Alumni 2026
Marten Scheuck (12.1.) - Christian Sorgenfrei (29.1.) - Katrin Lehle (24.4.) - Clara Escanuela Nieves (27.4.) - Julia Lienert (29.4.) - Lucia Haerer (19.5.) - Leon-Alexander Huehn (18.6.) - Marcelo Cortes Vergara (24.6.) - Taavet Kalda (1.7.) - Alexander Holas (1.7.) - Jonas Müller (2.7.) - Georg Schwefer (9.7.) - Roel Lefever (16.7.) - Mariia Demianenko (17.7.) - Frederik Wohlleben (20.7.) - Michael Cecil (21.7.) - Matheus Bernini Peron (22.7.) - Cyril Gapp (22.7.) - Nicolas Storm (23.7)
Nicolas Storm (Norway) - 23.07.2026
Chemical structure of the Galaxy and origin of s- and r-process elements ( thesis pdf, 40 MB )
Elements heavier than the iron peak are predominantly produced by neutron-capture nucleosynthesis, as they cannot be efficiently formed through charged-particle fusion reactions. These elements are synthesised via neutron captures followed by β− decays in high neutron density environments, and are classified into the slow (s-) and rapid (r-) neutron-capture processes depending on the neutron density. While the s-process is established to occur primarily in asymptotic giant branch (AGB) stars, the dominant astrophysical sites of the r-process remain heavily debated. Thus, Galactic Chemical Evolution (GCE) models are used to interpret stellar abundance trends and constrain production sites through Galactic archaeology. Here I present new model atoms for yttrium and europium and perform non-local thermodynamic equilibrium calculations using three-dimensional (3D) radiation-hydrodynamic model atmospheres to derive accurate stellar abundances of key neutron-capture and iron-peak elements. These state-of-the-art abundance determinations show the need for multiple r-process production sites, yet no existing GCE model can consistently reproduce all considered elements (Sr, Y, Ba and Eu) simultaneously. For Co and Ni discrepancies reach up to +0.6 dex at [Fe/H] < −3 compared with current GCE model predictions suggesting that nucleosynthetic yields of iron-peak elements may need to be revised.
Supervisor: Maria Bergemann (MPIA)
Matheus Bernini Peron (Brasil) - 22.07.2026
The winds of blue supergiants and hypergiants ( thesis pdf, 20 MB )
With their chemo-dynamical and ionizing feedback, massive stars are keystones to many areas of astrophysics. Yet, our quantitative understanding of their properties and evolution remains blurred. This thesis explores blue super- and hypergiants, a heterogeneous group at an evolutionary crossroad connecting the main sequence to red supergiants, luminous blue variables (LBVs), and Wolf-Rayet stars. Using detailed numerical models of stellar atmospheres, I analyze SMC B-type supergiants (BSGs) based on new UV and optical spectra. With hydrodynamically-consistent models, I analyze a prototypical hypergiant, establishing a framework to analyze hypergiants and further explore the mass-loss behavior in this regime. The BSG quantitative analysis shows no previously suggested sharp jumps in their mass-loss rates. However, hypergiants – closer to the Eddington limit – exhibit thicker, primarily iron-opacity-driven winds. Probing the hypergiants’ temperature range reveals jumps in the mass-loss trend, embedded in a complex relation between mass-loss rates and effective temperature. Progressively modulated by different iron ionization stages, the winds switch between LBV-like ("dense") and BSG-like ("rarefied") solutions. This work sets a new analysis standard for objects at the edge of the line-driven wind regime, stresses the need to differentiate between super- and hypergiants, and reveals a wind dichotomy currently unaccounted for in massive star evolution.
Supervisor: Andreas Sander (ARI)
Cyril Gapp (Germany) - 22.07.2026
Probing the atmospheric structures of transiting exoplanets using space-based transmission spectroscopy ( thesis pdf, 20 MB )
Most currently-known planets outside of the Solar System - exoplanets - transit their host star once per orbit as viewed from the Earth. In addition to the possibility to discover exoplanets through the periodic dimming of the host star’s light during transit, transits also offer the opportunity to study the planets’ atmospheres: A fraction of the star light transmits through the planetary atmosphere during a transit and by measuring how atmospheric constituents absorb parts of it, the chemical composition can be constrained. This method, transmission spectroscopy, was used in this thesis to study the atmospheres of exoplanets that range in size from smaller than Neptune to larger than Jupiter. The observatories used are located in space which eliminates distortions of the data caused by the Earth’s own atmosphere, but limits the spectral resolution to comparatively low values. Two transits of the ultrahot Jupiter WASP-121 b observed with the James Webb Space Telescope (JWST) revealed a highly heterogeneous atmosphere with a chemical composition distinct from those measured in other planets before. By measuring how the absorption of star light changes as the planet rotates during the transits, I reveal how the atmospheric temperatures drastically change between the eternal day- and nightside, driving the dissociation of water vapor. Due to the high temperatures, silicon monoxide is present in the gas phase which in principle allows to probe the amount of refractory material in the planet, delivering constraints on how much rocky material the planet accreted in its past. The observed gradients in temperature and water abundance, however, impose challenges for this measurement which need to be accounted for in future models. Exoplanets with radii between approximately 1.8 and 4 Earth radii — sub-Neptunes — are the most abundant currently-known ones with orbital periods shorter than 100 days in the Milky Way. Thus, inferring their interior structures and formation histories would shed light one of the dominant planet formation pathways in the Galaxy. The sub-Neptunes HD191939 b and TOI-270 c were observed with the Hubble Space Telescope (HST) and JWST, respectively, revealing no statistically-significant atmospheric signals. Both non-detections are consistent with the planets having a high mean molecular weight atmosphere, aerosols or disequilibrium chemistry masking atmospheric absorption. With these cases being degenerate and combinations being possible, the interior structures of the planets remain unknown. The comparison of the observations demonstrates that JWST delivers more constraining data than HST, opening the way for the atmospheric characterization of numerous sub-Neptunes in the future.
Supervisor: Thomas Evans-Soma (MPIA/Univ. Newcastle)
Michael Cecil (Austria) - 21.07.2026
Raving with the strars: Structure, evolution and (in)stability of the inner regions of protoplanetary discs ( thesis pdf, 9 MB )
The high occurrence rate of exoplanets on close-in orbits has raised fundamental ques- tions about their formation and has made the inner regions of protoplanetary discs a central subject of current planetary astrophysics. This thesis explores the structure and evolution of the inner disc regions over thousand-year timescales, with particular emphasis on the dynamics of the local turbulence transition. Two- and three-dimensional radiation hydrodynamic simulations are employed to capture the complex interplay of physical processes within the inner few astronomical units, including increasingly realistic descriptions of radiative properties and turbulence conditions. The results show that the conventional picture of the inner disc is generally not a static configuration but undergoes periodic phases of instability, triggered by the emergence of strong turbulence in otherwise weakly turbulent regions. This phenomenon substantially restructures the inner disc, destroying and re-establishing solid-trapping pressure peaks and generating observable stellar accretion outbursts. Starting from a simplified model, the triggering mechanism of instability cycles is characterised, and the resulting evolution, accretion signatures and consequences for planet-forming conditions are assessed. Subsequent refinements challenge typical simplifying assumptions about radiative and turbulent properties. They show, in particular, that the gas component’s opacity strongly affects the inner disc’s thermal structure, while the instability behaviour and outburst morphologies depend sensitively on the magnetic field strength and the conditions for turbulence activation. This work implies that the study of planet formation and migration close to the host star must be embedded within an analysis of the inner disc’s long-term stability. The periodic structural reshaping revealed in this thesis provides a new framework for understanding the emergence of close-in exoplanetary systems and may have implications for the origins of the terrestrial planets of our Solar System.
Supervisor: Mario Flock (MPIA)
Frederik Wohlleben (Germany) - 20.07.2026
Developing Instruments for Highest-Energy Gamma-Ray Astronomy in the Southern Hemisphere ( thesis pdf, 30 MB )
Gamma-ray astronomy will experience a significant step forward in the coming years. New observatories in the Southern Hemisphere will improve observations of the Galactic Center and shed light on unsolved questions in astronomy and cosmology, as well as particle physics. This thesis provides insights into the instrumentation for these observatories, with a particular focus on the detection of Cherenkov light using photo multiplier tubes (PMTs) and the processing of photodetector signals. The first part outlines the scientific context, relevant technologies, and planned observatories, including their projected performance, and the resulting requirements for the instruments. Building on this, a comprehensive study of different candidate PMT models, for the Southern Wide-field Gamma-ray Observatory (SWGO) is presented, with particular emphasis on reclaimed PMTs from previous experiments. The focus then shifts to SSTCAM, the camera of the Small-Sized Telescopes within the Cherenkov Telescope Array Observatory (CTAO), covering its design, subsystems, and assembly. A method for time reconstruction is introduced and its performance is evaluated through a detailed analysis of the camera’s timing accuracy and resolution. In addition, results from a prototype measurement campaign conducted in Tenerife in 2025 are presented. Finally, the findings are summarized to derivere commendations for the selection of PMT models for SWGO, as well as for time reconstruction and calibration strategies for SSTCAM.
Supervisor: James Hinton (MPIK)
Mariia Demianenko (Russia) - 17.07.2026
Observational constraints of intermediate-mass black holes ( thesis pdf, 35 MB )
Intermediate-mass black holes (IMBHs) may preserve the imprint of black-hole (BH) seed formation in the early Universe, yet convincing representatives remain rare. This thesis develops observational and simulation methods to identify and characterize this elusive population with present surveys, follow-up observations, and the future Extremely Large Telescope (ELT). I developed a post-processing pipeline to correct ZTF Forced Photometry light curves for a large sample of broad-line low-mass AGNs and active IMBHs, while also testing whether some candidates could instead arise from outflow-broadened narrow emission. The sample is presented on a dedicated website with interactive light-curve visualization. I selected feasible targets and then initiated a reverberation-mapping campaign of J1448+16, a highly accreting active IMBH, and measured the broad-line-region time delay at a BH mass and accretion rate that had not previously been explored. This provides a direct test of virial mass estimators in the low-mass, high-accretion regime. Finally, I developed mock images with anisoplanatism for the Multi-adaptive optics Imaging Camera for Deep Observations (MICADO) and multi-band Gaussian-process PSF interpolation, preparing for future ELT astrometry of dense stellar clusters. Together, these results link time-domain variability, reverberation mapping, and next-generation astrometry into a coherent effort toward revealing the bona fide IMBH population..
Supervisors: Joerg-Uwe Pott / Nadine Neumayer (MPIA)
Roel Lefever (Belgium) - 16.07.2026
A dynamically consistent Analysis of Wolf-Rayet Stars in different Environments ( thesis pdf, 9 MB )
Wolf-Rayet (WR) stars are a rare, yet highly impactful group of massive stars. These stellar powerhouses emit strong radiation which causes a large outflow of material from the surface of the star, moving away at high velocities. Via this “stellar wind”, these short-lived stars strongly impact their environment by injecting large amounts of (chemically processed) matter and kinetic energy into their surroundings while at the same time obscuring their hydrostatic surface. As the light from WR stars is distorted by their wind, stellar property inference requires tracing the light emerging from the wind back to the star itself. In this thesis, I demonstrate that traditional modelling of the outermost WR layers and wind, the co-called atmosphere, has numerous deficiencies such as significant degeneracies for the inferred stellar parameters. To overcome this, I employ hydrodynamically-consistent atmosphere modelling to derive a physically more realistic wind structure for different types of WR stars, study resulting mass-loss trends and apply these next-generation atmospheres for quantitative spectroscopy to get the first sample structurally-consistent sample of nitrogenrich, He-burning WR stars. Consequences and discrepancies with current stellar evolution and mass-loss modelling are highlighted and necessary future work to extend the concepts to more WR subtypes are discussed.
Supervisor: Andreas Sander (ARI)
Georg Schwefer (Germany) - 9.07.2026
Studying Galactic TeV Gamma-Ray Emission on the Smallest and Largest Scales ( thesis pdf, 20 MB )
The study of the Milky Way is of particular interest for TeV gamma-ray astronomy, especially on the smallest and largest scales. On the smallest scales, resolving the morphology of individual gamma-ray sources is crucial to understanding their internal workings. However, instrumental resolution often strongly limits these studies.
In this thesis, FreePACT, a novel gamma-ray reconstruction method for imaging atmospheric Cherenkov telescopes (IACTs) is presented. FreePACT combines existing image-likelihood fitting techniques with machine-learning methods from the field of likelihood-free inference. FreePACT is shown to significantly improve energy and angular resolution of the next-generation IACT observatory CTAO. It is then applied in a CTAO sensitivity study that uses pulsar wind nebulae to demonstrate the impact this improved resolution has on studying TeV gamma-ray sources.
On the largest scales, there is the Galactic diffuse emission of gamma rays. It is produced by cosmic rays propagating through the interstellar medium and therefore contains information on their global distribution in the Milky Way.
This thesis contains a study of the Galactic diffuse emission with HAWC Observatory data. It features an adapted data selection, a treatment of systematic uncertainties and considers the entire visible Galactic plane. The measured flux is consistent with results from the LHAASO experiment, but lower than that measured by the Tibet-ASγ experiment. Towards the Galactic center, the flux is in excess of predictions by conventional models, but in line with extrapolations from GeV measurements.
Supervisor: James Hinton (MPIK)
Taavet Kalda (Estonia) - 01.07.2026
Non-parametric recovery of the gravitational potential in the Milky Way ( thesis pdf, 6 MB )
Modern surveys of the Milky Way have delivered an unprecedented wealth of data on the positions, motions, and chemical compositions of billions of stars in the Galaxy. The methodologies used to infer the Galaxy’s gravitational potential – and through it, the distribution of dark matter – must adapt to the complexity the data reveals. Existing approaches typically rely on restrictive symmetry assumptions and simple parametric models that fail to fully exploit the complexity of these surveys. This thesis develops flexible, data-driven alternatives that apply modern machine-learning techniques to this classical inference problem. The method central to this thesis, Deep Potential, represents the stellar distribution function with a normalizing flow and the gravitational potential with a neural network, linking the two through the collisionless Boltzmann equation under a stationarity assumption. Because it imposes no symmetry on the potential and no functional form on the distribution function, it is well suited to the complex, non-axisymmetric structure of a real galaxy.
The thesis presents three contributions. First, I extend Deep Potential to rotating reference frames, enabling its application to galaxies with bars and spiral arms. I validate this extension on a self-consistent N-body simulation of a barred disk galaxy, recovering the gravitational accelerations to within ∼20%, the matter densities to within ∼50%, and the bar pattern speed to within ∼15%–20%.
Second, I apply the method to Gaia Data Release 3, using 5.6 million upper-main-sequence stars within one kiloparsec of the Sun, together with a smooth empirical neural-network model of Gaia’s selection function – the probability that a star with given observational parameters is included in the dataset. I measure a local pattern speed of Omega_p = (28.2 ± 0.1) km/s/kpc, and at the solar Galactocentric radius, I find a total matter density of rho = (0.086 ± 0.010) Msun/pc3, and a dark matter density of rho_DM = (0.007 ± 0.011) M⊙/pc3 (quoted uncertainties are statistical only).
Third, I introduce a methodological advance that addresses a central practical obstacle in dynamical modeling: the selection function. I show that by modeling the stellar distribution function conditional on position, rather than the full distribution function, one can simultaneously recover the true spatial density of the tracer population and the gravitational potential without ever modeling the selection function explicitly. The approach exploits the over-constrained nature of the inference problem and is demonstrated on a mock dataset with a complex three-dimensional dust distribution, where it recovers the potential accurately. This removes a key barrier to applying dynamical inference to the much larger stellar samples expected from future surveys. With these tools in hand, the application of flexible dynamical inference to larger Galactic volumes and ever-growing datasets from Gaia and to the chemically tagged stellar samples of upcoming spectroscopic surveys becomes a natural next step.
Supervisor: Gregory Green (MPIA)
Jonas Müller (Germany) - 02.07.2026
Visibilities of damped mixed modes in red giant stars ( thesis pdf, 6 MB )
The visibility of oscillation modes in red giant stars can be used to draw conclusions about the damping mechanisms operating in the stellar core. Recent observational studies demonstrated that although the damping rate of the oscillations in the core of many red giants appears to be negligible, other red giants exhibit high core damping rates that are sometimes consistent with the infinite value limit. However, some of these red giants with strongly damped oscillations show a clear mixed mode signature, suggesting that their damping rate in the core must remain finite. It is currently being debated whether a strong internal magnetic field could be the cause of this damping.
In the first part of this thesis, we present an analytical formalism that can be used to express the approximate power spectrum of red giants up to a proportionality factor, while taking into account the influence of damping on the oscillations. This formalism can be used to obtain a quantitative estimate for the visibility and to predict the detectability of the mixed mode signature under different core damping rates. In the second part, we fit an empirical scaling relation to the energy dissipation caused by a strong internal magnetic field as predicted by the Hamilton ray tracing method, in order to make the predictions testable on a larger scale. In the third part, we compare the visibilities calculated from theory with observational methods and discuss the effects of potential biases along the red giant branch. Finally, we show the evolution of the visibility and detectability of the mixed mode signature while testing different prescriptions for the potential energy loss caused by a strong internal magnetic field.
Supervisor: Saskia Hekker (HITS)
Alexander Holas (Germany) - 01.07.2026
The Explosive Fate of Intermediate Mass Stars ( thesis pdf, 20 MB )
Intermediate mass stars (8−12 M⊙) are thought to end their lives as electron-capture supernovae (ECSNe), forming low-mass neutron stars. In this scenario, electron captures on 20Ne trigger the collapse of the ONe core. Thermonuclear burning of 16O was long thought to be insufficient to halt this process, but modern 3D simulations indicate that thermonuclear burning can overcome the gravitational collapse in a so-called thermonuclear ECSN (tECSN). The conditions determining the outcome of ECSNe, and the observational signatures of tECSNe, remain uncertain. This dissertation addresses these questions in three studies. First, the asymmetry of white dwarf double detonations is investigated, highlighting the necessity of multidimensional simulations. Second, simulations of ECSNe find that tECSNe are possible for avariety of ignition conditions; more off-center ignitions allow for tECSNe at higher central densities, with conductive flame speeds playing a key role. Third, the first synthetic observables of tECSNe are calculated and compared to observations. Bare-core tECSNe resemble pure deflagrations in CO white dwarfs and do not exhibit any tension with current astronomical data. This thesis is an important step in unraveling the explosive fate of intermediate mass stars, and significantly advances our understanding of tECSNe.
Supervisor: Friedrich Roepke (HITS)
Marcelo Alberto Cortés Vergara (Chile) - 24.06.2026
Black hole formation in dense star cluster through stellar collisions ( thesis pdf, 20 MB )
Dense star clusters are prone to interactions between stars that can lead to the formation of a massive object through stellar collisions. These include different types of star systems, such as nuclear star clusters, globular clusters, ultracompact dwarf galaxies, and young massive clusters. This thesis presents comprehensive studies that combine numerical simulations and observational analyses to investigate a scenario in which dense star systems act as environments conducive to the formation of massive black holes through runaway stellar collisions. To evaluate this scenario, simplified equal-mass star models, observational analyses, and detailed, state-of-the-art simulations were used. First, we used idealized compact systems as a proof of concept, demonstrating that exceeding the critical mass, which depends on the cluster size, the number of particles, and the average mass of the stars, leads to global instability where cluster dynamics are dominated by frequent collisions. As a consequence, these models show a high black hole formation efficiency. Subsequently, a large database of published data from simulations and observations was used to study the dependence of black hole formation efficiency on the ratio of the initial stellar mass to the critical mass, considering a wide range of data from both numerical simulations and observations of dense stellar systems. Finally, one of the most computationally expensive million-particle simulations to date was performed, forming a very massive star, which subsequently collapsed into an intermediate-mass black hole seed of ~5 x 10^4 M_sun in ~5 Myr. This framework allows us to understand the formation of massive black holes in various stellar environments through a purely collisional channel.
Supervisor: Rainer Spurzem (ARI)
Leon-Alexander Hühn (Germany) - 18.06.2026
Environmental Interactions of Protoplanetary Disks – Early-Stage Planetesimal Formation and Late Infall ( thesis pdf, 40 MB )
Planets form in disks of gas and dust around young stars called protoplanetary disks. The classical method of treating planet formation assumes an isolated environment, where purely internal processes lead to the creation of planets out of the dust. However, this picture is increasingly being challenged. Substructures in young disks and the insufficient solid mass budget of older disks have highlighted the importance of considering the onset of planet formation before the commonly considered evolved Class II stage. Furthermore, ubiquitous evidence of material inflowing from larger scales, in the form of streamers, onto older disks is being discovered. Infall being important, even at later stages, has started to drive a paradigm shift in the field of planet formation, and it could have fundamental consequences for the theoretical and observational treatment of protoplanetary disks.
I investigate the impact of environmental interactions on protoplanetary disks from multiple perspectives. First, I consider young, embedded disks that undergo heavy accretion. Here, the turbulent conditions are believed to be detrimental to the formation process. I condense three-dimensional star-formation simulations, where such disks arise naturally as a byproduct, into smaller models containing the essence of the conditions in these disks. Contrary to previous studies, I find that planetesimals can form during this early stage.
I also find that the relevance of infall is not limited to the early stages. Using three-dimensional simulations, I show that late-stage infall can cause streamer structures that match the ambient emission seen in observations. I conducted this study using two different infall models, the capture of a cloudlet of gas, and sustained Bondi-Hoyle-Lyttleton accretion. The second scenario provides a natural pathway to forming the streamers found in observations. Infall can also act as an explanation for the configuration of the IRAS~04125+2902 binary system, where the misaligned disk around the primary star could be an accreted second-generation disk. Finally, I discover that infalling material can also create disk spirals without additional mechanisms.
Supervisor: Cornelis Dullemond (ITA)
Lucia Härer (Germany) - 19.05.2026
The non-thermal physics of young stellar ecosystems ( thesis pdf, 50 MB )
Young stellar ecosystems host star clusters and associations and are shaped by mechanical feedback from massive-star winds and supernovae. Stellar-wind interaction forms collective star-cluster scale outflows, which can terminate in strong shocks, and inflate hot, low-density bubbles. Degree-scale γ-ray emission from young stellar ecosystems is indicative of non-thermal particle populations accelerated in-situ. What role these regions play in the Galactic cosmic ray ecosystem, in particular in the PeV regime, is unclear. This dissertation constrains non-thermal physics in young stellar ecosystems based on γ-ray observations and simulations.
Physical models are developed for sources in the Westerlund 1 and Cygnus X regions. In Westerlund 1, the collective termination shock powers leptonic inverse Compton emission. While the TeV source is ring-like, surrounding the cluster, the GeV source traces an asymmetrically expanding bubble, or “nascent outflow”, suggested to eventually break free of the Galactic plane. In Cygnus X, a past supernova is identified as the accelerator at energies of 100 TeV–PeV, while the emission is powered by termination shocks of individual stars at lower energies. This framework quantitatively reproduces the GeV–PeV spectrum and the morphology across energy bands.
The interaction of stellar-wind flows and magnetic fields in compact star clusters is investigated with a custom-built setup using 3D magnetohydrodynamics. The collective outflow and its termination shock are characterised. The shock is found to produce steep, curved particle spectra.ery-high-energy gamma rays (> 100 GeV) reveal fundamental insights into the particle composition, acceleration mechanisms, and particle escape from cosmic sources. Unlike charged cosmic rays deflected by magnetic fields, gamma rays travel in straight lines, tracing their origin directly. Ground-based telescope arrays study these gamma rays by collecting Cherenkov light from secondary particles produced when they interact with the atmosphere. Current instruments such as the High Energy Stereoscopic System (H.E.S.S.) have made significant advances in this field. The Cherenkov Telescope Array Observatory (CTAO) will achieve unprecedented sensitivity through two large arrays of telescopes equipped with high-performance Cherenkov cameras and advanced data analysis techniques.
Supervisor: Brian Reville (MPIK)
Julia Lienert (Germany) - 29.04.2026
Chemical composition of protoplanetary discs: How disc winds shape the evolution of the inner region ( thesis pdf, 5 MB )
Protoplanetary discs are the birthplaces of planets. Not only do they contain the necessary building blocks to form planets, but their structure and evolution strongly impact the planet formation pathway. It is therefore of crucial importance to understand how the chemical composition and evolution of these discs are influenced by the presence of different types of disc winds. The goal of this thesis is to investigate through numerical simulations the effects of internal photoevaporation and magnetohydrodynamic (MHD) winds on the structure of protoplanetary discs, as well as on their evolution and composition. We show that photoevaporative winds open gaps in the discs that not only block inward-moving pebbles but also carry away any gas diffusing into them. The latter distinguishes them from gaps caused by growing planets, which still let gas pass through to the inner disc. As a result, the inner regions of protoplanetary discs with active internal photoevaporation are enriched with water and have a low C/O ratio. The latter separates them from discs with gaps caused by planets, which have high C/O ratios in their inner regions. Consequently, observations of protoplanetary discs with gaps could infer the cause of these gaps when measuring the C/O ratio of the inner region. Discs that are affected by MHD winds also show low C/O ratios in the inner disc if the mass loss from the winds is strong enough to create a pressure bump in the disc. We conclude that incorporating both photoevaporative and MHD winds is crucial when simulating the evolution of protoplanetary discs, as these winds strongly affect the structure of the disc as well as the inner disc’s chemical composition.
Supervisor: Thomas Henning (MPIA)
Clara Escanuela Nieves (Spain) - 27.04.2026
Improvements in Shower Reconstruction and Data Analysis Techniques for IACTs, and a Study of RX J1713.7–3946 ( thesis pdf, 45 MB )
Very-high-energy gamma rays (> 100 GeV) reveal fundamental insights into the particle composition, acceleration mechanisms, and particle escape from cosmic sources. Unlike charged cosmic rays deflected by magnetic fields, gamma rays travel in straight lines, tracing their origin directly. Ground-based telescope arrays study these gamma rays by collecting Cherenkov light from secondary particles produced when they interact with the atmosphere. Current instruments such as the High Energy Stereoscopic System (H.E.S.S.) have made significant advances in this field. The Cherenkov Telescope Array Observatory (CTAO) will achieve unprecedented sensitivity through two large arrays of telescopes equipped with high-performance Cherenkov cameras and advanced data analysis techniques.
The first part of this thesis presents and validates new methods for the CTAO data processing chain that improve the observatory’s sensitivity and precision, with particular emphasis on reliability under suboptimal observation conditions. The study encompasses photoelectron extraction, image cleaning, and an alternative image parameterisation method. An online data volume reduction algorithm considerably reduces stored data while maintaining the observatory’s performance. Simultaneously, a rapid online muon-tagging method identifies muon candidates for calibration. Together, these methods enhance the CTAO performance and reconstruction robustness.
Supernova remnants are theorised to be the primary sources of Galactic cosmic rays. RX J1713.7-3946 is among the brightest and most studied remnants at high energies. In the second part of this thesis, a new joint analysis of H.E.S.S. in conjunction with data from the Fermi Large Area Telescope (LAT) reveals significant gamma-ray emission outside the remnant. This provides strong evidence for protons escaping the acceleration site. Spectral differences within the remnant further indicate the acceleration of multiple particle types.
Supervisor: James Hinton (MPIK)
Katrin Lehle (Germany) - 24.04.2026
The Life Cycle of Galaxy Cluster Cores: Gas Physics and Evolution in Cosmological Simulations ( thesis pdf, 30 MB )
Gas is ubiquitous in the Universe and plays a fundamental role in shaping the Universe we observe today. While nearly all galaxy clusters are filled with large-scale, hot, and X-ray emitting intracluster gas, their central regions can differ significantly. Some clusters host low-entropy gas that can cool efficiently, forming cool cores, whereas others contain only high-entropy gas and lack a distinct core. Understanding why some clusters form and maintain cool cores, while others do not, is essential for constraining the complex interplay of the many physical processes operating in cluster centers. In this thesis, we analyze the properties and evolution of the gas in the centers of galaxy clusters, with a particular focus on the cool-core population, using the cosmological magneto-hydrodynamical simulation TNG-Cluster.
First, we analyze the population of 352 cool-core (CC) and non-cool-core (NCC) clusters, focusing on their thermodynamic properties. Using several criteria to classify clusters, we find that TNG-Cluster naturally produces unimodal distributions of these properties, whereby CCs and NCCs represent the two extremes. We show that the CC fractions as well as the radial profiles of thermodynamic quantities from z = 0 to z = 2 are in broad agreement with observations. This establishes TNG-Cluster as a laboratory to study the evolution of cluster cores and the transformations between CC and NCC states. To that end, we develop a method for identifying transformations based on the evolution of central entropy. We find that clusters frequently undergo such events; on average, clusters experience two to three transformations over their lifetime. Transformations can occur in both directions, but the ones to higher-entropy cores, i.e., in the qualitative direction from CC to NCC states, make up the overwhelming majority. We show that mergers occur statistically more often ∼ 1Gyr prior to a transformation toward a higher- (but not lower-) entropy core.
We also show that transformations to higher core entropy in some clusters are accompanied by an increase in AGN activity. Furthermore, we characterize the magnetic field properties of the halos with a focus on central magnetic field morphology in CC versus NCC clusters. Compared to low-redshift observations, TNG-Cluster produces reasonable magnetic field amplitudes in the central regions of clusters. The main finding of this analysis is that z = 0 cool-core clusters have preferentially tangential magnetic fields at a characteristic scale of ∼ 0.1r500c. In contrast, across the full cluster population, magnetic fields show isotropic configurations at all radii and redshifts. Moreover, we examine the cool gas content of galaxy cluster cores and its morphology at z = 0 and across cosmic time. We find that all clusters contain cool gas at some point in their history, with a wide variety of morphologies ranging from clumpy and filamentary to rotating disk-like configurations. The presence and amount of cool gas in cluster cores are linked to the thermodynamic state of the host cluster and to the transformations of the core.
Supervisor: Dylan Nelson (ITA)
Christian Sorgenfrei (Germany) - 29.01.2026
Determining the innermost structure of quasars by microlensing: Measuring, simulating and interpreting light curves of multiple quasars ( thesis pdf, 45 MB )
This thesis aims at measuring quasar microlensing light curves and applying them to constrain the structure of quasar accretion disks. Thus, data was taken in two photometric filters at the Las Cumbres Observatory, using their global network of 1m telescopes since 2014. In the first part, applying difference imaging analysis together with point spread function photometry aided with Gaia data, we measure the light curves of the multiple images of eight gravitationally lensed quasars in the R and V band, covering almost ten years with in total 1872 epochs. For each quasar, we determine difference curves of the time delay corrected light curves. This removes the intrinsic quasar brightness variations present in all images with only uncorrelated microlensing variability of the individual images remaining. We find these additional variations, attributed to the source size depended microlensing of the individual images by compact objects in the lens galaxy, throughout our whole data set.
For the second part of this thesis, we focus on the prominent microlensing signal in image B of the quadruple quasar HE0435-1223, revealed through our difference curves. The variations appear to be chromatic, i.e. depend on the filter, with higher amplitude fluctuations in the V band. This is expected, since the hotter central region of the accretion disk experiences more microlensing variation due to its smaller size. To quantify this observation, by means of microlensing simulations, we are able to infer that the accretion disk of HE0435-1223 is indeed larger in radius by factors of 1.24^{+0.08}_{−0.20}, 1.42^{+0.11}_{−0.22} and 1.43^{+0.10}_{−0.23} in the R with respect to the V band, depending on the disk model, in agreement with the expectation from thin accretion disk theory, though with a tendency towards a shallower temperature profile. Additionally, we find disk half-light radii of 0.7 to 1.0 Einstein radii, corresponding to average inclined disk scale radii of around log 〈R2500/cm〉 ≃ 16.4^{+0.5}_{−0.7} at 2500Å in the quasar rest-frame.
Supervisor: Jocahim Wambsganss (ARI)
Marten Scheuck (Germany) - 12.01.2026
The structure of the innermost regions of circumstellar discs with VLTI/MATISSE ( thesis pdf, 15 MB )
Circumstellar discs serve as the progenitors of planetary systems. Their inner regions (from <1 au to a few astronomical units) are where terrestrial (i.e. Earth-like) planets form. Structures closely connected to planet formation, such as spirals, rings, or gaps, have been observed in the outer disc regions (∼20– 400 au). These outer regions are being intensively studied with instruments like SPHERE at near-infrared wavelengths or ALMA in the (sub-)millimetre. The innermost regions, on the other hand, have only been made accessible recently by interferometers with high spatial resolutions in the near- and mid-infrared (e.g. AMBER, MIDI, PIONIER, and GRAVITY on the VLTI). In this thesis, I use spectro-interferometric observations with the VLTI/MATISSE instrument, covering a broad wavelength range in the mid-infrared from the L/M (2.8–4.2 µm/4.5–5 µm) up to the N band (8– 13 µm), to investigate the structures present in the innermost regions of planet-forming discs. MATISSE provides spatial resolutions sufficient to resolve the innermost disc regions for sources in nearby star formation regions (∼100 pc).
I apply data reduction and parametric modelling to give astrophysical interpretations of the interferometric observations. First, I explore the circumstellar discs of the HD 142527 binary system, where a close passing (periapsis∼5 au) stellar companion greatly perturbs the disc around the primary. Using a a disc model with an off-centre Gaussian asymmetric component, I find a time-variable structure around the primary at a separation of∼1– 1.2 au over the course of our observations (2021, 2022, and 2023) close to periapsis. The complex structure in the disc seems to be strongly influenced by the companion passage and warrants further observations to enable more detailed modelling. Subsequently, I perform a statistical analysis of∼60 sources from the MATISSE GTO YSO survey. I derive L- vs. N -band sizes from modelling and combine them with simple disc models. In doing so, I find that many discs in the survey sample show signs of gaps or truncations. Here, the sizes increase when the gaps in the disc become larger, up to the point of no significant N -band emission (i.e. truncated), which show smaller sizes than for continuous discs. The sample contains many strongly asymmetric sources from which disc substructures are deduced.
Supervisor: Thomas Henning (MPIA)