Vacancies

We offer a variety of interesting projects (Internship, Bachelor, Master, PhD) in the research fields of numerical astrophysics and computational physics in general! As an appetizer, please find below some brief project descriptions.

If you are interested in joining our research team, just drop by our offices and/or contact us via email!

Dynamical Friction: Free-floating Planets and Runaway Stars:

Planets and stars, which got ejected from their birth region, move at high velocity through space. Using multi-dimensional hydrodynamical simulations of the shock dynamics, we aim to quantify their impact on the interstellar medium and how the interstellar medium impacts the evolution of the moving object.

The simulation snapshot to the left shows the shocked gas density structure around a high-velocity, solid, spherical object.

These research projects are conducted at the Institute of Astronomy & Astrophysics at the University of Tübingen within the newly established Emmy Noether Research Group on Massive Star Formation (Dr. Kuiper) and the Department of Computational Physics (Prof. Kley).

Self-Gravitating Accretion Disks and Planet-Disk Interaction:

The collapse of a massive cloud of gas and dust ultimately yields the formation of an accretion disk surrounding a (proto-)star. Depending on the properties of the disk and its host star, the self-gravity of the gaseous disk can lead to the formation of spiral arms or even its gravitational fragmentation.
We are studying the stability of forming accretion disk around massive and sun-like stars in multi- dimensional self-gravity-radiation-hydrodynamical simulations.

These research projects are conducted at the Institute of Astronomy & Astrophysics at the University of Tübingen within the newly established Emmy Noether Research Group on Massive Star Formation (Dr. Kuiper) and the Department of Computational Physics (Prof. Kley).

The simulation snapshot above shows the formation of a spiral arm structure and a ring-like gap in an accretion disk due to the gravitational potential of an embedded planet orbiting its host star .

The Formation and Evolution of Planetary Atmospheres:

During their formation, proto-planets will become massive enough to gravitationally bind gas from the disks they are born in. For giant planets such as Jupiter the rocky or icy core even becomes so massy that it triggers an unlimited, runaway accretion of gas. Smaller planets such as Neptune or Earth instead are only able to bind more limited quantities of gas. Hence, the physical details of gas accretion in the disk-embedded phase yields a distinctive attribute of the observed planetary distribution.

The formation and early evolution of planetary atmospheres, embedded in their natal disk, shall be investigated using state-of-the-art modeling. As a pioneering study, we include both – the hydrodynamic as well as the thermodynamic – effects via two- and three-dimensional radiation-hydrodynamical simulations. Conducting these simulations in a local frame in spherical coordinates with the planet at the grid’s origin, allows to focus specifically on the small scales around the planetary body in very high resolution. Initial and boundary conditions for the local frame, representing the large-scale disk flow, can e.g. be extracted from preceding global disk simulation such as those described above.

This research project will be conducted in close collaboration with Dr. Chris Ormel from the Anton Pannekoek Institute, University of Amsterdam.

The Growth and Feedback of Expanding HII Regions:

Regions of ionized hydrogen, so-called HII regions, form around luminous massive main-sequence stars, which irradiate their environment with a bulk of extreme ultraviolet radiation. These expanding HII regions have strong impact on the stellar neighborhood due to the enhanced thermal pressure of the gas and radiation pressure.

In this project, the expansion of HII regions shall be investigated via direct modeling of the ionization-radiation-hydrodynamics. Simulation will be performed in spherical symmetry and three dimensions as well. In addition to hydrogen ionization, the modeling will successively include absorption and emission by dust grains, the diffuse EUV radiation field from direct recombination into the hydrogen ground state in already ionized regions, and the various components of radiation pressure, namely direct stellar irradiation of dust, stellar photo-ionization of hydrogen, dust (re-)emission, and recombination radiation.

The resulting growth rate and feedback will be quantified in their dependence of the initial cloud configuration and the parameterized incident radiation field. The effect of turbulence can be investigated in a follow-up study.

Visualization of a two-dimensional, axially and midplane symmetric test simulation of the expansion  of an HII region around an a priori fixed source of EUV photons. The initial growth speed follows the analytical estimate of Spitzer and/or Hosokawa-Inutsuka.

The growth of the HII region eventually stops at the so-called stagnation radius, which can as well be derived analytically from the pressure equilibrium condition at the expanding front. Shock waves are triggered and eventually the ionization front becomes unstable.

MHD-driven Jets from Massive Protostars:

Visualization of a two-dimensional, axially and midplane symmetric simulation of the launching, acceleration, and expansion of a magneto-centrifugally driven stellar jet. The jet is launched within a collapsing cloud core of gas and dust from a massive protostar and its surrounding accretion disk.

The visualization was created by Anders Kölligan, who successfully finished his PhD thesis in Nov 2018 in our research group.

We conduct magneto-hydrodynamic (MHD) simulations using the state-of-the-art code PLUTO, combining non-ideal MHD, self-gravity, and very high resolutions, as they have never been achieved before. Our default setup includes a 100 solar mass cloud core that collapses under its own self-gravity to self-consistently form a dense disk structure, launch tightly collimated magneto-centrifugal jets and wide angle tower-flows. Of these two outflow components, the tower flow dominates angular momentum transport. The mass outflow rate is dominated by the entrained material from the interaction of the jet with the stellar environment and only parts of the ejected medium was directly launched from the accretion disk. A tower flow can only develop to its full extend, when much of the original envelope has already dispersed.

Massive stars do not only posses slow wide-angle tower flows, but also produce magneto-centrifugal jets, just as their low-mass counterparts. The actual difference between low-mass and high-mass star formation lies in the embededness of the high- mass star which implies that the jet and tower flow interacts with the infalling large-scale stellar environment, potentially resulting in entrainment.

The interaction of the expanding jet into the stellar surrounding denotes one of the best observational signatures of the early phases of high-mass star formation.

Photoevaporation of Planetary Atmospheres:

The movie shown to the left is a visualization of a two-dimensional, axially symmetric preliminary hydrodynamics simulation of a photoionization-driven wind from a pre-existing planetary atmosphere around a super-Earth like planet.

The initial condition of this simulation describes a planetary atmosphere in hydrostatic equilibrium. Initially, the gas in the whole computational domain is neutral. A very strong photoionizing radiation flux (1050 EUV photons per second) is injected into the computational domain at the bottom boundary streaming upwards. The distance to the radiation source is set to be 0.1 astronomical units.

The hot, ionized gas forms a nearly spherically symmetric outflow, while the cold, neutral gas behind the planet, which is shadowed from the bottom photoionizing radiation flux, is compressed into a wind tail. The compression is the result of the thermal pressure gradient between the ionized and the neutral gas.

The color-coding shows the gas mass density (pink to red to yellow for high density, green to blue to black for low densities), while the arrows denote the velocity field (dark red for low velocities and white for high velocities). The brown sphere in the center resembles the solid, rocky planetary core.