Alumni 2022
Samantha Brown Sevilla (Mexico) 20.12.2022
High-contrast imaging study of exoplanets and circumstellar disks ( thesis pdf, 13 MB )
High-contrast imaging provides an excellent tool to detect and characterise exoplanets and circumstellar disks. Understanding the connection between them is key for the improvement of planet formation and evolution theories. In this thesis, I analyse near-infrared (NIR) observations obtained with the Spectro-Polarimetric High-contrast Exoplanet REsearch instrument (SPHERE) to look into various stages of the evolution of planetary systems. I combine the high-contrast imaging technique with observations in the millimetre continuum, hydrodynamical simulations, and radiative transfer models, as well as atmospheric retrievals and self-consistent models to analyse and interpret the different systems. Starting with protoplanetary disks as the birthplaces of planets, I study the morphology of the disk around WaOph 6 at different wavelengths (NIR and millimetre continuum) and find the presence of spiral arms in scattered light for the first time in such a young disk. Additionally, I test the hypothesis of a planet driving the architecture of the disk through hydrodynamical simulations and radiative transfer. Moving on to more evolved systems, I first demonstrate the use of the high-contrast imaging technique to characterise companion candidates and to determine their membership to the system. Furthermore, I analyse spectro-photometric data of the exoplanet 51 Eridani b and apply an atmospheric retrieval to estimate the physical parameters of the planet, revisiting previously reported values and finding a cloud-free atmosphere. Finally, I analyse a sample of debris disks with a double belt architecture inferred via SED modelling. I present mass and location estimates of planets that may be orbiting in the gaps between the belts, as well as detection limits from the observations and plans for future research. This thesis illustrates the current challenges in our understanding of planet formation and evolution and provides possible paths to overcome them.
Supervisor: Wolfgang Brandner (MPIA)
Theodoros Soultanis (Greece) 06.12.2022
Spectral properties of gravitational waves from neutron star merger remnants ( thesis pdf, 15MB)
We present a new analytic model describing gravitational wave emission in the post-merger phase of binary neutron star mergers. The model is determined by a number of physical parameters that are related to various oscillation modes, combination tones or non-linear features. The time evolution of the main post-merger frequency is incorporated. The model achieves high fitting factors for a sequence of equal-mass simulations of varying mass. All parameters of the model correlate with the total binary mass. For high binary masses, we identify new spectral features originating from the non-linear coupling between the quasi-radial oscillation and an antipodal tidal deformation of the remnant, the inclusion of which enhances the corresponding fitting factors. We find that subdominant frequency components are crucial for the construction of faithful gravitational wave templates. Because of the high fitting factors our model is particularly suitable for searches with upgraded detectors of the current generation (aLIGO+, aVirgo+) or future detectors. We find a quasi-universal relation for the proximity of a binary configuration to black-hole formation. We then develop a method for determining the threshold mass for prompt black-hole formation and the maximum mass of non-rotating neutron stars. This procedure relies on one precise measurement of the inspiral and post-merger phase.
Supervisor: Andreas Bauswein (HITS / GSI-DA)
Raul Dominguez Estaban (Chile) 30.11.2022
Embedded star clusters surviving gas expulsion with low star formation efficiencies ( thesis pdf, 12 MB )
Star clusters which are about to or have finished the process of star formation are commonly found still immersed in their natal gas clouds. This coexistence is known as embedded star clusters. During this state, the stars are continuously injecting energy to the surrounding gas due to feedback processes such as ultraviolet radiation and massive stellar winds from OB stars, or supernovae explosions which will eventually remove the natal gas. The bare star clusters expand, which in most cases, can lead to their complete dissolution. Observations and simulations agree about the strong effect that gas gravitational potential removal produces on the dynamics of the stars. However, recent observations of massive young bound star clusters challenge the idea that most embedded star clusters do not overcome the gas expulsion phase. In this thesis, we study the outcome of the interaction between stars and gas for different star cluster masses. By means of N-body simulations we explore several combinations of initial conditions. We let the simulated star clusters evolve until the moment when all the gas has been pushed away. We characterise the star remnants either by quantifying them by the bound fraction or by comparing their radial expansion with destroyed star clusters.his dissertation focuses on classical Cepheids and RR Lyrae stars as tracers of the recent star formation and the early assembly of the Milky Way, respectively.
Supervisor: Ralf Klessen (ITA)
Lizxandra Flores Rivera (USA) 30.11.2022
Connecting simulations of protoplanetary disks with forbidden emission lines observations ( thesis pdf, 25 MB )
The big picture of the planet formation scenario is sought by understanding the interplay between the gas and dust dynamics in protoplanetary disks. In particular, the gas dynamics in accretion disks have been carefully studied in a purely hydrodynamical set up where the vertical shear instability (VSI) play a major role in the transport of the angular momentum and turbulence throughout the disk. Though, it is very difficult to directly measure the angular momentum transport, mechanisms that affect it are the launch of magneto centrifugal winds (MHD winds), outflows, and the launch of thermal photoevaporative winds. One way to pin point these processes is by identifying forbidden emissions lines in the disk spectrum and analyze their velocity components that are originating in highly ionized gas. The exact thermochemical conditions in active accreting disks that cause the emission of different forbidden emission lines is very challenging to determine. To understand such outflows, the wind launching mechanisms and their effects to the angular momentum transport require the knowledge of a full global picture of hydro- and thermal- dynamical configurations.
The work presented here covers a broad range of scenarios that are important for the evolution and dispersal of the protoplanetary disk. Our focus relies more on the dynamics and the thermo-chemistry conditions at the surface of the disk. We found that the VSI is active until the location of the wind base. The characteristic vertical velocity in this region is about a fraction of the sound speed. For the first time, we report small scale vortices appearing in VSI active disks which influence especially the dust evolution. From spectral and imaging observations obtained from the Multi-unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope (VLT), high velocity components (HVCs; $>$100 km~s$^{-1}$) are detected at the inner parts of the disks as outflows/jets that are aligned within 1 degree with respect to the outer disk. With our current and upcoming global hydrodynamical models including thermochemistry and observations, we will reveal the detailed chemical and physical conditions of protoplanetary disks with active winds at the surface.
Supervisor: Mario Flock (MPIA)
Gustavo Medina Toledo (Chile) 03.11.2022
Unveiling the Milky Way’s History Using Young & Old Population Variable Stars ( thesis pdf, 40 MB )
This dissertation focuses on classical Cepheids and RR Lyrae stars as tracers of the recent star formation and the early assembly of the Milky Way, respectively.
I use data from the Gaia spacecraft to search for classical Cepheids in Galactic open clusters. I confirm (reject) several Cepheid-cluster associations considered in previous studies as bona-fide and identify new potential cluster Cepheid candidates. I also study the feasibility of using young cluster ages as tests of the Cepheid period-age relation, and conclude that their usage still faces difficulties due to their sparsely populated red giant branches, their stochastically sampled main-sequence turn-offs, and their quick dissolution.
I combine publicly available and proprietary data to study outer halo RR Lyrae stars, in the context of the Halo Outskirts with Variable Stars (HOWVAST) survey. Using Dark Energy Camera data, I search for distant halo RR Lyrae stars and detect ∼ 500 candidates, ∼ 25% of which are not reported in the literature, and 11 of which are new discoveries beyond 100 kpc. I use their pulsation properties and radial distribution to investigate the role that the accretion of satellites play in the formation and shape of the halo. Finally, I derive atmospheric parameters, chemical abundances, and kinematics for a sample of distant halo RR Lyrae stars using Magellan Inamori Kyocera Echelle spectra. I speculate about their origin, looking for hints of their association with satellites and stellar streams, and conclude that the accretion of sub-haloes contributes significantly to building up the outer halo.
The results of this thesis confirm the pivotal role of variable stars as tools to unveil the Milky Way’s evolution.
Supervisor: Eva Grebel (ARI)
Jacob Isbell (USA) 09.11.2022
The Dusty Hearts of AGN: Understanding the Central Parsec through Imaging ( thesis pdf, 25 MB )
Active galactic nuclei (AGN) play a crucial role in the formation and evolution of their host galaxies. Understanding the dust in the vicinity of supermassive black holes is key to understanding how AGN are fed and how they interact with their hosts. This circumnuclear dust traces both radiation-driven outflows and dense molecular gas which feeds the AGN.Circumnuclear dust structures are also thought to be responsible for determining AGN types and are a key feature of AGN unification. In this thesis, I use mid-infrared observations of circumnuclear dust to reveal its morphology and temperature. I use the new mid-infrared interferometer, MATISSE, to observe NGC 1068 and the Circinus Galaxy, two of the nearest AGN. Using these observations, I have produced the very first mid-infrared images of the circumnuclear dust. Analysis of these images in the L-, M-, and N-bands reveals the temperature and structure of the circumnuclear dust at sub-parsec resolution. In both galaxies, the images show a disk-like substructure which obscures emission from the accretion disk. Perpendicular to the disk -- in the polar direction -- warm dust extends for several parsecs. The temperature of this polar dust remains high (> 200 K) at large distances from the accretion disk, indicating significant clumpiness of the dust. Additionally, substructures revealed in the images provide direct evidence for clumpy or filamentary polar dust. To complement the high-resolution study of these two AGN, I compiled a low-resolution catalog of 119 AGN. The reported L- and M-band fluxes indicate 44 targets for further high-resolution study with MATISSE. The L-M and M-N colors are compared to various radiative transfer models of the circumnuclear dust and show that models with clumpy polar dust provide the best match. Together the high- and low-resolution studies indicate a disk+wind model of the circumnuclear dust: the disk is a geometrically-thin obscuring structure, which plays a role in the Seyfert 1/2 dichotomy and is associated with the inflowing material that feeds the AGN; the wind (shown via the polar dust) is clumpy, warm, and associated with massive outflows from the AGN to the host galaxy. The combination of low-resolution mid-infrared fluxes/colors with the first sub-parsec images of the circumnuclear dust places strong constraints on both future modeling and AGN unification.
Supervisor: Klaus Meisenheimer (MPIA)
Li-Hsin Chen (Taiwan) 09.11.2022
Stars and star formation in the Milky Way system ( thesis pdf, 6 MB )
Dwarf galaxies in the Milky Way (MW) system provide us with important information of the star formation in the early Universe. In particular, extremely metal-poor stars in them are likely to form in the environment that is directly influenced by the first stars. In this thesis, I utilise the semi-analytic code a-sloth to study the properties of these dwarf galaxies. A new star formation model that allows tracking of individual stars is implemented. The star formation efficiency is important in determining the stellar mass of ultra-faint galaxies at z = 0. To better quantify the goodness-of-fit of the physical models, I develop a new analysis method, which utilises the unsupervised clustering. With this method, the results suggest that the fiducial model cannot reproduce the mean metallicity of the dwarf galaxies. A temporary improvement of the model is implemented and the goodness-of-fit improves by a factor of 2, compared to the fiducial model. I further analyse the location of EMP stars of MW/M31-like systems in the hydrodynamical simulation TNG50. The analysis is based on the spatial and kinematic information of stars. I find that EMP stars reside mostly in the Stellar halo in the galaxy, regardless of the definition of EMP stars. The model and analysis methods introduced in this thesis provide a new framework for future works to understand dwarf galaxy formation and stars that reside in them.
Supervisor: Ralf Klessen (ITA)
Mischa Breuhaus (Germany) 28.07.2022
Towards an Understanding of Galactic Ultra-high Energy γ-ray Emission ( thesis pdf, 15 MB )
Just recently, we are starting to explore the γ-ray sky at energies above 100 TeV. A detailed understanding of the emission at these extreme energies is therefore of paramount importance. γ rays at these energies are produced by highly energetic particles, the so-called cosmic rays. This thesis investigates three different aspects related to the ultra-high energy emission.
The first aspect concerns the leptonic or hadronic origin of sources at these energies. While the suppression of inverse Compton emission at these energies disfavours a leptonic origin, it is shown that in environments with sufficiently high radiation energy densities or low magnetic fields, leptonic ultra-high energy emitters are possible. Furthermore, the viability of such leptonic emitters is confirmed by modelling newly detected sources in this energy regime.
The second aspect concerns hadronic emission, specifically the effects of different compositions of the hadronic particles producing γ rays at these energies. These effects are thoroughly investigated in this thesis. It is shown that the presence of heavier cosmic ray species decreases the resulting emission and shifts spectral features to lower energies. The influence of different compositions on the diffuse Galactic emission at ultra-high energies is investigated. For this use case, the composition can have an important influence on the resulting γ-ray and neutrino production. The models are compared to current data. Although current measurements do not allow to constrain the composition of the Galactic cosmic rays, future observations will be able to do so.
For the third aspect, the particle acceleration and γ-ray emission by colliding stellar winds is investigated. This is done in the case of the colliding wind binary η Carinae. The developed time-dependent model is able to explain the resulting flux and variability properties of the emission detected from η Carinae. The γ-ray emission is likely produced by the collisions of accelerated hadronic particles and not by electrons. The variability in the X-ray emission is explained by the inhibition of electron injection and heating during the closest approach of the two stars.
Supervisor: Jim Hinton (MPIK)
Giancarlo Mattia (Italy) 28.07.2022
Toward a consistent turbulence model for the origin of jet-launching magnetic fields: theoretical and numerical improvements ( thesis pdf, 20 MB )
Astrophysical jets, consisting of collimated high-speed outflows, are typically found in several astrophysical objects, e.g., young stellar objects, X-ray binaries, gamma-ray bursts, or active galactic nuclei. The formation of collimated outflows requires some common features, such as the presence of a central object, an accretion disk and a large scale magnetic field (whose origin is still unclear). Regarding the numerical aspects, we compared several solutions of the Riemann problem for ideal relativistic plasma in terms of accuracy and robustness against one – and multidimensional standard numerical benchmarks. We then performed non-ideal Magnetohydrodynamic simulations by employing the PLUTO code in order to investigate how the mean-field dynamo and the magnetic diffusivity affect the disk and jet properties. At first we have investigated a non-isotropic dynamo toy model in order to disentangle the effect of the different dynamo components on the launching process and on the disk magnetic field. Then, we investigated a disk dynamo that follows analytical solutions of the mean-field dynamo theory, essentially based mainly on the Coriolis number. We thereby confirmed the anisotropy of the dynamo tensor acting in accretion disks, allowing both the resistivity and the mean-field dynamo to be related to the disk turbulence. Subsequently, we studied the feedback of the generated magnetic field on the mean-field dynamo. We found that a stronger quenching of the dynamo leads to a saturation of the magnetic field at a lower disk magnetization. Nevertheless, we found that, when applying only a dynamo quenching, the overall jet properties do not depend on the feedback model. Finally, we present a feedback model which encompasses a quenching of the magnetic diffusivity. We find that after the magnetic field is saturated the Blandford-Payne mechanism takes place yielding to more collimated yet slower jets. We find strong intermittent periods of flaring and knot ejection for low Coriolis numbers.
Supervisor: Christian Fendt (MPIA)
Irina Smirnova-Pinchukova (Russia) 27.07.2022
Comparison of Different Star Formation Tracers in Nearby AGN Host Galaxies ( thesis pdf, 50 MB )
This work is a part of the Close AGN Reference Survey (CARS, cars-survey.org) that is intended to provide the most detailed view of the AGN - host galaxy connection and establish a reference for high-redshifts. The survey consists of 40 nearby Seyfert 1 galaxies with optical integral field unit datacubes and a multi-wavelength observational dataset. The first part of the thesis presents the star-formation-related analysis of the CARS first data release. The panchromatic spectral energy distribution is modeled with additional AGN constraints to infer stellar masses and infrared star formation rates. I also present the novel method of calculating the recent star formation rate from the Hα emission in the conditions of AGN contamination using spectral and spatial datacube information. Then I compare the resulting infrared and Hα star formation rates with CO(1-0) data to analyze the star formation history and the trends between AGN and host galaxies. The second part of the thesis explores the data from SOFIA airborne observatory. The [CII] far-infrared line is a bright coolant in the interstellar medium accessible with ALMA and often used as a star formation rate indicator. The discovery of one of the few known [CII] excess galaxies HE1353-1917 is introduced together with the discussion of its mechanisms and the meaning for the high-redshift objects. This is complemented by the follow-up SOFIA observations of HE0412-0803 that allowed to confirm the proposed [CII] excess mechanism and intrigued us with its puzzling VLA radio continuum data. Overall, in this thesis, I accurately infer, analyze, and compare star formation rate tracers for the CARS objects emphasizing the power of the multi-wavelength approach.
Supervisor: Bernd Husemann (MPIA)
Marcelo Barraza-Alfaro (Chile) 27.07.2022
Gas dynamics and kinematical signatures of turbulent planet-forming disks ( thesis pdf, 90 MB )
Gas dynamics and turbulence in protoplanetary disks play a crucial role in disk evolution and planet formation, not only by dictating the motion of solid particles through the planet-forming disk but also by allowing the transport of angular momentum and so the mass accretion onto the star. Significant are the effects of turbulence on dust settling, dust concentration and growth/fragmentation, fundamental ingredients for the dust to grow into pebbles and planetesimals, building blocks of planets. Therefore, exploring the gas dynamics and observational signatures of turbulence is essential to further understand its origin and impact in planet formation. In this thesis, I present a study on the gas dynamics and kinematical signatures of disks unstable to the vertical shear instability (VSI), a robust candidate to generate turbulence in the outer regions of protoplanetary disks. Via high-resolution 3D hydrodynamical simulations post-processed with radiative transfer predictions and synthetic observations, I show that the VSI produces observational signatures in CO kinematics, observable within ALMA’s capabilities. However, I also show that the interplay between the VSI and forming massive planets can substantially affect their kinematic signatures, by partially suppressing the VSI and sculpting a complex velocity structure. These predictions will help to interpret upcoming ALMA kinematical observations, potentially revealing a process behind gas turbulence in planet-forming disks.
Supervisor: Mario Flock (MPIA)
Ismael Pessa Gutierrez (Chile) 25.7.2022
Study of star formation and resolved stellar populations in nearby galaxies ( thesis pdf, 35 MB )
Observations have revealed a bimodality in galaxy properties such as color and morphology leading to a fundamental classification into passive red galaxies and star-forming blue galaxies, where the latter follow a tight correlation between their stellar mass and their star formation rate (SFR), known as star formation main sequence (SFMS). In this thesis, I use a sample of star-forming galaxies from the multi-wavelength Physics at High Angular resolution in Nearby GalaxieS (PHANGS) survey to study the physics that regulate the formation of stars and keep these galaxies on the SFMS, as well as to investigate how they have assembled their stellar mass, and thus, unveil their evolution through cosmic times. The PHANGS survey allows us to perform these analyses, for the first time, at a spatial resolution of ∼ 100 pc, thus, resolving individual star-forming regions and galactic morphological features. I find that correlations between stellar mass surface density (Σ∗), molecular gas sur face density (Σmol), and SFR surface density (ΣSFR) hold at these spatial scales, albeit with an increased scatter compared to lower-resolution measurements. The correlation between Σmol and ΣSFR is the most homogeneous across different galaxies and galactic environments indicating that the amount of molecular gas is regulating the formation of stars. The interplay between Σ∗, Σmol , and ΣSFR reveals significant variations across individual galactic environments implying that an additional mechanism(s) not captured by either Σ∗ or Σmol is playing a role in setting the level of SFR. Analysis of the age and metallicity distributions of the stars across the galaxies shows negative stellar age and metallicity gradients, consistent with an inside-out growth scenario. A clear dependency of the stellar velocity dispersion on age is present in the galaxies, where younger stellar populations at any given radius have lower velocity dispersion than older stars. Variations of the time-averaged SFR across the galactic disk reveal a diffusion of the galactic structure with lookback time – consistent with the progressive dynamical heating of young stellar populations through interactions with molecular gas and/or non-axisymmetric galactic features. The results presented in this thesis show how local processes shape the evolution of galaxies, driving the formation of stars, and modulating the local star formation histories across the galactic disk of nearby galaxies.
Supervisor: Eva Schinnerer (MPIA)
Alina Boecker (Germany) 19.7.2022
Tracing the Central Stellar Mass Assembly of Galaxies in a Cosmological Context: Insights from Observations and Simulations ( thesis pdf, 40 MB )
Galaxies grow in stellar mass either by turning their gas into stars, or by merging with other galaxies. Disentangling the relative contribution of these processes is essential to understand both the emergence of today’s diverse galaxy population and the cosmology in which galaxies form and evolve. In principle, the distribution of ages and metallicities of a galaxy’s stellar populations is informative about its assembly history. However, this rich information content is not yet systematically obtained for the majority of galaxies, as they are not resolved on a star-by-star basis. In this thesis, I build on a methodology that can extract age-metallicity distributions from observed integrated spectra alone. I verified that such a technique is robust by using observations of a nearby stellar system, where additionally the ages and metallicities of individually resolved stars are known. Using state-of-the-art cosmological, hydrodynamical simulations, I showed that interactions with other galaxies dictate the assembly of stars in the central few hundred parsecs of galaxies, which are thus ideal regions to study merger histories. The joint analysis of integrated spectra of galaxy centers from real and mock observations confirms the recovery of signatures in ages and metallicities reminiscent of past merger events. These promising results pave the way in measuring galaxy merger statistics from current and future spectroscopic data to understand their role in shaping galaxy properties over cosmic time.
Supervisor: Nadine Neumayer (MPIA)
Camille Bergez-Casalou (France) 13.7.2022
Formation of multiple giant planets and their impact on the protoplanetary disc structure ( thesis pdf, 30 MB )
The Solar System, together with some exoplanetary systems, is known to host multiple planets. These planets are believed to be born in discs composed of gas and dust in rotation around forming stars. These discs are called protoplanetary discs. The formation of such planets is studied via different, complementary, methods: theoretically, with the help of both hydrodynamical simulations to analyse the behavior of the fluids in presence of planets and N-body simulations to predict the dynamical interactions between these planets; and observationally with the recent detailed observations of protoplanetary discs.
In this thesis, I use a hydrodynamical approach to investigate the impact of multiple giant planets on the global disc structure. Previous hydrodynamical simulations focus on the local growth of single planets. Here, my collaborators and I start by analysing the impact that a single gas accreting planet has on its surrounding disc at a global scale. I show that the influence of planetary gas accretion on the gas disc structure depends on its viscosity. With that in mind, it is possible, in a second study, to determine how the gas is distributed in a disc hosting two accreting planets. By running long-term hydrodynamical simulations (up to 0.5 Myrs), we find that even if the planets do not start accreting simultaneously, they end up with similar masses. This has an interesting impact on our understanding of planet formation.
Finally, an observational approach is investigated by deriving synthetic ALMA images of the potential parental Solar system protoplanetary disc. I use two different simulation codes to determine the behavior of the gas and the dust in presence of multiple giant plan ets, before relying on a radiative transfer simulation that predicts how light is emitted from the disc. The resulting image is treated to mimic recent protoplanetary disc observations, allowing us to compare the observable features to known discs. With this project, we provide a way to put the Solar system in perspective with the known observed disc. This comparison allows us to better constrain the formation pathway of giant planets.
To summarize, this thesis uses a theoretical approach to investigate the formation of multiple giant planets in their protoplanetary disc. I finish by discussing several questions addressed in this work. The answers provided can be used as foundations to follow-up studies, improving our understanding of planet formation both theoretically and observationally.
Supervisor: Bertram Bitsch (MPIA)
Caroline Gieser (Germany) 13.7.2022
Physical and chemical properties during high-mass star formation ( thesis pdf, 25 MB )
This thesis is dedicated to the characterization of the physical and chemical properties in high-mass star-forming regions. I use interferometric observations at 1 and 3 mm wavelengths with the NOrthern Extended Millimeter Array (NOEMA) and Atacama Large Millimeter/submillimeter Array (ALMA) of a sample of high-mass star-forming regions at different evolutionary stages ranging from infrared dark clouds, high-mass protostellar objects, hot molecular cores, to ultra-compact HII regions. At angular resolutions <1 arcsec, the physical and chemical properties of individual fragmented cores can be studied on scales <0.1 pc using both continuum and molecular line emission. Molecule properties, for example, the column density and rotation temperature, are derived using the eXtended CASA Line Analysis Software Suite (XCLASS) of species such as SO, OCS, SiO, H2 CO, CH3CN, and CH3OH. I determine for a statistical sample of cores radial temperature and density profiles (T ∼ r^-q and n ∼ r^-p , respectively), masses M, and molecular column densities N. Chemical timescales τchem are estimated using the physical-chemical model MUlti Stage CLoud codE (MUSCLE). There is a high degree of fragmentation in the regions and the spatial morphology of the continuum emission is diverse, where in some regions there is a single isolated core, while in other regions, for example, filamentary structures that have many embedded cores are found. The molecular content of individual cores have local chemical variations and with MUSCLE this chemical differentiation can be explained by the cores being at slightly different evolutionary stages. By combining the results of the in total 31 high-mass star-forming regions that were observed with either NOEMA or ALMA at high angular resolution and that were analyzed within this thesis, evolutionary trends of the physical core properties are found. The temperature profile q steepens from q ≈ 0.1 to q ≈ 0.7 and the density profile p1 on clump scales (0.1 - 1 pc) flattens from p1 ≈ 2.2 to p1 ≈ 1.2 with time as the cores evolve. No evolutionary trend is found for the density profile p2 on core scales (<0.1 pc), with p2 ≈ 2, indicating that all of the analyzed cores are collapsing to form (high-mass) stars. These results provide invaluable observational constraints to test theoretical formation models of high-mass stars.
Supervisor: Henrik Beuther (MPIA)
Irham taufik Andika (Indonesia) 25.5.2022
The Hunt for Quasars at Cosmic Dawn ( thesis pdf, 40 MB )
Quasars in the early universe are excellent laboratories for analyzing the assembly of the first supermassive black holes and galaxies. In this work, we present an endeavor to discover z ≳ 6 quasars and lenses, expanding the selection space missed by many previous quasar surveys. Our strategy consists of three main steps: (i) candidates pre-selection based on their colors using catalog-level photometry, (ii) modeling the observed spectral energy distributions and calculating their probabilities to be a quasar plus lens galaxy or some contaminant, and (iii) image classification based on convolutional neural network analysis to detect potential lens configurations. Utilizing the combined optical multi-band images and infrared data, we validate
our selection pipeline and compile new high-probability (lensed) quasar candidates.
We then present the discovery of a remarkable weak-line quasar at z = 6.3401, i.e., PSO J083+11. Using the spectrum taken with Gemini/GNIRS, we fit the continuum and Mg II line emissions, resulting in a black hole mass of log(MBH ) = 9.30+0.16 −0.10 M⊙ and an Eddington ratio of Lbol /LEdd = 0.51+0.13−0.17 . The broad-line emissions of this source are inherently weak, where rest-frame equivalent widths Lyα + N V = 5.65+0.72−0.66 Å and C IV ≤ 5.83 Å. The spectrum reveals a small proximity zone size, suggesting a present quasar age of only ≈ 103 − 104.5 yr.
HST/WFC3–ACS imaging shows no strong lensing affecting the apparent flux. ALMA observations uncover a quasar host similar to hyper-luminous infrared galaxies having a star formation rate of ≈ 900–4900 M⊙ yr−1. Given its accretion lifetime and the timescale of the broad-line region formation, we propose that the PSO J083+11’s weak-line nature originates from a still-emerging broad-line region event.
In addition, Magellan/FIRE and VLT/MUSE spectroscopy reveal the presence of a z = 6.314 metal-poor sub-damped Lyα absorber toward PSO J083+11, having a neutral hydrogen column density of log NHI = 20.03±0.30 cm−2 along with an abundance ratio of [C/O] = −0.04±0.33 and a metallicity of [O/H] = −2.19±0.44. This absorber truncates PSO J083+11’s proximity zone and complicates its quasar age estimation. However, at the same time, this quasar exhibits no trace of an extended Lyα halo, with a 1σ surface brightness limit of 2.76 × 10−18 erg s−1 cm−2 arcsec−2 at 1′′ aperture, or corresponds to a Lyα luminosity of ≤ 43.46 erg s−1 . This non-detection provides alternative support for the notion of a young quasar with a short accretion lifetime. Further statistical analyses with more samples at the highest redshifts are critical for establishing the relations between typical quasars, young sources, and weak-line active galaxies. This study will help us apprehend the rapid growth physics of the earliest black holes.
Supervisor: Arjen van de Wel (Univ. Gent), Knud Jahnke (MPIA)
Mattis Magg (Germany) 25.5.2022
The Cosmological Transition to Metal-Enriched Star-Formation ( thesis pdf, 170 MB)
The first stars in the Universe mark the beginning of the epoch referred to as Cosmic Dawn. Due to their metal-free nature, they are believed to be distinctively different from present- day stars. While they are considered to be important in shaping the early Universe, little is known about their properties. This thesis addresses several indirect methods of constraining these properties, with a particular focus on the transition from metal-free to metal-enriched star formation. To this end, I employ and develop a variety of analytical estimates, semi- analytical models and a numerical simulation. By analysing the non-detection of metal-free stars, I confirm that these stars must on average have been far more massive than present- day stars. An analytical model and a careful review of numerical simulations demonstrate that the interaction between the first supernovae and their ambient medium is key to un- derstanding the most metal-poor stars known today. With a simulation of the massive metal-free stars exploding as pair-instability supernovae, I highlight the tension implied by the non-detection of their nucleosynthetic fingerprint. Finally, I use a semi-analytical model to reveal the sensitivity of future 21 cm absorption detections to the time-delay between the first supernovae and the second generation of stars. The results, tools, and methods of this thesis will find application in future theoretical studies and in the analysis of upcoming observations.
Supervisor: Ralf Klessen (ITA)
Peter Rodenkirch (Germany) 25.5.2022
Signatures of Planet-Disk Interaction and Disk Winds ( thesis pdf, 30 MB)
Protoplanetary disks, consisting of gas and dust, are thought to be the birthplace of planets. Recent observations have revealed a multitude of substructures in the dust continuum and scattered light emission. It is unclear if these gaps, rings and asymmetries are signposts of embedded planets interacting with their surroundings, or if different physical mechanisms, such as hydrodynamic instabilities, interactions with magnetic fields and disk winds could be the origin. Observations indicate that the lifetime of protoplanetary disks is limited to several million years and it is debated whether accretion due to turbulent processes, planet-disk interaction or disk winds might be the driving factor of disk dispersal. In this work a suite of numerical (magneto-)hydrodynamical simulations of planet-disk interaction as well as photoevaporation and magnetically driven disk winds was carried out. We found that asymmetries observed in the dust continuum can indicate the presence of an embedded planet, containing several earth masses of dust. Furthermore, we found that dust can efficiently be entrained in both thermal and magnetic disk winds, being potentially observable and assisting the identification of the physics driving the wind. Extending the wind models to transition disks revealed accretion rates compatible with recent observations with a similar dependence on the disk mass.
Supervisor: Cornelis Dullemnond (ITA)
Katja Stock (Germany) 20.4.2022
N-Body Simulations of Multiplanetary Systems in Star Clusters: The Effect of External Perturbations on the Dynamical Evolution of Planets ( thesis pdf )
Stars predominantly form in groups or clusters, which, however, only dissolve completely after hundreds of millions of years. Within such stellar overdensities, stars interact gravitationally with each other. On timescales of the order of millions of years, close flybys of neighbouring stars occur, which can significantly alter the orbital architecture of the planetary systems that have formed around the stars in the cluster. In this thesis, I simulate a total of four star cluster environments of different densities and subsequently numerically integrate different planetary system architectures over 100 million years, taking into account the gravitational forces that would have acted on them due to the motion of their central stars through the cluster. The results show that the gravitational perturbations from the birth environment can explain the large diversity in the orbital parameters of the observed exoplanet population. In particular, the simulation results show that about 1–2% of all planets adopt stable retrograde orbits due to external stellar perturbations or resulting interactions with other planets in the system. Furthermore, by taking into account tidal interactions between the host star and the planets, the formation of hot Jupiters can be observed in some systems as an indirect consequence of stellar encounters. Moreover, all simulated star cluster environments produce a significant percentage of unbound planets.
Supervisor: Rainer Spurzem (ARI)
Jonas Kemmer (Germany) 16.2.2022
Earth-sized planets orbiting M-dwarf stars: Detection and mass measurement with CARMENES and TESS ( thesis pdf )
The technical progress of the last decades has enabled us to study ever smaller planets with increasing precision. The characterisation of Earth-sized planets is particularly exciting, since they can provide important insights into the formation and development of our own Earth. M-dwarf stars are especially suited to search for Earth-sized planets with the radial velocity (RV) and transit method, because the planetary signals are more pronounced for these stars due to their larger planet-to-star mass and radius ratios. In my dissertation, I deal with the CARMENES spectrograph, which was constructed exactly for this purpose. The first part is an error budget for the visual channel of the spectrograph, in which I investigate what comprises the uncertainty of the measured RVs and what the technical limits of the instrument are. In the second part of the thesis, I present the discovery of two Earth-sized planets using CARMENES observations. The two short-period, Earth-sized planets GJ 3473 b and GJ 3929 b orbit M-dwarf stars and were initially identified as transiting planet candidates by the TESS survey satellite. Our follow-up observations using the RV method confirmed their planetary nature and revealed in the case of GJ 3473 another non-transiting, temperate planet with sub-Neptune mass, GJ 3473 c. Joint modelling of the transit and RV data made it possible to determine precise masses and radii for the two transiting planets. The densities determined from this are most consistent with a MgSiO3 composition for both planets. Due to their short orbital periods and therefore high equilibrium temperatures, both are particularly well suited for future atmospheric characterisation using transit spectroscopy.
Supervisor: Andreas Quirrenbach (LSW)
Paz Bluhm (Chile) 15.2.2022
Detection and characterization of exoplanets around M dwarfs in the presence of stellar activity ( thesis pdf )
M dwarfs are the most numerous type of stars in the Universe. Their low masses and their large planet-to-star size ratios offer big advantages to study orbiting exoplanets around these stars. On one hand, the radial velocities signals have greater amplitudes and are easier to identify than in FGK stars. On the other, the small size of M dwarfs leads to transiting planets leaving a stronger imprint in the light curve. Additionally, their low temperatures places their habitable zone close to the host star, which makes them excellent targets to search for temperate or habitable worlds. Despite the advantages that M dwarfs offer, these do not come for free. Typically they are active entities, and most of the M-dwarfs have strong magnetic fields, which can heat their stellar chromospheres, creating magnetic activity which drives the occurrence of stellar spots in their photosphere. Consequently, stellar activity can mimic the signal of an orbiting planet which is one of the most problematic sources of noise. This thesis is focused on the discovery and characterization of exoplanets around M dwarfs, where I highlight the importance of careful modeling of the intrinsic stellar noise present in the data for to avoid false planet claims.
Supervisor: Andreas Quirrenbach (LSW)
