Kat Lee & Thomas Nordlander (Uppsala University): The chemical composition of the oldest stars (PhD introduction seminar)

Thursday 24 Sept 2026, 14:00 → 15:00 Europe/Stockholm

Å100155

Title: The chemical composition of the oldest stars  
Speaker: Kat Lee & Thomas Nordlander 
Affiliation: Uppsala University 
Room: Å100155 
Time: 14:00-15:00 

Abstract:
The first stars and supernovae imprinted a distinct chemical signature on the early Galaxy, and this signature survives in the chemical makeup of subsequent generations of stars. Ancient metal-poor stars that formed early in the Milky Way’s evolution are expected to carry the clearest imprint of this chemical signature, and studying them can reveal details about the first stars to ever form in our Galaxy. This PhD thesis will focus on spectroscopic analysis of nearby metal-poor stars, and in particular will investigate the abundances of carbon, nitrogen, and oxygen, since the ratios of these elements can indicate the typical masses and rotational velocities of stars in the early Galaxy. Abundances will be measured using near-UV spectral lines of the OH and NH molecules, as well as optical lines of CH and Fe II, and 3D radiative transfer calculations will be used to ensure that abundance estimates will be reliable. The first project will focus on the ancient metal-poor red giant SMSS J2003-1142, which has previously been theorized to be the descendant of an unusual progenitor explosion: a magneto-rotational hypernova. Models of these hypernovae indicate unusually low oxygen abundances, but oxygen has not been previously measured in this star due to a lack of data; the first project of the thesis will fill this gap, allowing for a clearer picture of the properties of this star’s progenitor. Subsequent projects will expand the analysis to a sample of roughly 40 metal-poor stars in the Milky Way’s halo, including around 15 with iron abundances [Fe/H] below -3.0, giving a population-level look at our Galaxy’s early chemical evolution.