Master Defense by Kim Kobusch Simonsen
In this project I have been investigating the chemical evolution of yttrium mainly in metal poor stars. What we don't understand is why we see less yttrium than strontium and zirconium although they are all mainly produced by the s-process, and have a majority of their abundance in the magic number 50-neutron isotope. I have compared theoretical studies from stellar nucleosynthesis with a selection of abundance ratios for 407 metal-poor stars [Fe/H]∈ [−5,−1] determined by 9 different studies. I have also looked at how these studies differ in method and how they achieve different abundance ratios for the same star. My comparison of 407 metal-poor stars from 9 studies shows a majority of 77% yttrium-poor stars i.e. [Sr⁄Fe] ∼ [Zr⁄Fe] > [Y⁄Fe]. My comparison also shows a surprising high amount, 21%, of strontium-poor stars [Sr⁄Fe] < [Y⁄Fe] < [Zr⁄Fe]. Of these strontium- poor stars there is a gap of almost no stars around [Fe/H]∼−2:0. Recent studies indicate that this gap can originate from an r-process burst. Two comparisons of abundance ratios for the same star reveal a significant temperature sensitivity. In both cases, temperature differences of more than 100 K changes the abundance ratios significantly. The weak-line approximation and curve of growth used for these calculations also show the temperature sensitivity. This study therefore shows a need for better estimates of effective temperatures.