Victor Ksoll (Chile) 07.07.2021
Characterising Pre-Main-Sequence Stars in the Large Magellanic Cloud with Machine and Deep Learning Techniques ( thesis pdf, 70 MB )
The Large Magellanic Cloud (LMC) exhibits an extraordinary star-forming activity, providing excellent targets for star formation research. Photometric observations with the Hubble Space Telescope (HST) allow for deep, high-resolution studies of young stellar clusters and still-forming pre-main-sequence (PMS) stars in the LMC. In this thesis we study two LMC star-forming complexes, the Tarantula Nebula and N44. Using HST photometry of the Tarantula Nebula from the "Hubble Tarantula Treasury Project" (HTTP), we devise a machine-learning (ML) classification procedure to identify PMS stars from photometry and recover the PMS population captured by the HTTP survey. We introduce new HST observations of N44, the "Measuring Young Stars in Space and Time" (MYSST) survey, identify N44’s PMS content with our ML classification procedure, and conduct a clustering analysis of the identified PMS stars. Additionally, we develop a conditional invertible neural network approach to predict stellar physical parameters from photometric observations, based on the PARSEC stellar evolution models. We perform a test on HST observations of the Milky Way clusters Westerlund 2 and NGC 6397, and successfully confirm previous findings on e.g. the age of Westerlund 2. For NGC 6397, however, we identify discrepancies between the PARSEC stellar evolution models and HST observations that prevent accurate predictions.
Supervisor: Ralf Klessen (ITA)
Francisco Aros Pinochet (Chile) 06.07.2021
Towards a robust detection of intermediate-mass black holes in globular clusters ( thesis pdf, 20 MB )
Intermediate-mass black holes (IMBHs) with masses between $10^2$ to $10^5$ solar masses could be the key to explaining the formation of supermassive black holes in the centre of galaxies at high-redshift. The possible formation scenarios of IMBHs point towards dense stellar systems as favoured birthplaces. If an IMBH resides at the centre of such dense stellar systems, the surrounding stars under the IMBH’s dynamical influence will show characteristic kinematic signatures. During the last two decades, the community has made significant efforts to find IMBHs through these kinematic signatures, particularly in the centre of the Milky Way’s globular clusters (GCs). To date, however, no robust detection has been made.
In this thesis, we explore the internal dynamics of GCs to understand the current limitations for detecting IMBHs and propose alternative lines of evidence of their presence. With this purpose, we use numerical simulations of GCs with and without a central IMBH and apply commonly used dynamical models to estimate the mass of the possible central IMBH. We focus on the limitations of the dynamical modelling itself, particularly those due to common assumptions such as a constant velocity anisotropy and mass-to-light ratio. From an observational viewpoint, we explore the role of binaries in the observed kinematics and the connection between the observed binary fraction and the presence of an IMBH. Finally, we study the implications of the dynamical modelling limitations and the effects of binary systems on the scaling relations, which connect the mass of the central IMBHs with their host stellar system properties.
Supervisor: Glenn van de Ven (MPIA / U. of Vienna)
Felix Bosco (Germany) 01.07.2021
Probing the Growth of Black Holes at the Limit of Large Telescopes ( thesis pdf, 6 MB )
Direct kinematical measurements of black hole (BH) masses require to resolve the sphere of influence at sub-milliarcsecond scales. Here, two novel observational approaches are studied that exhaust the resolution limit of modern large telescopes in order to allow for tighter constraints on masses and growth mechanisms of BHs. The first approach applies holographic speckle imaging techniques to near-infrared images after partial atmospheric turbulence correction. Based on simulations and observational data, we show that the combination of techniques recovers the diffraction limit of 8 m-class telescopes for stars too faint for classical speckle imaging. This approach will allow for tracing BHs directly via stellar kinematics. The second technique uses the spectroastrometric signal of the quasar broad emission line region in order to constrain its geometric and kinematic structure. We extract this signal from adaptive-optics-assisted near-infrared spectroscopy with an 8 m telescope and carefully study the uncertainties. The comparison of the data to our model allows us to report on the first tentative detection of the spectroastrometric signal of a luminous quasar and thereby for constraining its BH mass, the first direct measurement beyond a redshift of z = 2 and out of the detection range of reverberation mapping or optical interferometry.
Supervisor: Joerg-Uwe Pott (MPIA)
Sabrina Gronow (Germany) 15.06.2021
Contribution of Type Ia supernovae to the chemical enrichment of the Milky Way: explosions of sub-Chandrasekhar mass white dwarfs ( thesis pdf, 10 MB )
Type Ia supernovae (SNe Ia) are important for galactic chemical evolution (GCE) because they produce heavy elements. Sub-Chandrasekhar mass carbon-oxygen white dwarfs with helium shells are favored progenitors for SNe Ia. This thesis investigates the double detonation explosion scenario. A focus lies on an accurate calculation of the detonation propagation in the white dwarf shell and the assumption of core-shell mixing. Parameter studies were conducted to analyse whether variations found in observables of SNe Ia can be reproduced and to provide (metallicity-dependent) nucleosynthetic yields for subsequent radiative transfer calculations and GCE models. Three-dimensional simulations were carried out using the Arepo code. A previously neglected carbon detonation ignition mechanism was found showing that the helium detonation wave convergence is sufficient to ignite carbon in a core-shell transition region. The study shows that various luminosities coinciding with SNe Ia can be reproduced. Metallicity-dependent yields illustrate that a high stellar metallicity shifts the production to stable isotopes while supporting the manganese production. GCE models suggest that the inclusion of this explosion type allows to account for about 80% of the solar manganese abundance. The correlation of [Mn/Fe] with metallicity in the solar neighborhood is supported by the inclusion of metallicity-dependent SNe Ia yields.
Supervisor: Friedrich Roepke (HITS)
Stephan Stock (Germany) 28.04.2021
Analysis of High-Precision Spectroscopic Data in Search of Exoplanet Signals Close to the Stellar Noise Limit ( thesis pdf, 70 MB )
Due to their low mass and low temperature, M-dwarf stars are promising targets for the detection of planets in the host star’s temperate zone with the radial velocity (RV) method. However, M dwarfs typically exhibit stellar activity, producing noise in the RV data which can mask the signals from existing planets or, as well as aliasing caused by data sampling, can lead to false candidates or to wrongly determined planetary parameters. I present RV analyses for four M dwarfs with the aim of detecting and characterizing potential planetary signals. To overcome the aliasing problems, I developed the public tool AliasFinder, which is an improvement of an already known method for distinguishing aliases from actual signals. For the disentanglement of stellar activity and planetary signals, and for their modeling, I used current state-of-the-art analysis techniques, e.g., Bayesian statistics and Gaussian process (GP) regression. The properties and the performance of GP models have been investigated and tested on simulated RV data of active M dwarfs. These GP models, along with additional data taken by CARMENES, have resulted in improved planetary parameters for four previously known planets (the multiplanetary system YZ Ceti and the single-planet system of Lalande 21185) and two new planet discoveries (the temperate super-Earth GJ 251 b and the warm super-Earth HD 238090 b). The analyses, results,and partially unique approaches in this thesis demonstrate that it is possible to detect and model planetary RV signals at the limit of the stellar astrophysical noise by applying rigorous statistical models, along with prior information from auxiliary data and further simulations.
Supervisor: Sabine Reffert (MPIA)
Victor Marian (Austria) 28.04.2021
The Intricate Connection Between Major Mergers and AGN with the Highest Eddington Ratios ( thesis pdf, 25 MB )
Over the past decade, research has revealed contradictory results on whether mergers of galaxies of similar mass, so-called major mergers, are the dominant or at least an essential mechanism for the formation of active galactic nuclei (AGN) and the growth of supermassive black holes (SMBH) in galaxies. In this work, it is investigated whether such a connection exists for the ‘most plausible’ objects – broad line AGN with the highest Eddington ratios. The first sample is based on HST/WFC3 observations and focuses on these objects that possess an Eddington Ratio >0.7 at z=2, the peak epoch of black hole activity. To validate the results and to identify possible discrepancies between observations and simulations, a comparison study is performed on similar objects modeled by the large-scale cosmological simulation ILLUSTRIS-TNG. An additional sample consists of local z=0.2 AGN with an Eddington ratio >0.3, observed with the VLT/FORS. All AGN samples are compared to inactive galaxies of similar stellar mass and redshift. For all three individual studies, the applied methodology is identical: sources are visually identified and classified into merging and unperturbed galaxies. An excess for the respective AGN host galaxies in the subsequently derived major merger fractions would then indicate that AGN are predominantly triggered by such galaxy mergers. The analysis of the observed galaxies at z=2 yields merger fractions of 0.24 ± 0.09 and 0.19 ± 0.04 for the AGN hosts and inactive galaxies, respectively. In addition to the visual assessment, the ILLUSTRIS-TNG data set is analyzed by evaluating the galaxies’ merger histories and accounting for typical observational uncertainties. The resulting best estimates give 0.27 ± 0.07 and 0.22 ± 0.01, indicating an excellent agreement between the two studies at this redshift. In contrast, with 0.41 ± 0.12 and 0.08 ± 0.06, the fractions at z=0.2 show a clear excess in the incidence of major mergers for the AGN host galaxies. The results are analyzed extensively, including an investigation of possible influences due to selection effects, a time lag between the visibility of galaxy mergers and the AGN, as well as the methodology used. While there is no significant evidence that such major mergers are the dominant process for the existence of the studied AGN at z=2, such gravitational encounters appear to be an important mechanism for comparable AGN at z=0.2. This may be due to the fact that the gas required for SMBH to achieve such high accretion rates is less abundant in galaxies at low redshifts, leading to major mergers playing a more dominant role in the local Universe. Nevertheless, regardless of redshift, for a minimum of 50% of this particular AGN population, the origin remains unclear.
Supervisor: Knud Jahnke (MPIA)