to assess correctly and accurately these findings. It is shown in Chapter 3 that because of inadequate statistical procedures applied to the data of polarimetric standards, many stars supposedly showing variations in their polarization and position angle, may still be considered as standards. We have performed, more rigid statistical tests on some previously published broad-band data. It is therefore concluded that, prior to this work presented here, all data involving standard polarimetric stars have not been statistically interpreted correctly. We have undertaken new observations on some well known polarization standards which are thought to have a variable polarization. Double channel polarimetry of 55Cyg, &phis;Cas, rhoCas, oCas, rhoLeo and 14Cep was performed and attempt were made to establish further standards in Cassiopeia and Perseus.

Item Type: Thesis (MSc(R))
Qualification Level: Masters
Keywords: Astronomy
Date of Award: 1991
Depositing User: Enlighten Team
Unique ID: glathesis:1991-78264
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 15:35
Last Modified: 30 Jan 2020 15:35

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