IMPRS-HD Alumni 2020

Alumni 2020

Aida Ahmadi  (10.1.)  -  Christos Vourellis  (21.1.)  Alexander Htgate (30.1.)  -  Daniel Haydon (3.2.)  -  Karan Molaverdikhani (5.2.)  -  Xudong Gao (6.2.)

Karan Molaverdikhani    (Iran)                                                                                                                                            05.02.2020

Characterization of Planetary Atmospheres ( thesis pdf, 65 MB )

After the discovery of the first exoplanet in 1990’s and a fast growing number of discoveries since then, there have been many attempts to observe and characterize their atmospheres. In particular, water and methane have been the focus of many investigations due to their relevance to the origin of life and habitability, as well as their major roles to shape the structure of planetary atmospheres. Abundances retrieved for these species can be also used as a tracer of carbon-to-oxygen ratio (C/O) and metallicity of these atmospheres; hence potentially linking the formation scenarios with the observations. Water’s spectral signature is everywhere, but despite many efforts, there has been only one robust detection of methane and only recently. The question is, “where is methane?”. By applying a hierarchical modelling approach (utilising more than 177,000 thermochemical equilibrium cloud-free, disequilibrium cloud-free, and thermochemical equilibrium cloudy models) we predict that there are four classes of irradiated gaseous planets; two of them (Class-I and Class-II; Teff<1650 K) likely to show signatures of CH4 in their transmission spectra, if cloudy-free and C/O above a certain threshold (aka the “Methane Valley”). The effect of disequilibrium processes on the classification found to be modest with a more continuous transition between Class-II and III planets. Clouds, however, heat-up the deeper parts of Class-I and Class-II planets; removing CH4 from the photosphere. Simultaneously, clouds obscure any molecular features; hence making the observation of methane even more challenging.

Supervisor:    Thomas Henning  (MPIA)

Daniel Haydon    (UK)                                                                                                                                                            03.02.2020

Visualising the Synthetic Universe ( thesis pdf, 45 MB )

Star formation cannot truly be understood from observational data alone;only with simulations is it possible to assemble the complete picture. Observations guide the physics we build into our simulations, yet the impact of different star formation and feedback models can only be investigated with simulations. Synthetic observations allow us to make are alistic comparison to true observations as well as teach us about the emission tracers we depend upon. Through coupling the stellar population synthesis code slug2 to galaxy simulations,we can generate synthetic star formation rate tracer maps. These maps assume differentstellarmetallicities,starformationratesurfacedensities,andsuffer fromvariedamountsofextinction. Thisallowsustoexploreandconstrainthe environmental effects on the characteristic emission lifetimes—the duration for which a tracer is visible. With these emission lifetimes and inconjunction with a new statistical method, the ‘uncertainty principle for star formation’, constraintscanbeplaceduponthedurationsofdifferentevolutionaryphases ofthestarformationprocess,allowingustobetterunderstandthephysicsof starformation and feedback on sub-galacticscales. Studying the interstellar medium can also reveal information about stellar feedback: the gas density structure is altered as a result of the injected energy,momentum,and matter. Surveys of the COemission in galaxies can tell us how the properties of this medium have evolved over cosmic time. Using despotic to model CO line emission of gas found within the Illustris TNG50cosmological simulation,we produce an equivalent synthetic survey. This synthetic survey can be used as a basis for comparison and predictor of observational trends.

Supervisor:    Diederik Kruijssen  (ARI)

Christos Vourellis    (Greece)                                                                                                                                              21.01.2020

GRMHD Launching of Resistive and Dynamo Active Disks  ( thesis pdf, 60 MB, corrigdm )

Astrophysical jets appear as linear collimated objects of high speed that are typically found in young stellar objects, X-Ray binaries, gamma-ray bursts, or active galactic nuclei. The physical procedures that lead to the development of these jets have been studied extensively in the past years. We believe that the launching of highly relativistic jets requires the existence of an accretion disk threaded by a strong magnetic field that rotates around a black hole. We perform general relativistic magnetohydrodynamic simulations of outflow launching from thin accretion disks. As in the nonrelativistic case, resistivity is essential for the mass loading of the disk wind. We implemented resistivity in the ideal GRMHD code HARM3D, which allows us to run simulations with larger physical grids, higher spatial resolution, and longer simulation time. We present the numerical details of the code and we show numerical test in the resistive regime that prove the robustness of the code. As a reference simulation, we consider an initially thin, resistive disk orbiting the black hole, threaded by a large-scale magnetic flux. As the system evolves, outflows are launched from the black hole magnetosphere and the disk surface. We mainly focus on disk outflows, investigating their MHD structure and energy output in comparison with the Poynting-dominated black hole jet. The disk wind encloses two components -- a fast component dominated by the toroidal magnetic field and a slower component dominated by the poloidal field. The disk wind transitions from sub- to super-Alfvenic speed, reaching velocities approximately 0.1c. We provide parameter studies varying spin parameter and resistivity level and measure the respective mass and energy fluxes. A higher spin strengthens the disk wind dominated by the toroidal component of the magnetic field along the inner jet. We disentangle a critical resistivity level that leads to a maximum matter and energy output for both, resulting from the interplay between reconnection and diffusion, which in combination govern the magnetic flux and the mass loading. For counterrotating black holes the outflow structure shows a magnetic field reversal. We also show the structure and direction of the electric field and its connection with the velocity and magnetic field vectors. Finally, we present the first fully dynamical simulation of dynamo generated poloidal magnetic field in a GRMHD environment. We simulate cases of both accretion tori and disks and we find induced magnetic field with both dipolar and quadrupolar structure. We follow the evolution of the field structure and strength and we show the launching of outflows from the torus/disk surface and the black hole magnetosphere.

Supervisor:    Christian Fendt  (MPIA)

Aida Ahmadi   (Canada/Iran)                                                                                                                                                  10.01.2020


This thesis is dedicated to the search and characterization of disks in high-mass star formation. The work presented is part of the CORE survey, a large observational program making use of interferometric observations from the NOrthern Extended Millimetre Array (NOEMA)for a sample of 20 high-mass protostellar objects in the 1.3 millimetre wavelength regime. An in-depth analysis of the W3(H2O) star forming region examines its fragmentation into two hot cores, separated by 2300 au and engulfed in a rotating circumbinary envelope of dense gas. Higher resolution observations reveal that embedded within each core is a rotating disk-like structure with outflows being ejected along the disk rotation axes. Studying the stability of the disk-like structures confirms that they are gravitationally unstable and prone to disk fragmentation. In an effort to understand the uncertainties involved, we created synthetic observations of a high-resolution 3D radiation-hydrodynamic simulation that leads to the fragmentation of a massive disk at different inclinations and distances. We find that the kinematics of differentially rotating disks resemble rigid-body-like rotation in poorly resolved observations, leading to overestimation of protostellar masses. Despite the lack of resolution, we find that the stability analysis correctly predicts disk fragmentation regardless of the uncertainties. Studying the kinematics of the full CORE sample, we find rotational signatures in dense gas perpendicular to bipolar molecular outflows in most regions. Modelling the level populations of various rotational transitions of the dense gas tracer CH3CN, we find the disk candidates to be on average warm (200 K). Applying the robust stability analysis, we find that most high-mass young stellar objects are prone to disk fragmentation early in their formation due to high disk to stellar mass ratio. Since most high-mass stars are found to have companions, disk fragmentation seems to be an important mechanism by which such systems may be formed.

Supervisor:    Henrik Beuther   (MPIA)

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