Topics for Bachelor, Master and PhD Theses

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

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.

The circumgalactic medium is composed of warm-hot gas and surrounds the stellar disk of galaxies but is still within the galaxy's gravitational influence. This topic has recently attracted much attention because of its important role in our understanding of galaxy evolution owing to rapid advances in cosmological simulations and observational break-throughs that gave us access to this diffuse, previously nearly invisible medium. In this project, the student will use state-of-the-art cosmological simulations (the HESTIA simulations) to analyze the hot X-ray gas in-between the Milky Way and Andromeda in the Local Group. It will be determined whether the hot gas traced by Oxygen (the OVII transition) can best be described by the "superposition" of spherical density profiles around Milky Way and Andromeda, or whether gas compression and shocks additionally enhance the density. Furthermore, supernova remnants in the interstellar medium are able to accelerate elementary particles to large relativistic energies, making up a population of so-called cosmic rays. As this cosmic ray population is escaping from the disk into the circumgalactic medium, it can substantially contribute to the overall pressure balance and hence modify the thermodynamics of the hot phase. The student will compare simulations with and without cosmic ray feedback, and thus determine how different models for cosmic ray transport modify the hot gas and the X-ray emission of the circumgalactic medium of galaxies.

References: Buck et al. 2020, MNRAS, 497, 1712, arXiv:1911.00019, Libeskind et al. 2020, MNRAS, 498, 2968, arXiv:2008.04926

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

One of the most important discoveries to emerge from the ESA Gaia astrometric survey is the “phase spiral” pattern detected in the z − Vz plane throughout the solar neighbourhood in the Milky Way (MW). Since its discovery two years ago, up to a dozen independent research papers seek to explain or shed light on the phase-spiral phenomenon. Vertical and in-plane oscillations induced by a massive disc-crossing perturber has received the majority of attention to date, not least because the Sagittarius (Sgr) dwarf spheroidal galaxy is observed to be undergoing disruption as it crosses the disc. In this project we aim to develop a wide range of models of the MW - Sgr dwarf interaction, taking into account uncertainties in the mass and orbital parameters of the perturber and at the same time varying the MW density and velocity ellipsoid. We plan to study the MW phase-space evolution and connection of the 'phase spiral' to the galactic spiral arms and bending waves.

References: Antoja, T., et al 2018, Nature, 561, 360

Unlike the Milky Way and some nearby galaxies, due to large distances, individual stars can not be resolved in external galaxies. Therefore, our knowledge about their structure, dynamics and chemical composition is based on averaged properties of stellar populations extracted from spectra of groups of stars captured in the same location. However, over their lifetime stars can migrate far away from their birthplaces thus being mixed together representing different epochs of evolution and various galactic environment. The project aims to understand the impact of stellar radial migration on the properties of unresolved stellar spectra which is the key tool in reconstruction of star formation histories in external galaxies. We plan to analyze a set of galaxies from cosmological simulations where radial migration is a natural outcome of internal (spiral arms, bar) and external (interaction with satellites and mergers). In these models we will use the information about chemical abundances, ages and kinematics to create a mock spectra catalogue where knowledge of stellar birthplaces will allow to highlight the impact of stellar migration on spectra of galaxies to be compared with IFU observations (CALIFA, MANGA surveys).

References: Minchev, I., et al. 2013, A&A, 558, 9; Sánchez, S. F., et al. 2012, A&A, 538, 8

The Milky Way is the only spiral galaxy where we can obtain full phase-space information for a significant number of stars. The Gaia satellite mission is giving us parallaxes, proper motions and line-of-sight velocities with unprecedented precision and therefore allows us to quantify the complex dynamical processes driving the evolution of the Milky Way. The first two Gaia releases discover that the MW stellar halo is not only formed by the oldest stars in the Galaxy but is also made up of stars that have the age and abundance typical of disc populations. Running a series of major merger simulations between the Milky Way progenitor and a massive satellite (Gaia-Enceladus-Sausage, GES) we will attempt to understand how compact and on what orbits the merger remnant stars can be distributed and what is the impact of the merger on the further chemical evolution of the MW disk.

References: Belokurov, V., et al. 2018, MNRAS, 478, 611; Haywood, M., et al. 2018, ApJ, 863, 113

The basic idea of the survey is to characterize potential planet-host candidates for the upcoming NASA/TESS mission by providing precise spectroscopic parameters for stars in the satellite’s continuous viewing zone, which is around the two ecliptic poles. The North survey field comprises of approximately 800 square degrees with 311 main-sequence stars V=8.5 mag and cooler than F0. Two R=200,000 spectra will be obtained for each of these stars at different epochs and shall deliver the following parameters: Two radial velocities good to 2-3 m/s, precise global stellar parameters like effective temperature, gravity, metallicity, micro- and macroturbulence, vsini, and radii in combination with GAIA data, high-precision abundances of all important chemical elements including the α-elements (Mg, Si but also Ca, Ti) as well as CNO. Isotope ratios like ⁶Li/⁷Li and ¹²C/¹³C if applicable, average and specific line bisectors as a function of excitation as a measure for convective blueshift, and magnetic-activity signatures like absolute Ca II IRT, Hα and Hβ line-core fluxes. The PhD is focused towards the determination and analysis of chemical abundances, but not necessarily. The survey observations with the VATT (Vatican Advanced Technology Telescope, 1.8m diameter, with a 450m fiber link to PEPSI) formally start on May 27th, 2018 and last for 3 years, 50 nights per year.

This thesis shall expand our iMAP code to molecular Doppler-Imaging and apply it to low-mass M stars and possibly to fully convective L dwarfs and Brown Dwarfs. The scientific goal is to detect surface inhomogeneities due to magnetic fields (or density clouds in case of cold Brown Dwarfs) and to find evidence for differential rotation and interpret these with the predictions of concurrent turbulent dynamo models. The iMAP inversion shall consider the reconstruction of surface temperature maps either solely from molecular features of, e.g., TiO, CO, OH, and CN bands, and possibly also simultaneously with an unlimited number ofatomic lines. The central scientific objective is to find out whether, and if yes, how different, the surface topology of fully-convective M and L dwarfs appears compared to solar-like interface-type dynamo stars. The long-term goal is to pave the way to quantify the structure and dynamics effects of stellar activity at the lower end of the main sequence, and its consequent implementation into stellar evolutionary models.

The idea of a PEPSI “deep spectrum” is to provide the highest quality optical spectra ever obtained for any star other than the Sun. A signal-to-noise ratio of thousand at a spectral resolution of 1.3 km/s covering the entire optical spectrum from 384 to 912 nm is what we can get with PEPSI at the LBT. One such spectrum was published recently for the planet-host Kepler-444 (see https://pepsi.aip.de). The thesis includes new observations and shall address some stellar properties in great detail, e.g., chemical abundances of all sorts of elements, stellar surface velocity fields like granulation and supergranulation or, in general, turbulent convection signs. This kind of information is essential for our understanding of the nature of ‘turbulence’ in stellar atmospheres, and for the validation of current 3D hydro-dynamical models of stellar convection. The accurate determination of chemical abundances and isotope ratios is a fundamental building block of our knowledge about stellar nucleosynthesis and the chemical evolution of the Galaxy. This is in particular exciting if the star hosts a planetary system. Are the refractory elements overabundant with respect to the volatile elements?

Magnetic fields likely play an important role in almost any astrophysical target, from the early Universe to the Sun, Earth, and its environment. While numerical 3-D MHD simulations became more and more sophisticated in the previous years, magnetic-field observations are still extremely rare (except for the Sun). In this PhD, we will carry out such measurements with the high-resolution full-Stokes-vector spectropolarimeter PEPSI of the 11.8 m Large Binocular Telescope (LBT). A central scientific question in this thesis is to survey the magnetic fields of stars in open clusters and, possibly, map the distribution of magnetic flux for a representative target and qualify its impact on stellar evolution. Are there observable signs when the surface field transfers from dynamo-generated morphology like in our Sun to fossil fields like in white dwarfs? Is the angular momentum loss from magnetized winds and its associated braking of stellar rotation just a brief epoch on or near the ZAMS? This PhD topic is very flexible depending on what targets one would choose for PEPSI observations. Some observations already exist.

This topic is structured for three PhD theses over the years to come:

1. The flare statistics and non-thermal energy budget of M dwarfs hosting planets (Klaus G. Strassmeier, with Dr. A. Warmuth)
2. A search for CMEs in nearby stars with LOFAR (Klaus G. Strassmeier, with Drs. A. Warmuth & C. Vocks)
3. Observations and analysis of the Vegetation Red Edge in Earthshine Observations during Total Lunar Eclipse (Klaus G. Strassmeier, with Drs. I. Ilyin and M. Mallonn)

Stellar magnetic activity influences the non-thermal particle environment of potentially habitable planets in three different manners: by modulation of the galactic cosmic ray environment through changing magnetized stellar winds and stellar mass ejections (CMEs), by energetic particles accelerated by stellar flares, and by CME-magnetosphere interactions resulting in geomagnetic storms and associated particle acceleration. Biomarkers in general, e.g. like the strength of the vegetation red edge (VRE), could be severely affected by such non-thermal emission from the planet's host star. It is therefore very relevant to incorporate stellar non-thermal processes that eventually impinge on a planet. The consequence of these processes for habitability would be particularly important in the case of M dwarfs, partly due to the close proximity of any potentially habitable planets, but also due to the fact that M dwarfs tend to be more magnetically active than earlier-type stars. Solid number statistics are still missing and an observing campaign for selected targets with our APT in Arizona or STELLA and – in the near future- BMK10k in Chile could be done.

Doppler imaging is an inversion technique to recover a 2-D image of a rapidly rotating star from a series of high-resolution spectral line profiles. The inverse problem for stars with cool spots amounts to solving the integral equation relating the surface temperature distribution to the observed line profiles and light and color curve variations, while controlling the effects of noise in the data through a regularizing functional. Our group had developed two inversion codes, TempMap and iMAP, that we would want to apply to new, unpublished time-series of high-resolution stellar spectra. Thanks to our automated telescopes we have several targets with fully reduced spectra that are ready to be inverted into surface maps. The master student basically takes care of one star and goes through the entire process of the line-profile inversion including an interpretation of the surface map obtained. Auxiliary data, e.g. like continuum photometry, will be either taken from the literature or can be reobserved. The goal is to obtain a well-constrained Doppler image of the target in question, or even a series of images if the data allow, and to place the results in context to other stars. We have currently binary stars with giant or main-sequence components, stars with high lithium abundances, single class III giants as well as very young main-sequence stars and a few pre-main sequence targets.

High-precision photometry can be used to time a starspots repeated appearance on the visible stellar hemisphere, and thereby obtain stellar rotation periods ten times more precise than from spectroscopic measurements. It also allows determining differential surface rotation in case the latitude of the spot can be obtained, or at least constrained to a certain range. The rotation of the star can also be used to infer the surface brightness distribution and thereby obtain a spot map of the (unresolved) stellar surface. The inversion module of our code iMAP shall be applied to existing Kepler or K2 light curves. Combined with automatic telescopes, like our APTs and the STELLA-WiFSIP facility, photometry is unbeatable in obtaining long-term information on the growth and decay of spots and even on decades-long activity cycles.

Several Master theses will be possible with the survey data. We are observing 10 open clusters with ages between 30 Myrs and 1.7 Gyrs. So far, four clusters were analysed and published. We employ AIP’s Wide Field STELLA Imaging Photometer (WiFSIP) on the STELLA-I telescope in Tenerife with the Sloan r filter for the “monitoring” mode and in Strömgren uvbyβ in “deep-field” mode. 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. A separate thesis is envisioned for new observations that include a narrow H-alpha filter and/or radial velocity observations with multi-object spectrographs at larger telescopes. The 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.

The STELLA Control System (SCS) is a java-based software written in-house and currently runs three telescopes (STELLA-I, STELLA-II, and RoboTel), their instruments and their buildings. Combined with other software packages it also takes care of data reduction and pipelining the data to the user = astronomer. An interface within the Virtual Observatory is in progress. Within this field of “(Astronomical) Software engineering” we offer master theses in the following subtopics

• Interfacing the STELLA cloud monitor to the SCS. We run an automatic IR cloud imager (at 8μm plus visual) in Tenerife and want to interface it to the SCS scheduler for possible target interference with moving (night) clouds.
• Currently located at AIP in the dome on top of the Schwarzschild-Haus, RoboTel is used as a test bed for new software. It is an 80cm-diameter copy of STELLA-I and also carries a copy of the WiFSIP instrument with a 4k CCD imaging photometer. It could be shipped to Tenerife and/or Chile for a new science case if it had been thoroughly tested and documented and equipped with a modified version of the STELLA-I/WiFSIP-I operation’s GUI.
• Design and implement an interface of the SCS for a robotic-telescope network as part of the e-science initiative.