Investigation of Abundance Planes to Isolate Accreted Stars

Abstract

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.

Popular Abstract

It is in human nature to be curious, to wonder where we came from and what exists outside of our small corner of the Universe. While some people are content to just wonder, others take it upon themselves to try to find answers to the many mysteries surrounding us. The vastness of space invites research from various angles to increase our understanding the Universe. The formation of galaxies is one of the major mysteries that has puzzled astronomers for ages. But as human technology has become increasingly advanced, we are now able to study the Universe in even better detail. And to get a closer look at galaxies we only need to look at the space around us, as we are surrounded by our home Galaxy, the Milky Way. It is the host of numerous stars, planets and various other astronomical objects that can be used to investigate its formation history.

So how can we gain an understanding of something so big as a galaxy? Just like archaeologists use the remnants of civilisations past to piece together the history of the Earth, astronomers can use data collected from the various objects in space to unveil the history of the Universe. A major piece of the puzzle is the origin and evolution of the chemical elements. The processes which allow the elements to form and how they then are distributed across the Galaxy allow for a closer insight into how the Galaxy was formed and became what it is today.

Stars are born from regions of dust and gas, fusing together elements to sustain enough outgoing pressure to stave off a collapse. As these stars grow older and die, the elements produced during their lifetime—i.e. their chemical makeup—will be returned to the surroundings, and a new generation of stars can be born from the remnants. Different types of explosive endings release different elements, which can act as a special fingerprint defining where a star was born and how old it is. This fingerprint can be used to discover foreign stars, those not formed by the dust and gas composing the Milky Way, instead having been poached from other galaxies. These are called accreted stars, which were left behind following mergers between smaller galaxies and the Milky Way, during the tumultuous formation of our Galaxy. These stars will have the chemical makeup of their original galaxy, which is often distinct from the chemical fingerprint that defines the Milky Way.

Using the data collected from surveys of stars in the Galaxy we can look at the amount of each element contained in each star, also known as their abundances, and categorize them based on their properties. This is instrumental to understanding the formation history of the Milky Way, as this information combined with the star positions in the Galaxy will give valuable insight into the potential origin of the star. However, while there are certain well known abundances that will be more prevalent in accreted populations, they are still not enough to fully isolate the populations. Thus it is prudent to explore whether there exist other elements, or ratios of specific abundances, to figure out if there is a way to bring more certainty to the chemical analysis of accreted stars.

• author: Tellander, Elise
• supervisor: Thomas Bensby; Joshua Povick
• organization: Department of Physics; Astrophysics
• course: FYSK04 20261
• year: 2026
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: Chemical abundance, Abundance planes, Accreted stars
• report number: 2026–EXA263
• other publication id: 2026–EXA263
• language: English
• id: 9239601
• date added to LUP: 2026-06-17 14:36:50
• date last changed: 2026-06-17 14:36:50