Alumni 2025
Zhang-Liang Xie (3.2.) - Jan Henneco (6.2.) - Rahul Ramesh (24.4.) - Max Häberle (24.4.) - Lynn Bychele (28.5.) - Klaus Paschek (10.6.) - Dane Späth (11.6.) - Dhruv Muley (25.6.) - Nils Hoyer (2.7.) - Xiangyu Zhang (3.7.) - Carolin Kimmig (7.7.) - Helena Ren (10.7.) - Sophia Krieger (14.7.) - Quentin Coppée (15.7.) - Molly Wells (17.7.) - Yash Sharma (17.7.) - Jan Eberhardt (22.7.) - Rhys Seeburger (22.7.) - Junia Göller (25.7.)
Junia Göller (Germany) - 25.07.2025
Stars in the Making: Probing Star Formation and its Effects in Simulated Galaxies ( thesis pdf, compressed to 10 MB )
Up to today, the formation of stars in the broader galactic context remains poorly understood. The role of the external forces – such as ram-pressure stripping – and internal properties of galaxies – including the gravitational potential and largescale gas dynamics – are a topic of active debate. In this thesis, we study star formation across two contrasting galactic environments: jellyfish galaxies and the Milky Way. These example systems allow us to investigate both the extragalactic and intragalactic mechanisms affecting star formation. In the first part of this thesis we use the magnetohydrodynamical cosmological simulation TNG50 to probe a sample of jellyfish galaxies for their star formation, asking if gas compression from ram-pressure increases the star formation rate (SFR) in such galaxies, as some observations suggest. We find no such enhancement in the general population, but instead for the majority of jellyfish see a phase of enhanced SFR in their individual evolution. In the second part we introduce the Rhea simulation suite, a set of elaborate hydrodynamical simulations conducted by us to model the Milky Way galaxy. We study the importance of the galactic gravitational potential for star formation, and find the bar to have the strongest influence, as it changes the location and properties of star-forming regions. A spiral arm potential merely reorganizes gas in the galaxy and thus increases SFR in its potential wells. We also find the extend of the star-forming disk to divide the galactic stellar disk into a region dominated by in-situ formed stars, and an outer migrator-dominated region. This duality, in our simulations, affects the stellar density distribution and thus the stellar galactic rotation curve. If no distinction is made between the two regions, the resulting velocity curve can mimic a cut-off in the galactic mass distribution.
Supervisor: Ralf Klessen (ITA)
Luzian “Rhys” Seeburger (Austria) - 22.07.2025
Hunting for dormant Black Holes in the Milky Way ( thesis pdf, 30 MB )
Stellar population models predict 107 stellar mass black holes (BHs) in the Milky Way, with an unknown fraction in mostly dormant binary systems. To date, less than a handful of dor- mant BH binaries have been identified, and post-interaction star-star systems masquerading as BH systems — so-called impostors — have greatly complicated this identification. In this thesis, we develop a set of tools to identify and characterise these impostors using spectroscopy. We demonstrate the power of spectral disentangling using both simulated and real data. Applying the tools to multi-epoch spectra of a sample of impostor systems, we confirm their post-interaction nature and place constraints on their stellar parameters. Critically, we now ascertain their rotation rates and can make improved inferences on their mass transfer history. We also develop single-epoch high-resolution spectra as a diagnostic tool for Galactic binaries identified by Gaia , showing the capabilities of such data to determine fundamental properties, in particular the flux ratio, of these systems. We have been able to add flux ratio measurements or constraints to 80,000 spectra of Gaia binaries, which are crucial for the interpretation of the Gaia data. These toolsets will prove invaluable when analysing upcoming data releases, allowing us to constrain important processes in (binary) stellar evolution and BH formation.
Supervisor: Hans-Walter Rix (MPIA)
Jan Eberhardt (Germany) - 22.07.2025
Discovery and dynamical characterization of exoplanetary systems with the transit and radial velocity methods ( thesis pdf, 30 MB )
This thesis is dedicated to the discovery and characterisation of exoplanet systems using the radial velocity- and transit methods. This study focuses on the detailed characterization of several Jovian mass planets discovered in warm orbits (P > 10 days) as part of the Warm gIaNts with tEss (WINE) collaboration, which aims to validate exoplanet discoveries made with the Transiting Exoplanet Survey Satellite (TESS). The physical and orbital properties of these exoplanets have significant implications for planet formation- and evolution theories. Within the WINE collaboration, I studied five different exoplanets in four systems, TOI-2373b, TOI-2416b, TOI-2524b as well as TOI- 6695 b and c. The planets have been detected by TESS as candidates and later confirmed by myself and my collaboration with ground- based follow-up observations using the Fiber-fed Extended Range Optical Spectrograph (FEROS). TOI-2373 b, TOI-2416 b and TOI-2524 b orbit G-type solar analog stars on close orbits, with periods shorter than 15 days each, while TOI-6695b and c orbit a late F-type star on wider orbits of ∼ 80 and ∼ 240 days.
The planets presented in this thesis mark important discoveries, with TOI-2373b, TOI-2416b and TOI-2524b residing in the transition region between hot and warm Jupiters, TOI-2373 b being the second most massive exoplanet known to orbit a solar analog star and TOI-2524 b having an inflated radius. The pair of planets in the TOI-6695 system is a rare case of two planets with a period ratio of close to 3, that is, close to the 3:1 resonance. Beyond my work with the WINE collaboration, I studied the case of the young T-Tauri star TWHya. In 2008, an exoplanet discovery was presented for TW Hya. The discovery was based on radial veloc- ity observations with FEROS, which revealed a 3.57 day periodicity. Shortly after, this signal was suggested to be caused by a long-lived stellar spot instead of a planet. In this thesis, I present a detailed radial velocity and photometry analysis, including a spot-induced ra- dial velocity model. I found no further evidence for an exoplanet, but instead attribute the radial velocity signal to a large polar spot, fueled by stellar accretion.
Supervisor: Thomas Henning (MPIA)
Yash Sharma (India) - 17.07.2025
Decoding the Reionization Epoch Intergalactic Medium with Most Distant Quasars ( thesis pdf, 20 MB )
At high-redshifts z>= 6, the neutral hydrogen column densities in the IGM are so large that almost all of the Lyman-alpha photons around the resonance are absorbed. At those redshifts, the Lyman-alpha damping wing signals have proven to be instrumental in studying the epoch of reionization. With the recent advancements in the discovery and measurement of high-redshift sources (galaxies and quasars), it is becoming ever more crucial to explore what this new set of sources might reveal regarding the reionization history and its topology. Hence, in this thesis, we aim to comprehensively study signatures of astrophysical, IGM, and reionization source parameters on the reionization topology as seen through the spectra of an ensemble of damping wing profiles. For this, in chapter one, using 21cmFAST we generated the reionization models subjected to a large set of astrophysical parameters. We found that the neutral fraction, x_HI, quasar lifetime, t_q, quasar host halo mass, M_QSO, and minimum halo mass that can support star formation, M_min, significantly impact both the median signal and the scatter of the ensemble of damping wings. But just the idea of an ensemble is not sufficient. We need to quantize it and confirm if such an ensemble is observationally possible. That's why, in the second chapter, we studied the constraining power of damping wings over the parameters filtered in chapter one. We showed that the constraints provided by only 64 quasars at z= 7, x_HI = 0.5^(+/-0.02), log(M_min/ M_sun) = 8.78^(+/-0.53), log(t_q/yr) = 6.0^(+/-0.12), and log(M_QSO/ M_sun)= 11.52^(+/-0.32) are comparable to the results from other observables like 21cm signal, and our methodology works for as low as 32 quasars.
Even though this proved the strength of our damping wing analysis, it did not show that of our suite of models. Hence, using the similar reionization models developed in chapters one and two, in chapter three, instead of calculating Lyman-alpha optical depth, we looked at the Lyman continuum optical depth at z = 6. Subsequently, we studied the ionizing photons' mean free path (MFP) dependency on the filtered set of parameters from chapter one. We then plotted our models of transmission flux in the Lyman continuum regime against the stacked quasar spectra to estimate the range of these parameters that best fit the spectra. Our results imply that to explain the short MFP from the late reionization models, we would need x_HI >= 0.35 and a very short t_q of 10^4 yrs, or x_HI ~ 0.5 for t_q = 10^6 yrs. Thus, indicating that the late reionization models are insufficient to explain the short MFP of ionizing photons, we need some additional Lyman limit systems within the quasar proximity zone to explain the short MFP of the ionizing photons at z = 6.
Supervisor: Frederik Davies (MPIA)
Molly Wells (UK) - 17.07.2025
The Role of Filamentary Flows in High Mass Star Formation ( thesis pdf, 30 MB )
Accretion flows onto dense structures in star-forming regions plays a crucial role in the formation and growth of stars. These flows, which occur on multiple spatial scales -- from large-scale filaments down to small cores -- are responsible for channelling material onto protostellar objects. Understanding the properties of these flows is vital for developing a comprehensive picture of star formation, yet the detailed mechanisms governing their dynamics remain unclear. In this thesis, I investigate the properties of accretion flows in high-mass star-forming clusters, with a focus on understanding the interplay between gas dynamics, environmental conditions, and the evolutionary stage of the systems. Using interferometric data from the ALMA evolutionary study of high mass protocluster formation in the Galaxy, single dish data from the IRAM 30m observatory and synthetic observations from magnetohydrodynamic (MHD) simulations, I explore how material is transported along filamentary structures and accumulated onto dense cores. The results reveal that accretion flow rates are closely tied to the mass of the cores, following a M^2/3 relationship that supports the tidal-lobe accretion model. I find that flow rates have an increasing trend with respect to their evolutionary stages, suggesting a connection between accretion dynamics and the age of the star-forming system. Additionally, feedback from newly-formed stars is shown to significantly affect the flow structure. In feedback-dominated regions, feeder filaments sustain material flows onto the central clump, emphasising the importance of environmental conditions in shaping the accretion process. Through a multi-scale analysis of accretion flows, I find that the flow rates decrease slightly as we move from large to small spatial scales. This thesis contributes to our understanding of the complex dynamics driving star formation. By integrating observational data with theoretical models, it bridges the gap between the large-scale collapse of molecular clouds and the detailed accretion mechanisms moving material onto forming protostars, offering new insights into how this material is accumulated in star-forming regions.
Supervisor: Henrik Beuther (MPIA)
Quentin Coppee (Belgium) - 15.07.2025
Characterising the suppression of non-radial modes in red giants ( thesis pdf, 5 MB )
Magnetic fields are thought to play a significant role in various phenomena in the Universe, in particular in the evolution and the structure of stars. With asteroseismology, it is possible to probe the inner structure of stars. In this thesis, we investigate the properties of the oscillations of red giants that show signs of energy loss. We observationally constrained the mechanism causing this energy loss and found that it is consistent with the predicted effect of magnetic field in the core of the red giant. Using recent developments, we were also able to observe frequency shifts due to a magnetic field in the core of stars. We analysed stars with both magnetic frequency shifts and energy loss in the oscillations and showed that both phenomena can be explained by a magnetic field with the same field strength located in the core of the stars..
Supervisor: Saskia Hekker (HITS)
Sophia Krieger (nee Stuber) (Germany) - 14.07.2025
Clouded by Molecules - An Unparalleled View of the Molecular Interstellar Medium in M51 ( thesis pdf, 70 MB )
Star formation is fueled by dense molecular gas residing in the dense and cold in- teriors of molecular clouds. For decades, the emission of selected molecules, such as HCN and HCO+, has been used to trace this dense molecular gas phase within the Milky Way and external galaxies. Studies have revealed a tight link between star for- mation rate and the emission of these molecules, observed on both sub-cloud scales in the Milky Way and at kiloparsec scales in other galaxies. However, it has been recently established that the properties of molecular clouds significantly depend on the galactic environment they are residing in. Yet, observations of particularly the dense molecular phase – the direct fuel of star formation that resides within those clouds – are often limited to kiloparsec-scale resolution insufficient to access cloud properties or resolved studies of molecular clouds in a single environment only. To understand how exactly the large-scale galactic environments within galaxies are able to regulate the properties of star formation within molecular clouds, it is essential to gain a cloud-scale view of dense gas across a range of galactic environments.
This thesis provides the first piece in understanding this link, with the most compre- hensive analysis of common dense gas tracers on cloud-scales across a diverse set of galactic environments. “Surveying the Whirlpool galaxy at Arcseconds with NOEMA” (SWAN), is the largest cloud-scale mapping of 3-4 mm emission in an external galaxy to date and the focus of this thesis. The detailed view provided by SWAN shows that dense gas is not only confined to spiral arms, but that its tracers emit brightly in the interarm region and galaxy center, suggesting future star formation across the disk. The first time in-depth comparison between common extragalactic dense gas tracers and the Galactic ‘gold standard’ tracer N2H+ reveals significant variations with both large-scale environment and local cloud- scale regions, which are undetected at coarser kiloparsec-scale resolution. Particularly, the center of M51 emerges as an extreme environment in which the relation between these dense gas tracers varies strongly. The utility of HCN emission in tracing gas den- sity, breaks down when tested in M51’s (extreme) environments and HCO+ presents itself as a more robust dense gas tracer.
While studies from Milky Way clouds have questioned the use of HCN in tracing high gas density regions, this is the first time it has been tested at cloud-scales for entire cloud populations and the first time this could be placed into the context of galactic environments. Our new insights on the physical conditions that drive the dense gas emission reveal that gas density is not the sole driver of their emission. This implies that second order dependencies on other physical parameters like dynamical equilibrium pressure, star-formation rate and stellar mass surface density have a pronounced effect and cannot be neglected in further analysis. This thesis demonstrates the importance to place the star formation laws into an environmental context.
Supervisor: Eva Schinnerer (MPIA)
Helena Xiaya Ren (China/Spain) - 10.07.2025
Very-High-Energy and Ultra-High-Energy Gamma-ray emission from the Galactic Centre ( thesis pdf, 30 MB )
The Galactic Centre is a unique and complex astrophysical region, characterised by dense molecular gas, bright point-like sources in very high energies, and a potential PeVatron cosmic-ray accelerator. It is also a key target for indirect dark matter searches, due its high dark matter density and its proximity. However, the region’s complexity, along with challenges posed by line-of-sight superposition of gas and dust emissions, complicates efforts to isolate gamma-ray sources and study cosmic-ray propagation. This thesis presents a comprehensive study of the Galactic Centre environment using multi-wavelength data, focusing on the distribution of gas and dust and the analysis of gamma-rays. A refined decomposition of the interstellar medium is achieved through employing a new method in decomposing molecular and atomic line emission, from HI and several CO isotopologue data. By distinguishing the gas in the central molecular zone and the Galactic disk as well as incorporating atomic and molecular contributions, a more accurate description of the gas distribution is derived. This improved gas template allows for a more realistic estimate of cosmic-ray energy density across the Galactic Centre. The dust thermal emission is fitted using single- and two-component modified Planck functions. A clear deviation from linearity has been observed for the dust opacity as a function of the hydrogen column density, indicating that a simple correlation between dust emission and gas column density is not sufficient.
The second part of the thesis includes gamma-ray data from the H.E.S.S. telescopes, an array of imaging atmospheric Cherenkov telescopes. A novel background rejection technique, ABRIR, is developed and implemented within the H.E.S.S. analysis software. ABRIR uses time-based image cleaning and muon tagging to improve signal extraction from high-energy gamma-ray data. Its performance is validated through a detailed comparison of Galactic Centre observations with and without the method, showing enhanced source significance and reduced spectral uncertainties. Finally, the thesis explores the future observational potential of the Southern Wide-field Gamma-ray Observatory (SWGO), a ground-based water Cherenkov detector. Currently under development, SWGO will be the first large-scale water Cherenkov detector in the southern hemisphere. Using updated instrument response functions and simulated observations of the Galactic Centre, the expected performance of various array and tank configurations is assessed. SWGO’s sensitivity to WIMP dark matter annihilation is also evaluated. In summary, this thesis contributes with a new description of the gas and dust in the Galactic Centre, an improved background rejection method for H.E.S.S., and prospect estimates for the Galactic Centre observation with SWGO, to the field of Galactic Centre studies at very-high and ultra-high energies.
Supervisor: James Hinton (MPIK)
Carolin Kimmig (Germany) - 07.07.2025
Warped Disks in 3D – From the formation and evolution to synthetic observations ( thesis pdf, 30 MB )
Planets are born in disks around young stars called protoplanetary disks. It is therefore essential to understand the physical and dynamical conditions dominating these disks. In the past decade, highly resolved observations of protoplanetary disks have transformed the research field. Many observations revealed striking non-axisymmetric structures. Some of these structures can be explained by a shadow cast from a possibly unresolved misaligned inner disk region. The abundance of such observed shadows suggests that misalignments might be common.
In this thesis, I investigate the formation, evolution and appearance of warped protoplanetary disks. To study their dynamics, I use three-dimensional grid-based simulations, a method which is not commonly used in previous models of warps. I therefore extensively test its applicability to disks misaligned with the intrinsic geometry of the grid. I find that warps can be modeled accurately under the condition of a sufficient vertical resolution. My simulations show the expected wave-like warp evolution and additionall reveal an internal twisting of the disk, which is not included in standard one-dimensional models. My various tests suggest that the twisting is a physical effect intrinsic to the three-dimensional warp evolution.
I apply the grid-based method to a formation scenario of warps: inclined stellar fly-bys. I first investigate different configurations of the fly-by trajectory with respect to the disk. The simulations show that inclined fly-bys can excite a warp of a few degrees which can last for over ten thousand years. I then model RW Aur A, an observed star-disk system that recently experienced a close encounter with another star with a well constrained trajectory. My models show that a warp of about 5° misalignment is excited, which is consistent with the observations.
Shadows, mainly observed in scattered light observations, can give insight into the warp shape. In order to find limits on the observability of warps, I investigate the shadows in radiative transfer simulations of disks viewed edge-on, where warps can cause asymmetries. I find that under optimal conditions, small warps with a misalignment of only 2° can create observable asymmetries. As the strength of the asymmetry depends on the orientation of the warp with respect to the observer, it remains challenging to infer constraints on warps from observations. However, rare orientations can lead to a brightness swap between the disk surfaces, a clear indication of a warp, which is observed in a handful of disks.
Supervisor: Cornelis Dullemond (ITA)
Xiangyu Zhang (China) - 03.07.2025
On the Variation of Interstellar Dust Extinction ( thesis pdf, 25 MB )
Dust obscures astronomical observations and plays a important role in the evolution of galaxies. Mapping dust in three dimensions not only allows accurate correction of observations but also constrains the evolution of the interstellar medium (ISM). Extinction is the central observable to trace the density and properties of dust, and is widely employed in dust mapping. Most existing dust maps assume a universal extinction curve, treating the properties of dust as uniform. This thesis introduces variations in the slope of extinction curves, parameterized by R(V). I construct a data-driven forward model that predicts low-resolution spectra as a function of stellar parameters (effective temperature, surface gravity, and metallicity), parallax, and extinction properties. This model is applied to all 220 million Gaia XP spectra, yielding precise extinction curves for 130 million stars in the Milky Way, LMC, and SMC. From these, I construct the first 3D all-sky map of extinction curve variations. Unexpectedly, I find extinction curves steepen, rather than flatten, with increasing dust density in translucent dust clouds. I propose a theoretical explanation, attributing the steepening to the growth of polycyclic aromatic hydrocarbons (PAHs). The results also provide implications for future observations in the era of the JWST.
Supervisor: Gregory Green (MPIA)
Nils Hoyer (Germany) - 02.07.2025
The assembly of nuclear star clusters ( thesis pdf, 10 MB )
The evolution of galaxies represents a hitherto unsolved area of modern astrophysics. Various dynamical effects play an important role in shaping galaxy centres making them extremely interesting to study in detail. Nuclear star clusters are often a key element of galaxy centres and feature both diverse and complex formation histories as well as extremely high stellar densities. Many aspects of nuclear star cluster formation are still uncertain and require new constraints to make full use of their ability as ideal laboratories for studying galaxy evolution and the growth of massive black holes that are located in their centres. An improved knowledge of nuclear star cluster assembly is especially relevant today in order to properly interpret the vast amount of upcoming data produced with all-sky surveys. I present in this dissertation various analyses of high-resolution observational data sets from space-based missions ranging between the X-ray and mid-infrared regimes and a new computer simulation of the assembly of massive star cluster populations. The first analysis reveals tight correlations between the properties of nuclear star clusters and old globular star clusters in the Milky Way indicating a potential common formation mechanism of the two cluster types. In a second analysis I identify nuclear star clusters with variable accretion signatures from massive black holes within them and provide upper limits for lower-mass systems in case of non-detections. I demonstrate in a third project that analysing the spectral energy distribution of the nuclear star cluster in the nearby massive spiral galaxy Messier 74 constrains the assembly history of both the host galaxy and a potential massive black hole in the star cluster’s centre. In addition to the projects that rely on observational data sets, I introduce a new simulation that is based on a dark matter-only computation and considers the co-formation of galaxies and massive star clusters. I show that my simulation can reproduce a number of observational quantities such as the mass function of young massive star clusters or the metallicity distribution of old globular clusters, both in nearby galaxies. I conclude by summarising the contents of this dissertation and by presenting future efforts that build on the presented observational and numerical approaches.
Supervisor: Nadine Neumayer (MPIA)
Dhruv Muley (USA) - 25.06.2025
Three-temperature radiation hydrodynamics: A powerful tool for investigating protoplanetary disks ( thesis pdf, 60 MB )
High-resolution observations of protoplanetary disks over a range of wavelengths have uncovered a wealth of large-scale substructures---including gaps, rings, and spirals---often attributed to the gravitational influence of nascent planets. This problem has long been studied using numerical hydrodynamics, with recent works demonstrating that the thermodynamic properties of the disk play a defining role in substructure morphology. To better model these properties within simulations, we have developed a ``three-temperature" (3T; gas, dust, radiation) scheme for the PLUTO hydrodynamics code, including absorption/emission of radiation (principally by the dust, which supplies most of the opacity) and collisional thermal relaxation between dust gains and gas (which contains most of the mass/heat capacity). Although dust and gas are thermally well-coupled in the dense midplane of a typical disk, thermal relaxation times in the more rarefied disk atmosphere reach order-unity of the dynamical time, allowing perturbations driven by an orbiting planet to decouple the dust and gas temperatures. We apply 3T to open questions inspired by disk observations, finding that planet-driven gas-kinematic and temperaure spirals (such as that seen in TW Hya) are strengthened by planetary accretion luminosity, and that large-scale, double-armed spirals in the near-infrared (such as those in SAO 206462 and V1247 Ori) may be induced not by a planet's gravity directly, but by the effect of planet-carved gaps on shadowing and illumination of the outer disk.
Supervisor: Hubert Klahr (MPIA)
Dane Späth (Germany) - 11.06.2025
A Critical Appraisal of Planets Orbiting Giant Stars ( thesis pdf, 50 MB )
Giant stars provide a unique opportunity to study planets around intermediate-mass stars through radial velocity surveys and examine the impact of stellar evolution on planetary systems. Despite their growing numbers, few planets have been found with very short or long orbital periods. Furthermore, detections around luminous giants are debated as intrinsic mechanisms can mimic planetary signals. In this thesis, I rule out two transiting planet candidates from TESS via spectroscopic follow-up. To address the planet controversy surrounding luminous giants, I develop the simulation tool pyoscillot, showing that the radial velocities and activity indicators of the false-positive planet host NGC 4349 No. 127 can be reproduced by a non-radial oscillation model. I further combine data from the Lick, SONG, and CARMENES spectrographs to analyze ten planet candidates with intermediate orbital periods. I find that testing an extensive baseline of radial velocities for consistency with Keplerian orbits is crucial to rule out or confirm planets around luminous giants. While the signals of seven stars are intrinsically induced, I identify a long-period planet candidate orbiting HIP 64823. Finally, I present first results of the new échelle spectrograph mounted at the Waltz telescope at Landessternwarte, Heidelberg, demonstrating its potential for detecting planets around giants.
Supervisor: Sabine Reffert (LSW)
Klaus Paschek (Germany) - 10.06.2025
From Space to the Hadean Earth: Prebiotic Synthesis and the Origins of Life ( thesis pdf, 20 MB )
For life to emerge, a variety of different molecular building blocks must be available in sufficiently high concentrations to increase the chances of their assembly into the highly complex macromolecular machinery required without the aid of elaborate biological enzymes and compartments. In this thesis, we explore different environments in which prebiotic synthesis of these building blocks is feasible. This ranges from the interplanetary space of the early solar system in the interior of meteorite parent bodies to the surface of the early Hadean Earth in small bodies of water, so-called warm little ponds (WLPs), on the first landmasses emerging from the primordial ocean. The idea is to combine the exogenous and endogenous processes into a consistent and realistic scenario of the early solar system and the Hadean Earth within it. This includes bombardment by meteorites from space, which release excessive amounts of energy during impact, leading to the generation of reducing gases that are injected into the Earth’s atmosphere. Meteorites can also deliver prebiotic molecules formed in space, seeding the early Earth with the building blocks of life. In addition, geologic activity within the Earth’s mantle is included, leading to serpentinization of upwelling mantle material by chemical reaction with water that penetrates the Earth’s crust from the surface, releasing further reducing gases into the atmosphere. The goal is to assess the potential contributions and interactions of these different processes in setting the stage for the origins of life. We find that aqueous chemistry inside meteorite parent bodies can accurately describe and explain the prebiotic synthesis of the various biomolecules found in the meteoritic subclass of carbonaceous chondrites, with simulated and measured abundances of nucleobases, the sugar ribose, and vitamin B3 in good agreement, shown here for the first time. The key to achieving this was the introduction of a proper understanding of volatile depletion in the source material of meteorite parent bodies. If delivered by meteorites to the Hadean Earth surface, our simulations show that extraterrestrial biomolecules formed in space could have significantly elevated the concentrations of prebiotic molecules in WLPs. In addition, a carbonaceous bombardment in a reducing atmosphere is capable of releasing sufficient amounts of the reducing gas HCN (hydrogen cynanide) to rain-out into WLPs and drive the prebiotic synthesis of sufficient amounts of the building blocks of RNA, one of the most commonly favored starting points for the origins of life.
The most important finding of this thesis is that serpentinization alone can reduce an initially oxidizing atmosphere with initial 90 % CO2 when modeled in a manner appropriate for the Hadean Earth. This might provide a solution to one of the oldest problems in the origins of life sciences, allowing effective prebiotic synthesis despite an initially oxidizing atmosphere that would otherwise be considered highly unfavorable. Not only that, but we were able to show that an initially oxidized atmosphere is actually a desirable state for prebiotic molecules, provided that high enough fluxes of reducing gases are emitted by serpentinization, as we indeed anticipate in the Hadean. The oxidizing atmosphere is reduced and exploited as a carbon source. The modeling of these various contributing mechanisms in this comprehensive scenario for the Hadean Earth and its host planetary system as a whole paints an optimistic picture for the preparation of a potent prebiotic soup on the Hadean Earth and beyond.
Supervisor: Thomas Henning (MPIA)
Lynn Buchele (USA) - 28.05.2025
Asteroseismic Structure Inversions of Solar-like Oscillators ( thesis pdf, 30 MB )
The study of stars through their global oscillations, i.e., asteroseismology, has provided unprecedented insight into stellar interiors. One of the most powerful techniques of asteroseismology is that of structure inversions. This technique can localize and quantify differences in sound speed between a star and its best-fit model. These differences, then, are a direct test of the accuracy of our stellar models.
The first part of this thesis outlines the specifics of the inversion procedure and then applies it to 55 main-sequence stars observed by the Kepler mission with masses between 1 and 1.6 solar masses. This discussion is split into stars with radiative and convective cores. Overall, the inversions reveal that our best-fit models match the sound speed profile in around half of the stars studied. In the remaining half of the sample, there is an even split between cases where the model sound speed is too high and cases where it is too low.
The second part of this thesis explores whether current inversion techniques are suitable for subgiant stars. These stars exhibit mixed modes that are sensitive to deeper regions of stellar cores. As the sensitivity of these mixed modes changes on a very short timescale, obtaining reliable inversion results will require modifications to current techniques.
Supervisor: Saskia Hekker (HITS)
Max Häberle (Germany) - 24.04.2025
A new kinematic view of the globular cluster ω Centauri ( thesis pdf, 90 MB )
Omega Centauri (ω Cen) is the most massive globular cluster of the Milky Way. Due to its complex stellar populations, it is believed to be the stripped nucleus of an accreted dwarf galaxy, making it not only an important witness of the Milky Way’s merger history but also the closest galactic nucleus to us on Earth. Many of its properties are still under active investigation, including the origin and kinematics of its multiple stellar populations, the mass distribution in its center, and the potential connection with the debris of its former host galaxy. As a stripped nucleus, ω Cen has also been a candidate for hosting an intermediate-mass black hole, but previous searches have remained inconclusive. In this thesis, I present a novel, comprehensive astrometric and photometric catalog based on 20 years of archival and new Hubble Space Telescope observations. The underlying dataset contains over 800 individual images and allows for the measurement of precise proper motions for around 1.4 million stars within the half-light radius (r_HL=5’) of ω Cen. Within the new dataset, I discovered 7 fast-moving stars within a radius of just 3 arcseconds (0.1pc) around the cluster center. These stars move faster than the expected local escape velocity, and their presence can be best explained by being bound to a black hole with a mass of at least 8200 solar masses. These results make a strong case for ω Cen hosting an intermediate-mass black hole, a long-sought missing link between stellar mass and supermassive black holes. To study the overall kinematics of ω Cen, I combined the astrometric dataset with a recent spectroscopic catalog, allowing the determination of the full 3-dimensional velocity vector for thousands of stars. This analysis provides improved measurements of the velocity dispersion profile, the rotation curve, and the state of energy equipartition of ω Cen. By combining proper motions with line-of-sight velocities, I calculated an improved kinematic distance estimate of (5445±41) pc, the most precise distance to ω Cen available. In the final scientific chapter, I present preparatory efforts for the guiding system of the new SDSS-V Local Volume Mapper (LVM). This novel wide-field integral field spectrograph celebrated its first light in 2023. While the primary goal of this instrument is to map interstellar gas within the Milky Way and the Magellanic Clouds, it can also study the integrated light of star clusters such as ω Cen, benchmarking unresolved extragalactic studies. I present preliminary results of the overall line-of-sight kinematics measured with the LVM, which agree with our resolved results. The results of this thesis are an important step towards a better understanding of the formation history of ω Cen and low mass galactic nuclei in general. The large astro-photometric catalog is already being used for detailed studies of the age-metallicity relation, the abundances, and the subpopulations of ω Cen. The precise kinematic measurements will be followed-up with detailed dynamical models. Finally, the detection of fast-moving stars marks a breakthrough in the search for local intermediate-mass black holes in star clusters and can serve as a blueprint for future searches in other clusters, although new instrumentation such as ELT MICADO will be required.
Supervisor: Nadine Neumayer (MPIA)
Rahul Ramesh (India) - 24.04.2025
Deciphering galaxy evolution through the baryon cycle and circumgalactic medium in cosmological simulations ( thesis pdf, 50 MB )
Observations and theoretical studies suggest that galaxies are surrounded by a halo of diffuse gas that extends far beyond the extent of the central stellar component. This region, known as the circumgalactic medium (CGM), serves as the area through which gas from larger scales accrete due to the gravitationally-driven growth of cosmic structures. Additionally, it contains gas that has been expelled from the galaxy due to feedback processes, as well as gas that circulates in the halo through recycling or fountain flows. The CGM is thus believed to be critically linked to the evolution of galaxies.
The non-trivial interactions of these various physical processes result in a complex structure within the CGM: while most of the volume is dominated by a warm-hot phase, there are also clouds of cooler gas that coexist. Despite significant progress over the recent past, many open questions remain regarding the origin of the multi-phase nature of this gaseous reservoir, its properties, and the role of different processes in shaping its existence. In this thesis, we explore various such puzzles using cosmological magnetohydrodynamical simulations run with \textsc{AREPO} and the IllustrisTNG galaxy formation model.
We begin by analyzing the publicly available TNG50 simulation, with a particular focus on Milky Way-like (MW-like) galaxies in most cases. Our findings suggest that, among other processes, feedback driven by galactic processes may significantly impact the CGM, including its overall temperature and velocity structure, the number of cold clouds, and the angular structure of magnetic fields. In the later parts of this thesis, we introduce and analyze the new GIBLE suite of simulations, also run with the same IllustrisTNG galaxy physics model, but exclusively simulating MW-like galaxies at ultra-high CGM gas mass resolutions. As a first scientific exploration with GIBLE, we study the draping of magnetic field lines around cold clouds -- a phenomenon simply absent in simulations run at lower resolutions, highlighting the power of these new numerical experiments.
Supervisor: Dylan Nelson (ITA)
Jan Henneco (Belgium) - 06.02.2025
The Progenitors and Products of Stellar Mergers ( thesis pdf, 20 MB )
A considerable fraction of stars live in binary systems and may exchange mass with each other. One of the outcomes of this mass transfer is a contact phase in which the two stars share a common envelope. If this contact phase is unstable, the binary components will merge and leave behind a merger product. In the first part of this thesis, we evolve several thousand binary systems with varying initial parameters and assumptions regarding the efficiency of mass transfer to trace which binaries evolve into a contact phase and which of these may merge. We identify the mechanisms leading to contact and stellar mergers, assess the influence of our assumptions, and compare our population of contact binaries to observations. In the second part of this thesis, we predict the pulsations of stellar merger products to investigate whether they differ significantly from those of genuine single stars. We find that the peculiar internal structure of these merger products affects their predicted asteroseismic signatures, both for merger products on the main sequence and more evolved merger products. With these predictions, we demonstrate the potential of asteroseismology to distinguish merger products from genuine single stars.
Supervisor: Fabian Schneider (HITS)
Zhang-Liang Xie (China) - 03.02.2025
Tracing Jets Across Redshifts: Blazars and the Impact of Jets in High-z Quasars ( thesis pdf, 30 MB )
Blazars, a subclass of radio-loud Active Galactic Nuclei (AGN) with jets aligned close to our line of sight, offer valuable insights into jet physics and demographics of jetted AGN. In this thesis, we present a new algorithm for distinguishing blazars from non-blazars by analyzing their radio morphology with data from the Very Large Array Sky Survey (VLASS). Applying this algorithm to existing catalogs, we reveal that over 12% of previously classified blazar candidates exhibit non-blazar-like morphology. Remarkably, we find that 3% of previously "confirmed" blazars are likely misclassified. A case study of J0643--3314, initially identified as a blazar candidate, was rejected as a blazar by our algorithm,which is supported by observational evidence. This demonstrates the reliability of our morphology-based approach.
Expanding our scope to high-redshift quasars, we present optical and near-infrared spectroscopy of four radio-bright quasars at z=5.7-7.0. We estimate black hole mass, bolometric luminosity, and Eddington ratios of these z>5.7 quasars. Although our initial sample is limited in size, these results suggest that high-z radio-loud quasars may not commonly tend to host super-Eddington accreting black holes compared to their high-z radio-quiet or low-$z$ radio-loud quasar counterparts. This hypothesis will be tested further with an expanded sample of 22 additional known z>5 radio-loud quasars. We present preliminary outlooks of their spectra obtained with the Large Binocular Telescope (LBT). These spectra provide us with a larger dataset to determine whether high-z radio-loud quasars are indeed accreting at a faster rate more conclusively.
Supervisor: Eduardo Banados (MPIA)