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 Spaeth (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)