MSc defence by Johan Sommer Byrgesen

Title: A New Metallicity Tracer for Carbon-Enhanced Metal-Poor Stars

Co-author: Daniel Singh

Supervisor: Camilla Juul Hansen

Abstract: Carbon-enhanced metal-poor (CEMP) stars is a relatively new field of study. These old stars are of great interest since they are regarded as second generation stars, hence their chemical composition provides us with constraints on the early galactic chemical evolution. CEMP stars are galac- tic eld stars, and they are often observed at large telescopes with low resolution instruments resulting in low-resolution spectra. Furthermore, they are very metal-poor thus making measurements of iron absorption lines a very challenging task.

Knowing the iron abundance of a star is crucial for further in- vestigation of its chemical composition. Therefore our aim is to develop a relation between the abundance of iron and the equivalent width of another element's spectral feature. We thereby avoid measuring a lot of blended iron lines. We use a sample of 30 low resolution spectra observed by the spectro- graph XSHOOTER/VLT. We determine stellar parameters by using the infrared ux method to calculate temperatures, isochrones for obtaining the surface gravity. We use 1D LTE spectral line analysis for the metallicity ([Fe/H]) and spec- trum synthesis to derive elemental abundances. From low resolution (R∼5400) and low signal to noise (∼6-27), we have determined the abundances for C,Mg, Sr, Ba and Eu, thereby allowing us to classify the stars from their Fe, C, Ba and Eu abundances. We have successfully developed a relation be- tween the abundance of iron and the equivalent width of the nickel spectral feature at 5477 Å. The relation is valid for de- termining the iron abundance (metallicity) for giant, CEMP stars. Our method makes it possible to measure the equiva- lent width of a single line and immediately obtain the metal- licity with the same precision obtainable from the analysis of iron lines. This is important in the era of large surveys, where fast and e cient methods to derive stellar metallicities for thousands to millions of stars is needed.