Vorträge, Seminare, Ereignisse

Abstract
I will carefully explain what the published astrometric Gaia data for any given star mean in detail, and how they can be used in practice. In particular, I will elaborate on the precision estimates and on the quality/reliability flags given for each star in the Gaia data releases. In addition, I will give a preview of the forthcoming epoch astrometry data, i.e. of the individual astrometric measurements for all Gaia sources, to be published for the first time in Gaia DR4 in 2026. Since Gaia mainly does one-dimensional measurements, the structure and usage of these epoch data are not intuitive for an astronomer.

Abstract
The large collecting area, diffraction-limited optics, low background, and continuous near- to mid-infrared sensitivity of the NASA/ESA/CSA JWST gives it unique capabilities for making detailed studies of the early stages of galactic star and planet formation, including dense embedded young clusters, passive and actively-irradiated circumstellar disks, and shock-excited jets and outflows. In this talk, I will present an overview of initial results from my GTO  programme as an Interdisciplinary Scientist on the JWST Science Working Group. These include imaging and spectroscopy of the inner Orion Nebula and Trapezium Cluster, and of the young low- and intermediate-mass protostellar outflows HH211, HH212, and HH288. Some of the discoveries made so far, including candidate Jupiter-mass binary objects in Orion, are controversial, while others remain mysterious, awaiting further analysis and follow-up observations.

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There is no one way to live a life. How true this statement is also for stars is well reflected in the zoo of H-deficient stars, and strikingly beautiful and diverse planetary nebulae morphologies. In this talk, I will explore how late thermal pulses, stellar mergers, Type Ia supernovae, and magnetic fields can dramatically alter the observable characteristics of evolved stars. Understanding these processes helps us piece together the various evolutionary pathways that sun-like stars can follow.

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Massive stars drive galactic chemical evolution and are precursors to compact objects and gravitational wave sources. Our understanding of massive stars relies on matching observations to detailed synthetic observables computed using methods that model the star's atmosphere and winds. Traditionally, most modeling efforts of hot massive stars presume 1D spherically symmetric, steady-state atmospheres. These simplifications allow for more computational resources to be spent on accurate NLTE radiative transfer calculations. However, newer models suggest that these simplifications are not sufficiently accurate for stars at the higher end of the mass scale. In reality, multi-dimensional effects create turbulent regions with complex density and velocity structures within both the atmosphere and wind of the star. Our new method simplifies the complex NLTE radiative transfer but incorporates these multi-dimensional, time-dependent dynamics. This approach allows us to capture complex behaviors that have implications for the general atmospheric and wind structure. Applying our method has yielded several successful results. After explaining our methodology, in this talk I will highlight how multi-D models explain the winds of WR stars and predict macro-turbulent broadening of O-stars.

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In the context of low-viscosity protoplanetary discs (PPD), the formation scenarios of the Solar System should be revisited. In particular, the Jupiter-Saturn pair has been shown to lock in the 2:1 mean motion resonance while migrating generally inwards, making the Grand Tack scenario impossible. We explore what resonant chains of multiple giant planets can form in a low-viscosity disc, and whether these configurations can evolve into forming the Solar System in the post gas disc phase. 
We found that resonant chains formed in low viscosity discs are hardly stable, because the gaps opened by giant planets are wider and deeper, hence reducing the damping effect of the disc on the resonances.
Exploring numerous configurations, we found five stable resonant chains of four or five planets. In a thin (cold) PPD, the four giant planets revert their migration and migrate outwards. After disc dispersal, under the influence of a belt of planetesimals, some resonant chains undergo an instability phase while others migrate smoothly over a billion years. For two of our resonant chains, about ?1% of the final configurations pass the four criteria to fit the Solar System. The most successful runs are obtained for systems formed in a cold PPD with a massive planetesimal disc.
This work provides a fully consistent study of the dynamical history of the Solar System's giant planets, from the protoplanetary disc phase up to the giant planet instability. Although building resonant configurations is difficult in low-viscosity discs, we find it possible to reproduce the Solar System from a cold, low-viscosity protoplanetary disc.

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To arrange a visit with the speaker during the visit, please contact their host: Fabian Schneider

Abstract
This talk has two topics. The first part is about planetary systems in star clusters. Several tens of planetary systems, including our Solar System, contain both planets and debris structures. Most stars are believed to be born in clustered environments, such as in star clusters. In such environments, debris discs evolve through interactions with stellar neighbours and planets. I use gravitational N-body simulations to investigate how the joint effect of star cluster environments and planets affects the dynamical evolution and stability of debris discs. I focus on how (i) the presence of a planet, (ii) the density of the star cluster, and (iii) the orbit of host stars within the cluster affect the stability and evolution of debris discs, as well as the characteristics of escaping particles and remaining discs. The second part of my talk is about globular clusters. They are abundant in galactic disks and spheroids, serve as ideal laboratories for studying stellar evolution alongside Newtonian and relativistic dynamics. The previous study of Dragon-II (Arca Sedda et al. 2023) successfully revealed astrophysical details of these dynamical systems, including gravitational wave signals from compact object mergers that would be measured by LIGO/Virgo/KAGRA. As a continuation of DRAGON-II, I present the DRAGON-III project and report on its preliminary results, which focuses on the simulations of million-body globular clusters and million-body nuclear clusters over 10 Gyr.

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KoCo Signature Speaker

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Binary interactions shape the evolution of the most massive stars, leading to significant deviations from the evolutionary pathways possible in single star evolution. These processes impact the universe at large scales and result in high energy events such as peculiar supernovae and gravitational wave sources. To understand these outcomes, it is important to assess binary evolution in early stages ranging from pre-interaction, roche-lobe overflow and post-interaction phases. I will discuss the current progress in our understanding of mass-transferring binaries, covering the impact of this process on the donor star (with the possible production of a stripped star), as well as the response of its companion. Of particular importance in recent years is the identification of bloated stripped stars caught immediately after interaction which provides a snapshot of the end-states of mass transfer, and I will discuss how their properties constrain orbital evolution and the efficiency of mass transfer. I will also emphasize that many of the uncertain processes in massive binary star evolution can also be assessed through the study of intermediate mass systems, for which the physics in early evolutionary phases does not differ significantly. To arrange a visit with the speaker during the visit, please contact their host: Jaime Villaseñor (MPIA)

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Giant low surface brightness galaxies (gLSBGs) have the largest discs in the Universe with the radii up to 130 kpc. The formation of such enormous discs is a stress-test for the hierarchical galaxy formation paradigm and without clarifying it we cannot paint a coherent picture of galaxy evolution. In the talk I will give the answers to the following questions. How rare are gLSBGs? What are the formation scenarios of gLSBGs? And how does it all correspond to the results of modern cosmological simulations? These answers are based on both in-depth study of 8 gLSBGs, including the results of our deep spectroscopic and photometric observations, HI data collected in the framework of our observing programs and complemented by archival datasets. Finally, we used deep optical images from HSC Subaru Strategic Program and publicly available redshift catalogs, estimated the volume density of gLSBGs in the local Universe and compared it to state-of- the-art numerical simulations.

Abstract
To arrange a visit with the speaker during the visit, please contact their host: Dominika Wylezalek

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DRAGON star cluster simulations have provided the first fully realistic long-term simulations of globular star clusters, have reproduced LIGO/Virgo observed binary black hole mergers, and are now entering into the next phase to simulate more massive, young and nuclear star clusters. They are based on the direct N-body simulation code Nbody6++GPU. In the talk an introduction and overview to direct N-body simulation and DRAGON simulations is given. Two current new applications are then shown, first initially very dense star clusters which form quickly an intermediate mass black hole of order 50.000 solar masses, which could be a seed for massive black holes in the early universe. Second, a still ongoing project is discussed, in which an already pre-existing supermassive black hole in a nuclear star cluster is followed, how it tidally disrupts stars, and captures low-mass stars, white dwarfs, neutron stars and stellar mass black holes.

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