Spectropolarimetry of the Sun and F, G and K Type Stars

Fullerton, Stephen Robert (1994) Spectropolarimetry of the Sun and F, G and K Type Stars. PhD thesis, University of Glasgow.

Abstract

In this thesis the role of polarimetry in investigating the effects of active regions on the Sun, and similar magnetic regions on late-type stars, have upon the emitted radiation is pursued, and which forms an important part of understanding solar-stellar connections. A whole-disk solar polarimeter incorporating a tilt-tunable interference filter was used to measure the polarization at the centre of the absorption profile due to Fe I lines near 4782A and at Hbeta. Significant polarizations of up to 2.10-4 were measured with accuracies of 7.10-5 or better in p for both spectral regions, and were found to vary between the days of observations. It was postulated that the positions of active regions on the visible hemisphere of the Sun were causing these variations. A theoretical argument was that a combination of magnetic intensification in magnetically sensitive lines and of resonance scattering were producing the measurable polarization. A model was devised that was consistent with the observed data, and was extended to predict how the observed polarization from the Sun, or any star upon which magnetic regions exist, would vary as these regions traversed the star's visible hemisphere. To attempt to show observational evidence that these postulations were correct a double beam polarimeter was designed and constructed for use in measuring the polarization from small areas on the disk of the Sun, including sunspots and areas of quiet photosphere. However this investigation did not come to fruition. It is clear that further studies in many areas of solar polarization are required to confirm the variations, with more advanced detection systems and procedures of investigation into magnetic and scattering effects. A number of F, G and K type stars were observed to investigate whether similar polarimetric variations existed for them. Measurements made with the Glasgow polarimeter showed that alpha Tau was polarized at 4500 A, while alpha Boo exhibited different levels of polarization at four wavelengths, and evidence of temporal variations were found. The current instrumental polarization of the instrument was also found. On finding that the results of stellar observations showed large anti-correlations between the two channels of the Glasgow polarimeter, it was argued that atmospheric effects could cause these correlations when the design of the instrument and the data acquisition method were taken into account. Although the argument seemed sound, an attempt to show these effects numerically by using dummy data was not conclusive. xi Boo A, for which magnetic fields have been reported and which has a 6.2 day rotation period, and chi Her were observed with the Multi-purpose Fotometer at the Jacobus Kapteyn Telescope. However the instrument appeared to have a. large and varying instrumental polarization. An attempt was made to formulate this variation and subtract those measurements of an unpolarized standard star from those of the target stars, but no reasonable formulation could be estimated, and the polarization measurements of xi Boo A and chi Her are therefore inconclusive. The effect of dead-time upon polarimetric measurements acquired using photometric techniques was considered, and it was found that for increasing photon count rates the observed polarization and its error were underestimated. A correction procedure was formulated, and the dead-time of the solar whole-disk instrument was estimated to be 100ns. It is of great importance to follow up the findings in this thesis with improved polarimetric observations of both the Sun and solar-type stars to further understand the mechanisms behind the observed polarizations, and to explore "solar-stellar connections".

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: David Clarke
• Keywords: Astronomy
• Date of Award: 1994
• Depositing User: Enlighten Team
• Unique ID: glathesis:1994-76046
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Nov 2019 17:04
• Last Modified: 19 Nov 2019 17:04
• URI: https://theses.gla.ac.uk/id/eprint/76046