Axel Hahlin: Multi-scale magnetic field analysis of single and binary late-type stars

Abstract

Stellar magnetism is the driving force behind a large number of surface phenomena on stars. This means that the presence of stellar magnetic fields can influence the stars themselves, as well as their surroundings. For this reason, magnetic fields are an integral part of stellar physics that is often neglected when studying different properties of stars. This can pose a problem, particularly for stars hosting strong surface magnetic fields.      

To better understand the magnetic influence on stars and their surroundings, we must characterise the magnetic fields on the stellar surface. The methods used in this thesis rely on high-resolution spectroscopy and spectropolarimetry to measure magnetic field effects on the shape and polarisation of spectral lines from the Zeeman effect. The polarisation of spectral lines can be used to measure the global magnetic field geometry by employing the directional sensitivity of the polarisation signal to the field. The total magnetic field strength can be measured with spectra using the Zeeman broadening and intensification of spectral lines. We have used these methods to characterise magnetic fields for a variety of late-type single and binary stars. 

In our studies on binary stars, we investigated the possible influence of magnetic fields on stellar structure and evolution. We found an agreement between the magnetic field strengths measured from observations and introduced by theoretical stellar models to reproduce the stellar structure. This shows that magnetic fields could influence stellar structure and evolution.        

When investigating single stars, we found that the measured magnetic field strength depends on the choice of spectral lines. This seems to be caused by formation depth and surface structure effects as spectral lines originate from different layers within magnetically active regions on the stellar surface. If characterised in more detail, we could better understand the magnetically active regions on the stellar surface.