Experimental and Computational Atomic Spectroscopy for Astrophysics : Oscillator strengths and lifetimes for Mg I, Si I, Si II, Sc I, and Sc II

Abstract: Atomic data such as oscillator strengths and wavelengths are important for astrophysical applications as they have a crucial role in determining abundances of specific elements in a star, a galaxy, or any object emitting radiation in the space. Stars in the Galaxy mostly keep the composition of the interstellar gas from which they were formed, therefore studying stellar abundances helps us understand how the Milky Way was formed and how it evolved. However, atomic data of most chemical elements are incomplete and/or have low quality, particularly for the infrared wavelength region and for the highly excited levels. This PhD project focuses on completing missing atomic data for the infrared region in addition to the optical and UV regions, and improving the existing data. In order to achieve this, I have performed both laboratory measurements and large scale atomic structure calculations.Experimental oscillator strengths have been derived by combining measured branching fractions with radiative lifetimes. A hollow cathode discharge lamp has been used as a light source to produce free atoms in a plasma and a Fourier transform spectrometer has recorded the intensity-calibrated high-resolution spectra. In addition, atomic structure calculations have been performed using the multiconfiguration Hartree-Fock programmes ATSP2K and GRASP2K to determine oscillator strengths and lifetimes. Combining the experimental work with the computational approach allows determining a large set of accurate and validated atomic data.In this thesis, an evaluated set of atomic data for Mg I, Si I, Si II, Sc I, and Sc II has been provided for astrophysical applications. The experimental oscillator strengths in the infrared region have been measured for the first time. The uncertainties in the experimental oscillator strengths are as low as 5% for strong transitions. The theoretical oscillator strengths are validated with the experimental values and with internal investigations of the length and velocity forms. The small uncertainties in the values allow accurate astrophysical abundance determinations within the 0.1dex uncertainty.