Johannes Esser (Germany) 18.05.2020
Physical properties of the circumnuclear cloud distribution in Active Galactic Nuclei ( thesis pdf, 5 MB )
In this thesis the structure of the broad line region (BLR) and the inner dust torus of Active Galactic Nuclei (AGN) is studied. In a first project we have carried out a multiwavelength reverberation mapping campaign of hot dust in AGN for a sample of 25 nearby AGN with redshifts below 0.2. Reverberation mapping allows to measure the radius of the dust torus which, in relation to the accretion disk (AD) luminosity, can be used as a cosmological standard candle. Despite the radius the multiwavelength approach allows for the investigation of other dust properties like the temperature. An influence of dust temperatures on the relation between the dust radius and the AD luminosity is expected, as hotter dust should be located closer to the heating source at smaller radii. We were able to determine 14 reliable dust radii and 20 reliable dust temperatures for our AGN. This is the largest sample of homogeneously reverberation mapped inner dust tori using a multiwavelength approach and among the largest dust reverberation mapping samples overall. We got tighter constraints on the luminosity radius relation when a novel temperature normalization was applied. Especially the slope of the relation is only in good agreement with the expected value of 0.5 if the temperature normalization is taken into account. Additionally we can determine the surface area of the dust only when the temperature is known. We found that the radial extend of the dust torus behaves comparable to the delay with respect to luminosity.
In the second project we compare concurrent changes of the dust radius to shape variations of broad emission lines (BELs) for NGC 4151 observed from 2004 to 2006. These simultaneous changes are discussed in a variety of dust and BEL formation schemes. Furthermore we use the shape variations to assess possible (especially azimuthal) cloud distributions, which could be responsible for the observed variations. A dust inflated AD provides the framework best suited to explain our findings. The changes in the BELs suggest that this dusty cloud formation happens in spatially confined areas on rather short timescales.
Supervisor: Joerg-Uwe Pott (MPIA)
Zdenek Prudil (Czech Republic) 27.04.2020
RR Lyrae stars as tracers of substructure and Galactic archaeology ( thesis pdf, 30 MB )
The submitted thesis encompasses several topics linked to the usage of RR Lyrae stars in various astrophysical applications related mainly to Galactic archaeology. In addition, projects related to pulsation and physical properties of RR Lyrae variables are also discussed, e.g., uncertainty in the mass of RR Lyrae stars and distortion of photometric light curves due to the shocks.
In our first study, we employed several classes of variable stars (including RR Lyrae variables) to examined a small overdensity found north from the Small Magellanic Cloud – SMCNOD. Using variable stars spatially associated with the SMCNOD we linked this overdensity with the Small Mag- ellanic Cloud. We also speculated about its origin from the Small Magellanic Cloud, due to one of the interactions with the Large Magellanic Cloud.
Two subsequent projects focused on the spatial and kinematical study of the Galactic bulge using the RR Lyrae stars. In the central region of the Milky Way, we found two distinct groups of RR Lyrae stars that differ in their pulsation properties. We associated them with the Oosterhoff groups pre- viously found in the Milky Way’s globular clusters. Both populations are evenly distributed in the Galactic bulge and neither of them can be spatialy and kinematically associated with the Galactic bar.
The RR Lyrae stars in the solar neighbourhood and their association with the Galactic disc is discussed in the following project. We found a small population of the local RR Lyrae stars (up to ≈ 3 kpc distance from the Sun) that kinematically and chemically resembles the thin disc population of stars. Our finding challenges our understanding of the Galactic disc formation, which if these RR Lyrae stars are truly members of the Galactic thin disc, happened more than 10 Gyr ago.
The last two studies focus on the properties of RR Lyrae stars as variable objects and expand our current knowledge about their pulsation and physical properties. First, we expanded the current number of candidates for binary systems among RR Lyrae stars. We analyzed the variation in their ephemerides and estimated the physical parameters of a possible binary companion. Second, for the first time, we provided an extensive photometric study of the atmospheric shocks among RR Lyrae stars. We link the prominence of shocks in RR Lyrae phased light curves with their pulsation properties and discuss their selective behavior inside the Instability strip.
Supervisor: Eva Grebel (ARI)
Xudong Gao (China) 06.02.2020
Low-mass Stellar Evolution Traced with Non-LTE Abundances ( thesis pdf, 5 MB )
The detailed chemical composition of stellar atmospheres can reveal the structure and evolution of the stellar interiors, otherwise hidden from direct site, as well as the structure and evolution of our entire Galaxy. The advent of several large-scale stellar spectroscopic surveys promises breakthroughs in our understanding of the physical processes that shape stellar surface abundances. However, the full potential of these extremely large and precise surveys is not yet being reached, as standard elemental abundance determinations today are based on the simplifying and incorrect assumption that the stellar atmosphere is in local thermodynamic equilibrium (LTE).
In this thesis I have employed non-LTE radiative transfer methods to tackle two outstanding astrophysical problems. The first problem is related to the chemical homogeneity in the open clusters, which for example is very important to understand how disrupted clusters have formed the Galactic disk and pinpoint the birth location of field stars. Abundance trends with stellar effective temperature have been found in all the analysed elements, indicating that the chemical abundance varies along with evolutionary phase past the turn-off. The overall agreement between our measured abundance patterns and the predictions by the stellar models with atomic diffusion and mixing, implies that the process of atomic diffusion poses a non-negligible effects during the main-sequence phase, which leads to the inhomogeneities in the abundances of open clusters.
The second problem is related to lithium evolution in low-mass main-sequence stars. The primordial elemental abundances predicted by Standard Big Bang nucleosynthesis (SBBN) generally show good agreement with observations. However, a glaring exception is the cosmic abundance of lithium, which SBBN estimates to be three times higher than what is observed in the atmospheres of metal-poor stars in the Galactic halo (i.e. stars on the so-called Spite Plateau). This long-recognized discrepancy has become known as the Cosmological Lithium Problem. In this thesis, I present observational evidence, based on a state-of-the-art non-LTE spectroscopic analysis of more than 100,000 stars from the large-scale spectroscopic “Galactic Archaeology with HERMES"(GALAH) survey, that the surface lithium abundances of these Spite Plateau do not in fact reflect their initial (SBBN) lithium abundances; rather, they have been depleted by a factor of three. This further strengthens the case for an astrophysical solution to the cosmological problem, reconciling tension with predictions of the SBBN.
Supervisor: Karin Lind (MPIA)
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)
Alexander Hygate (UK) 31.01.2020
The Physics of Cloud-scale Star Formation and Feedback Across Cosmic Time ( thesis pdf, 60 MB )
Stars are an important visible and massive constituent of galaxies. They form out of cool, dense molecular gas regions known as molecular clouds and in turn impact this gas by emitting energy and mass known as "stellar feedback". Therefore, understanding the formation of stars and the feedback they generate is crucial for understanding galaxy formation. As a result of modern telescope facilities, high sensitivity, high resolution (cloud-scale) imaging of molecular gas is becoming available in an increasing number of galaxies. Analysis of this data with matched resolution observations of recently-formed, massive stars allows the characterisation of the star formation process on the cloud scale. The "uncertainty principle for star formation" is a statistical method for measuring the relative duration and spatial distribution of evolutionary phases of the star formation process. When applied to observational images that trace molecular clouds and regions of young stars, the method measures the duration of molecular cloud lifetimes, the timescale of their destruction by stellar feedback and the mean separation length between star forming regions. In this thesis, I investigate the physics of star formation and feedback on the cloud scale and present contributions to the development of methods for this analysis. First, I assess the impact of noise, astrometric offsets and diffuse emission on measurements made with the "uncertainty principle for star formation". I present a physically motivated method for separating emission from compact structures and diffuse extended structure in an image. The method separates diffuse and compact emission via filtering in Fourier space, with a filter defined by the mean separation length between star forming regions. This method enables the determination of the molecular cloud lifecycle and the mean separation between star forming regions with the "uncertainty principle for star formation" in data containing a diffuse background component. Second, I present the application of the "uncertainty principle for star formation" to determine the lifecycles of molecular clouds in the nearby flocculent galaxy M33. These measurements indicate that clouds in M33 have lifetimes approximately once or twice the timescale for their collapse due to gravitational freefall. Subsequently clouds are dispersed by stellar feedback over a timescale that could allow the earliest supernovae to explode whilst still embedded in their natal clouds. Third, I present the decomposition of tracer images of the molecular and ionised gas in nine nearby galaxies into compact and diffuse components and thus determine the fraction of emission coming from these components. I then present a correlation analysis between these emission fractions and a number of parameters characterising the galaxies in the sample. Last, I summarise the work of the thesis and present some future prospects for extending analyses such as the work presented in this thesis to other galactic environments in the Nearby Universe and further out into cosmic history.
Supervisor: Diederik Kruijssen (ARI), Fabian Walter (MPIA)
Christos Vourellis (Greece) 21.01.2020
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
IN SEARCH OF DISKS IN HIGH-MASS STAR FORMATION ( thesis pdf, 35 MB )
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)