Topics for Bachelor, Master and PhD Theses

We employ AIP’s Wide Field STELLA Imaging Photometer (WiFSIP) on the STELLA-I telescope in Tenerife for continuous monitoring of selected Open Clusters. Possible Master topics include the membership determination/verification based on metallicities and gravities (in the absence of radial velocities), the determination of rotational periods for selected clusters, the search for L-dwarfs from combined (stacked) CCD images, or the search for transits from extra-solar planets, a.o. The overall goal is to determine a mass-rotation and a rotation-activity relation and, later on, a mass-rotation-activity-age relation that shall constrain models of angular-momentum transport and evolution in the early phases of stellar evolution.

PLATO (PLAnetary Transits and Oscillation of stars) is ESA’s M3 mission for launch in 2026. It will search for extrasolar planets by means of ultra-high-precision transit photometry and addresses fundamental questions in exoplanet science. With our new robotic telescope BMK10k in Chile (see https://bmk10k.aip.de/), we aim supporting PLATO from the ground. A key planet parameter for the confirmation and characterisation
of planets is their mass, which can be determined by means of high-resolution spectroscopic observations providing precise radial velocities (RV). But additional photometric facilities will as well be useful to discard false positive scenarios. We will start monitoring the southern deep field with BMK10k by the end of 2024 and continue well beyond satellite launch. Individual Mastertheses are envisioned for several sub-aspects of the BMK10k data ranging from false positives detection, eclipsing binary statistics, periods for rotating and pulsating stars, to transient phenomena like stellar flares a.s.o., depending upon preference.

Solar limb darkening, or center-to-limb variation (CLV), is a well-known effect that causes the limb of the Sun to appear darker than its center. This phenomenon is caused by the variation in optical depth along the line of sight. It is strongly dependent on the wavelength, the observed target, and the spectral line being observed. While the CLV of the quiet Sun is well characterized, the limb darkening of active regions such as sunspots and plage remains poorly defined. Proper modeling of the CLV is critical for a wide range of astronomical quantities, such as exoplanet atmosphere parameters, stellar abundances, and rotation rates. A recent study by an undergraduate student in this department used data from the Solar Dynamics Observatory (SDO) to calculate the limb darkening coefficients of sunspots in a specific spectral line (Fe I 6173 Å). The current project aims to extend this work to other wavelengths using additional data from both space-based and ground-based telescopes.The project will use existing Python frameworks to analyze the limb darkening of active regions at multiple wavelengths. While some of the necessary code is already developed, some data sets will require careful handling to remove systematic effects. The project is modular so that more spectral lines can be analyzed as it progresses, and has the potential to result in a publication.

Ellerman bombs (EBs) are small-scale reconnection events found in active regions of the solar photosphere. These events are generally short-lived, with lifetimes of about a minute, but can last longer depending on their environment. They are thought to contribute to the heating of the higher layers of the solar atmosphere and are used as a tracer for the evolution of the active regions from which they originate. Despite their importance, they are poorly understood. Recent work by a master's student in this department has identified several EBs with extraordinarily long lifetimes of an hour or more, two orders of magnitude above the average. In this project, these long-lived EBs will be studied using satellite data from the Solar Dynamics Observatory (SDO) to analyze their intensity variations and those of their surroundings. The project involves the use of Python as an image processing and analysis tool, and has the potential to result in a publication.

The evolution of satellite dwarf galaxies depends strongly on the interaction with their host environment. In particular, tidal and ram pressure stripping are effective mechanisms for quenching star formation in gas-rich satellites. The strength of these mechanisms depends on the orbital history of the satellite, its distance from the host, and the host-satellite mass ratio. An additional quenching channel may be radiative feedback from the central supermassive black hole. Such feedback is thought to produce low-density bubbles in the circumgalactic medium along the minor axis of the host, where quenching should be less effective. Both observations and simulations have shown that satellites distributed along the minor axis do indeed have a higher star formation rate than those in other directions. However, it is unclear whether this effect might instead be due to the accretion history of the host satellite, or whether there is any dependence on the host mass. This project will exploit publicly available catalogues of satellite systems in the Local Universe, from Milky Way-like hosts (e.g. ELVES, Saga-II surveys) to more massive systems like Centaurus A, to search for possible anisotropies in the spatial distribution of quenched and star-forming satellites, and ultimately compare them to expectations from cosmological simulations.

References: Martin-Navarro I., et al. 2021, Nature, 594, 7862; Karp J., Lange J., Wechsler R. 2023, ApJ, 949, 1; Greene J., et al. 2023, 949, 2.

Note: this topic has been allocated to a student and is not available any more.

Over 70% of all stars in existence are low-mass M dwarfs. Their lifetimes are longer than the age of the universe, and each of them hosts at least 1-2 planets. Some of these planets orbit in the habitable zone, where the basic conditions for life as we know it are met. The youth of young, low mass stars is of particular interest to astronomers because it sets the stage for the main sequence development of the entire star-planet system, and helps understand the conditions under which it formed.
TIC 206544316 is a young mid-M dwarf with a light curve that puzzles observers with its complexity. Several hypotheses exist to explain the persistent yet variable modulations of its optical emission. Some include magnetic spots on the rotating surface of the star and/or material in a disk or co-rotating clouds. Flares are a powerful natural spotlight that illuminates these local, dynamic magnetic structure as well as the material along the line of sight. In a recent observation by the Transiting Exoplanet Survey Satellite (TESS), a giant flare erupted on TIC 206544316. This presents a unique opportunity to discriminate between the proposed ideas about the nature of its magnetism and immediate surroundings.
The master project will involve analyzing this flare with an existing Python framework that was designed to localize flares on the surface from optical light curves of rapidly rotating stars. The student will then expand that framework to include the presence of material in the line of sight either in form of clouds or a misaligned disk that obscures the flare.
Basic scientific Python skills are required, i.e., familiarity with numpy, scipy, pandas, and matplotlib. Foundations of stellar magnetism and rotation are a bonus, but not mandatory.

Observational studies of pairs of galaxies have uncovered that their differential line-of-sight velocities indicate the presence of a peak in their three-dimensional intervelocity distribution at 130-150 km/s. Modified Newtonian Dynamics (MOND) predicts such a preferred intervelocity for paired galaxies, which was initially presented as a success of MOND over the standard model of cosmology, Lambda Cold Dark Matter (ΛCDM ). However, a detailed comparison to galaxy pairs selected from a ΛCDM simulation analogously to the observational studies has also uncovered a preferred intervelocity that is compatible with the observed one. The existence of the observed intervelocity thus does not directly challenge ΛCDM. Developing the galaxy pair intervelocity into a test of gravity in the low acceleration regime will therefore require more detailed studies to identify measurable differences in the models. This goal requires to answer numerous questions that currently remain open: What is the exact origin of the peak? What sets the position of the peak, and how sensitive is this to the implemented sub-grid physics and resultant scaling relations (e.g. SMHM, BTFR) in the simulations? How does the intervelocity distribution evolve with time/redshift? How can the data analysis be improved to fully exploit the observational information (e.g. by forward-modeling the line-of-sight velocity difference instead of de-projecting it)? And ultimately, can differences between ΛCDM and MOND expectations be identified in the detailed properties of the intervelocity distribution which could be used to empirically differentiate between the two approaches? The project will address these questions. This will involve further investigations of a range of publicly available cosmological simulations, and require coding, data analysis, and scientific interpretation of the results.

Reference: Pawlowski et al. (2022), A&A in press, arXiv:2207.09468

Note: this topic has been given to a student and is no longer available.

In this thesis the student will learn about the wealth of information that is encoded in high-resolution spectra of exoplanetary systems. In real high resolution observations however, one is confronted with the complex problem of detecting and retrieving the tiny spectral signatures of an exoplanet. Advanced simulation will allow the student to model the spectral signatures of exoplanets around magnetically active host stars. Deep learning techniques offer a potential benefit to explore and analyze theses tiny spectral signatures. In a proof of concept case study based on simulated data and spectra the student will train and apply machine learning methods to retrieve characteristic parameters and information of the exoplanet and its active host star.

The observed satellite galaxy systems of the Milky Way, Andromeda, and Centaurus A are known to host flattened distributions of coherently orbiting satellites, called Planes of Satellite Galaxies. Similarly extreme structures are rare in cosmological simulations that model the formation and evolution of galaxies in the universe, giving rise to the Planes of Satellite Galaxies problem. However, it is thus far unclear whether similar structures might exist around lower-mass galaxies. For this project, the student will address this question by studying the Large Magellanic Cloud, the Milky Way’s largest satellite galaxy, and it’s companions. The work will involve assembling data on the current satellite galaxy system of the Milky Way, and on which of these are potentially associated to the LMC. Positional data will be used to study the spatial distribution and look for a satellite plane, and proper motion information will be used to investigate kinematic correlation. By generating and comparing with mock satellite systems, the significance of any potential alignments will be measured. Time permitting, the past evolution of the LMC satellite system can be modeled with simulations to determine the impact of tidal effects and to judge whether the presence or absence of a satellite plane is robust.

References: Pawlowski, 2018, MPLA, 3330004; Jethwa et al. 2016, MNRAS, 461, 2212

Note: this topic has been given to a student and is no longer available.

Stars in wide binary systems are formed at the same time, which provides us with two same-age (but not necessarily same-mass) stars. It is of high interest to stellar and exoplanetary science how the magnetic activity and X-ray emission of stars evolves over their main-sequence lifetime. In particular, the intrinsic scatter in this evolution of two stars that are otherwise very similar is important to understand the spin-down processes of stars. The goal of this thesis is to find archival X-ray data of wide binary stars and to measure the X-ray luminosity and other coronal properties of those stars. The student will then characterize the typical scatter within pairs of same-age stars and interpret this in the context of magnetic braking and spin-down of cool stars.

Different master thesis projects become available depending on the current research projects in the section. Please contact Prof. Poppenhaeger to check if thesis projects are currently available.

About half of the satellite galaxies of the Andromeda galaxy are aligned in a narrow plane that is seen edge-on from the Milky Way. Spectroscopic measurements of their blue-/redshifts indicate that those in the North recede and those in the South approach, as if the satellite plane were rotating. The M31 satellite distribution, especially for the on-plane satellites, is also heavily lopsided, meaning that more satellites are situated on one side of their host than on the other. Except for this, little is known about the orbits of most of these satellite galaxies. The known correlations in their positions and velocities do, however, hold the potential to reveal more about their orbital properties. To investigate this possibility, the student will set up and run computer simulations of satellite galaxies as test particles orbiting a host galaxy. These simulated satellite systems will then be mock-observed and analyzed for the presence of trends and correlations with the known input properties of the satellite systems. Questions the project might address are: How well can we constrain the orbital eccentricity of satellite galaxies if they orbit in a common plane seen nearly edge-on? Is it more likely to see a lopsided satellite distribution if the satellites preferentially co-orbit? Does having similar orbital properties among a satellite galaxy population make it more likely to find a lopsided distribution?

References: Pawlowski, 2018, MPLA, 3330004; Ibata et al. 2013, Nature, 493, 62

Different stellar populations in the Milky Way can be discriminated by their chemistry and by their kinematics. Thanks to the Gaia astrometric satellite mission, it is for the first time possible to track larger number of stars by their 3-dimensional orbits. This enables us to directly analyse the orbits of stars as function of their chemical abundance signatures. The project foresees to perform this analysis and compare findings with analogue relations seen in large-scale computer simulations of galaxy formation.

References: Buck, T., 2020 MNRAS 491, 5435; Guiglion, G., et al, 2020, A&A 644, 168; Steinmetz, M., et al, 2020 AJ 160, 183; Wojno, J., et al., 2018, MNRAS, 477, 5612

Central bars are present in about 2/3 of disk galaxies, including our Milky Way. Numerical simulations have shown that they are very important for the galactic chemo-dynamical evolution. This project will study the conditions under which bars form, using cosmological disk formation simulations. The time evolution of bar parameters, such as the pattern speed, length, and strength, will be linked to the disk phase-space structure discovered in the Gaia data, for the first time in the correct Local Group environment. The angular momentum redistribution induced by the bar in conjunction with the inner spiral structure will also be quantified. An ultimate goal of the project will be to resolve the longstanding controversy about the Milky Way’s bar length and pattern speed – is it long and slow or short and fast?

References: Minchev, I., et al., 2012, A&A 548, 126; Sanders, J. L., et al. 2019, MNRAS 488, 4552

The asymptotic giant branch (AGB) is the last evolutionary stage of intermediate-mass (1-8 M_sun) stars. The stars in this stage are extremely bright, so AGB stars are important luminosity contributors in stellar systems with intermediate ages (1-3 Gyrs). The luminosity contribution of AGB stars becomes even larger in near-infrared wavelengths due to their red colours. Models predict their luminosity and colour evolution, but they have significant uncertainty due to the complex internal stellar structure evolution, their rather cool atmospheres resulting in absorption lines of many elements and molecules, and circumstellar dust absorption effects. However, in preparation for the James Webb Space Telescope era it is important to calibrate the near-infrared luminosity and colours onto the models. Star clusters are useful tools to investigate the evolution, as they are assumed to have approximately the same age. There are resolved star photometry of stars clusters in the Large Magellanic Cloud and Andromeda galaxies, publicly available in the literature. We will select AGB stars in the star clusters in each galaxy, and investigate their luminosity and colour evolutions in Colour-Magnitude Diagrams. Detailed age-dependent properties, such as luminosity and colour of individual AGB stars, and total luminosity of AGB stars in star clusters in a given age bin, will be derived.

References: Radburn-Smith, D. J., et al. 2014, ApJ, 780, 105

Metal-poor stars in our Galaxy (those stars with at least 10 times smaller metal fraction than the Sun) hold important clues on the first steps of the formation and assembly of the Milky Way. It is now possible to use Gaia Mission Data Release 2 information combined with photometric information to find good candidates of metal-poor stars. This can be tested with samples for which the metallicity is known via spectroscopy (from many publicly available surveys). We will also study the effects of the different stellar evolutionary tracks on the outcome of the low metallicity candidates near and far way. This is a project to be carried out with the StarHorse code designed to determine stellar parameters (e.g., temperature, metallicity) from photometric and astrometric input values of millions of stars.

References: Queiroz, A., et al. 2018, MNRAS, 476, 2556

The stellar structure in the outermost regions of disk galaxies contains important clues about the disk formation and evolution. This project will involve examining a set of ~33 high-resolution disk formation simulations in the cosmological context, aiming to understand how the outer regions of galactic disks form and evolve with time. The student will write a code to split simulated stars by angular momentum, age, and chemical composition, and study the evolution of these parameters with cosmic time. The goal of this work will be to (1) understand the physical causes for the formation of different types of observed stellar density profiles: Type I (single exponential), Type II (down turning), and Type III (upturning) and (2) to find observational relations that can be used to distinguish different formation scenarios in observations of external galactic disk outskirts.

References: Minchev et al. 2012, A&A 548, 126