Constraining the Properties of Multiple Populations in Globular Clusters
by Sven Martens
Date of Examination:2025-03-11
Date of issue:2025-07-31
Advisor:Prof. Dr. Stefan Dreizler
Referee:Prof. Dr. Stefan Dreizler
Referee:Prof. Dr. Martin Roth
Persistent Address:
http://dx.doi.org/10.53846/goediss-11413
Files in this item
Name:thesis.pdf
Size:34.2Mb
Format:PDF
The following license files are associated with this item:
Abstract
English
Classically, all stars in a globular cluster are assumed to be very similar in age and chemical composition because they are thought to originate from the same giant molecular cloud. In recent years, researchers have identified multiple populations of stars in globular clusters. Stars from population 1 (P1) have abundances similar to field stars, whereas population 2 (P2) stars differ in light element abundances. The origin of these multiple populations is still a topic of debate. Many formation scenarios have been proposed, but none of them can explain all available observations. In this thesis, we analyze the kinematics and binary content of stars in globular clusters to further characterize multiple populations and provide additional constraints on their formation. We use spectroscopic data from the Multi Unit Spectroscopic Explorer (MUSE), which is an integral field spectrograph that is operated at the Very Large Telescope. For dense stellar objects, such as globular clusters, a single MUSE observation can provide spectra of thousands of stars. Our analyses are based on observations from the MUSE Galactic globular cluster survey of 25 clusters. As the spectra of stars from different populations change little in the spectral range of MUSE, we use pseudo color diagrams to separate the giant branch stars in our sample. These diagrams are constructed from HST photometry and can be used to separate the multiple populations in globular clusters because they trace the chemical differences between them. To characterize the kinematics of each cluster and its populations, we determine the rotation and dispersion as a function of distance from the cluster center from averaged MUSE radial velocities. Based on these profiles, we measure the rotation strength for each cluster and its populations. We detect rotation in all but four clusters, and the rotation strength generally scales with the dynamical age of the cluster. Any differences in the rotation amplitude between populations, would allow us to distinguish between multi-epoch formation scenarios and single-epoch formation scenarios. However, we find no significant differences between populations for clusters in our sample. There are several outliers that could be attributed to statistical variations because the differences are below the 3$\sigma$ detection limit. Differences in the binary content of P1 and P2 would allow us to infer the initial spatial distribution of P1 and P2 stars. To quantify the number of binary stars in globular clusters, we developed a new statistical method to calculate binary probabilities for each star in the sample that has been observed at least four times. Our binary detection method is comparing the measured variation in radial velocity for each star to what we expect to find for a sample of single stars. This allows us to detect binary stars that show larger variation in radial velocity than single stars. As this analysis is based on individual MUSE observations instead of averages over all observations, we first needed to gain a deeper understanding of the radial velocities derived from MUSE observations. We found that there are steps during data reduction that could introduce artificial variation in radial velocity caused by contamination from neighboring stars, that we discuss in detail. To remove the affected stars from our sample, we applied additional data quality filters to the MUSE radial velocities. We analyze the binary content of NGC 104 and NGC 3201 by applying our new binary detection method to the filtered dataset of each cluster. We find observed binary fractions of $7.9^{+1.7}_{-1.3}\,\%$ for NGC 104 and $9.2^{+1.3}_{-0.8}\,\%$ for NGC 3201. When differentiating for multiple populations, we find that P1 contains significantly more binary stars than P2 in NGC 3201, with an observed binary fraction of $25.0^{+1.5}_{-1.1}\,\%$ in P1 and $10.4^{+1.6}_{-1.1}\,\%$ in P2. For NGC 104 we observe a similar trend, but the difference is not significant. There, we observe $8.4^{+1.8}_{-1.4}\,\%$ of P2 stars to be variable in radial velocity and $10.8^{+1.8}_{-1.4}\,\%$ in P1.
Keywords: Spectroscopy; Astronomy; Astrophysics; Globular Clusters; Binaries; Stellar Dynamics; Radial Velocities; NGC 104; NGC 3201; Stellar Abundances
