|M.Sc Student||Peretz Uria|
|Subject||The First lonization Potential Effect and Stellar|
|Department||Department of Physics||Supervisor||Professor Ehud Behar|
|Full Thesis text|
This work presents a short review on element fractionation in stellar (i.e. of the star) coronae and provides new measurements that expand the view and challenge old assumptions. We analyze six stars with supersolar abundances and compare existing photospheric abundance measurements with coronal abundance measurements in order to shed further light on the physics of the interface between the photosphere and the corona.
We first define basic terms used often and commonly in the field of stellar physics. In general, the outer light emitting part of the star (e.g. our sun) is called the photosphere. The photosphere is the part of the star visible in optical light as hinted by the name.
Once the photosphere ends, an extended plasma layer - lengths of order of the star's radius - continues the final transition of the star into space. This very hot and sparse plasma is dubbed the corona, and is observed primarily in X-rays.
The conditions in the corona allow simple approximations in the modeling of the X-rays emitted by its ion populations, which enables temperature and chemical diagnostics. The fact we can differentiate the corona from the photosphere makes it possible to measure different parameters in the corona and in the star (photosphere) separately.
The next important term, First Ionization Potential (FIP), refers to the minimal energy needed to ionize a neutral element. This is where the thesis begins. As soon as X-ray measurements allowed the determination of elemental populations in the solar corona and could be compared to the solar abundances previously measured, a dependence of the difference between the two on the FIP was observed. This solar FIP effect was initially thought to be a general behavior across the stellar population. Over time the solar trend was understood to be non-unique and more trends were observed, connecting other stellar parameters to the behavior of the fractionation of elements between the photosphere and the corona.
The physics behind the transport of ions from the photosphere to the corona may affect the outer stellar structure through coronal mass ejections. In addition it may shed light on the diverse coronal activity, as well as the very large magnetic structures that, e.g., produce stellar winds and affect earth's space weather.
Here we extend the research on this subject to measure abundances in three new stars, and analyze the FIP behavior of these and three additional stars with previously measured abundances. Our interest in this specific sample arises by merit of different (super solar) abundances of the stars themselves. In addition we wish to check the validity of the often used solar abundance assumption, where solar photospheric abundances are assumed when no actual measurements are available. The results extend a previous observed dependence on spectral type, and show the importance of knowing actual photospheric abundances to the understanding of the true physics governing the element fractionation.