Investigation of Abundance Planes to Isolate Accreted Stars

Sammanfattning

The formation of our Galaxy has long been a rewarding area of research within astronomy. A large part of analysis includes using kinematics to separate the different structures in the Milky Way and also to discover accreted stars. While it is highly useful to understand the motions of populations there are systematic errors that follow, due to the difficulties of getting high quality data for them. There is also the issue of stars having their kinematics perturbed due to interactions with other objects. Abundances do not have those problems, as stars retain the chemistry of the gas cloud they formed out of. This makes them excellent tracers for events concerning the formation of the Milky Way, since the chemical makeup will act as a fingerprint, revealing where it formed and if it originates from another galaxy. The aim of this project is to evaluate different combinations of abundance planes in order to examine whether there exist additional ways to identify and isolate accreted stars. By using abundance data from APOGEE DR19, regions associated with accreted stars were isolated in the [Fe/H]-[Mg/Fe] and [Al/Fe]-[Mg/Mn] planes, where the stars were further divided into velocity groups. These velocity ranges represent the structures of the Milky Way, thus they can give an indication to whether a potential accreted population has settled into a certain region of the Galaxy. Considering the processes of chemical evolution in stars, the abundance planes of [Fe/H]-[Nd/Ce], [Nd/Si]-[Ti/Mn], [Ce/Mg]-[Mg/Fe], [Nd/C]-[Mn/Fe], [Nd/H]-[Mg/Mn] and [Co/Si]-[Ce/Mn] were investigated. The sample of stars were then plotted in these planes and observed how they moved depending on the abundances. Additionally some other groups were isolated in the hopes of observing clear trends across the planes. Two distinct regions were observed, the first occupying -1.5<[Nd/Si]<-1, the second being able to be separated into three groups defined by group 1: -0.5<[Nd/Si]<-0.3 & [Ti/Mn]<0, group 2: -0.2<[Nd/Si]<-0.1 & [Ti/Mn]<0 and group 3: 0.05<[Nd/Si]<0.15 & [Ti/Mn]<0. The isolated stars display clear dependence on Nd, separating into distinct groups when an abundance plane includes Nd but appear to be clustered together when displayed in other abundance planes. While this analysis can not definitively say that these stars are relics from an accretion event, it has been shown that Nd can be a powerful element to use when further separation of an accreted population is needed.