Towards a high resolution view of infrared line formation

Sammanfattning: Observing in the infrared has many benefits, such as seeing through the interstellar dust in the galaxy, easier use of adaptive optics, and better flux ratios for direct observations of exoplanets. Before the infrared spectral range can be utilised for highly accurate stellar spectroscopy there is a need for a better understanding of both stellar modelling and the atomic physics that go into forming spectral lines. In this thesis I aim to evaluate to what extent abundances derived from infrared spectra agree with optical values, and which aspects of the line formation cause eventual discrepancies. The aspects investigated in this study are: macroturbulence determination; astrophysical linestrengths; NLTE corrections; and hyperfine structure splitting of atomic energy levels. For this purpose I have analysed H band (1.49 to 1.80 $\mu m$) spectra of 34 K giants, taken with the spectrometer IGRINS. The elemental abundances derived from these infrared spectra are benchmarked against results from high resolution optical spectra of the same stars. This study uses stellar parameters derived from the same optical spectra, as techniques for determining them from infrared spectra are still being developed. My results for the elemental abundances of C, Na, Mg, Ca, Ti, V, Cr, Co, Ni, Cu and Ce show good agreement with the optical values. Significant differences are seen in the abundances of Al, Si, Mn and Nd. I have also measured the abundances of P, S, K and Zn, elements which lack optical benchmark values. No measurements have been possible for Ge and Rb. Different factors affecting the analysis have been studied in more detail. I show that the method used for determining macroturbulence is an important factor in abundance determination, especially for stars with supersolar metallicity. The lack of accurate measurements of spectral linestrength is addressed in the thesis by my astrophysical measurements of the quantity, using a method developed in the thesis that accounts for the relative strengths of fine and hyperfine structure lines. For elements with NLTE corrections from Amarsi et al. (2020) I show how these corrections improve the agreement between optical and infrared results. For elements with data on the hyperfine structure splitting of the energy levels, I have assessed the impact of including the additional transitions, showing the importance of doing so.