Vorträge, Seminare, Ereignisse

Abstract
Magnetic fields are ubiquitous in the Universe. The Sun’s magnetic field drives the solar wind and causes solar flares and other energetic surface phenomena that profoundly affect space weather here on Earth. The first magnetic field in a star other than the Sun was detected in 1947 in the star 78 Vir. Today, we know that about 10% of these intermediate-mass and high-mass stars have strong, large-scale surface magnetic fields whose origin has remained a major mystery till today. In this talk, I will present the first 3D magneto-hydrodynamical simulations of the coalescence of two massive main-sequence stars and 1D stellar evolution computations of the subsequent evolution of the merger product that can explain the origin of strong magnetic fields in massive stars. I will argue that such magnetic massive stars are promising progenitors of those neutron stars that host the strongest magnetic fields in the Universe, so-called magnetars, and that may give rise to some of the enigmatic fast radio bursts.

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Celebration of the Awardees and short presentations by each of them.

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High-energy non-thermal astrophysical environments - such as Active Galactic Nuclei, Pulsar Wind Nebulae and Gamma Ray burst - involve magnetized relativistic flows, whose energy flux can be partly converted, at dissipation sites, into random relativistic motion of particles that lose their energy through a variety of non-thermal processes and give rise to the observed radiation. Understanding of these extreme environments requires a detailed description of the highly nonlinear interactions between plasmas, non-thermal relativistic particles and radiation taking place on a wide range of spatial and temporal scales. This has forced practitioners in the field to take a sectorial approach. On the one hand, models at the large scale can be obtained through relativistic magnetohydrodynamical (RMHD) numerical simulations using a fluid model, albeit neglecting the relativistic particle component. On the other, Particle In Cell (PIC) codes allow a deeper comprehension of kinetic phenomena taking place at much smaller scales. As a consequence, the resulting picture is incomplete and fragmentary. In this talk, I will discuss some of the present and future computational perspectives which intend to bridge this gap. This task requires the coupling between large scale dynamics with a detailed treatment of the microphysics at dissipation sites, an extremely challenging task that only now is becoming feasible thanks to the advancements of high performance computing.

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One of the possible ways of creating the supermassive black hole (SMBH) is hierarchical merging scenario. Central SMBHs at interacting and coalescing host-galaxies are observed as SMBH candidates at different separations from hundreds of pc to mpc. One of the strongest SMBHs candidates is ULIRG galaxy NGC6240 which was X-ray spatially and spectroscopically resolved by Chandra. Researching of central SMBHs merging in dense stellar environment allows to retrace their evolution from kpc to mpc scales. The main goal of our dynamical modeling was to reach the gravitational wave (GW) emission regime for the multiple BHs model. We present the direct N-body simulations with up to one million particles and relativistic post-Newtonian corrections for the SMBHs particles up to 3.5PN. From our models we found the upper limit of merging time for NGC6240 central SMBHs is less than ~50 Myr.

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I will start with a broad review of the field of star formation and galaxy evolution, and some pressing open questions. I will then dive into the star-forming interstellar medium (ISM), asking the question, what regulates the star formation process on galactic scales? I will discuss the multiphase structure of the ISM, heating-cooling processes, and turbulence, all of which may play an important \;role in regulating star formation.  \;I will focus on a particularly appealing theory in which far-UV radiation from massive stars introduces a natural \;self-regulation process for star-formation, and will present recent results \; (Bialy 2020) for the intimate link between star-formation rate and the far-UV radiation intensity in the ISM. I will conclude by discussing promising future directions: (1) charting new ways for constraining poorly known interstellar properties: turbulence, 3D ISM structure, the nature of low energy cosmic-rays, and (2) an ongoing effort to construct an improved star-formation model for next-generation cosmological simulations (i.e., IllustrisTNG successors).

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Globular clusters (GCs) exhibit star-to-star variations in specific elements (e.g., He, C, N, O, Na, Al) that bear the hallmark of high-temperature H-burning. These abundance variations can be observed spectroscopically and also photometrically, with the appropriate choice of filters, due to the changing of spectral features within the band pass. This phenomenon is observed in nearly all of the ancient GCs, and has recently been found in many younger clusters as well. Many scenarios have been suggested to explain this phenomenon, with most invoking multiple epochs of star formation within the cluster; however, all have failed to reproduce various key observations. I will review the state of current observations and outline the successes and failures of some of the main proposed models. The traditional idea of using the stellar ejecta from a first generation of stars to form a second generation of stars, while conceptually straightforward, has failed to reproduce an increasing number of observational constraints. I conclude that the puzzle of multiple populations remains unsolved, hence alternative theories are needed, and will present new HST and VLT/MUSE results that suggest that we may be finally closing in on origin of this enigmatic phenomenon.

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On December 3, 2020, the first (early) part of Gaia’s third star catalogue will be published. Based on 34 months of Gaia observations it contains a significant increase in the number of entries compared to Gaia DR2: position measurements of more than 1.8 billion stars as well as proper motions, parallaxes, and broad-band photometry for about 1.4 billion stars. Additionally, both the precision and accuracy of the astrometric data has significantly improved, especially the determination of proper motions. Together with the release of the star catalogue, four papers are published, impressively demonstrating the scientific potential of the Gaia EDR3 catalogue.

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Did you know that we can probe the core of a red-giant star better than the core of the Sun? Actually, we do thanks to oscillations that are sensitive to the core that can be observed on the stellar surface. Over the past decade this has led to several groundbreaking discoveries. In this talk I will discuss these discoveries and explain how it is possible to look into the core of a red giant and not into the Sun.

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The above was the subject of a Marsilius Project at Heidelberg University undertaken jointly by the art historian Prof. Henry Keazor and myself, triggered by my experiences in the DFG commission for investigations of scientific misconduct and his work on exposing art forgeries. I will present a summary of our findings on the different types of such misconduct across different disciplines, discuss the motivation of the perpetrators and ways of preventing misconduct, as well as talk about the consequences for science, scientific credibility, political and economic decisions, and public perception in general.

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Active Galactic Nuclei are an interesting source not only for photons in a very wide energy range, but also for cosmic rays and neutrinos. The emission of these different messengers is most likely connected to each other. To understand the physics behind AGN, emission models have to be constructed. Nowadays observational data is available from radio frequencies to TeV energies and neutrino observatories like IceCUBE may provide ultra-high energy neutrino counts. Unfortunately long-term simultaneous multi-wavelength observations are still scarce. I will present results from time-dependent and spatially resolved AGN emission models that allow for a self-consistent treatment of emission and particle acceleration within AGN. These models may help to distinguish different emission processes using the timing of lightcurves in separate energy bands. But they may also provide answers to questions like "What is the shape of an AGN jet?" or "Is there really one neutrino per one high-energy photon?"

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Over the past decade the space missions CoRoT, Kepler and TESS have revolutionised the field of asteroseismology ? the study of the internal structures of stars through their global intrinsic oscillations. A particular steep increase in our knowledge has been possible in stars cooler than ~ 6700 K, which exhibit convection in their outer layers. Oscillations are excited in these turbulent layers that allow us to probe large parts of the stars. In this talk I will present recent results of asteroseismic inferences of the stellar structure of these cool stars.

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Stellar feedback connects the smallest, densest scales of the galaxy to the largest, most diffuse components of the galaxy halo. The detailed, fractal structure of the interstellar medium (ISM) and the evolution of stars sets the local coupling efficiency of energy from stellar winds, radiation and supernovae. This feedback can then drive outflows from most galaxies, removing mass and metals to the diffuse, hot circumgalactic medium. This low-density halo can act as a reservoir for material, slowly allowing it to re-accrete and fuel ongoing star formation. This in turn influences the formation of new molecular clouds within the ISM, connecting the process of star formation to all scales of the galaxy's gas content. In this talk, I will present recent results that have helped to clarify the details as to how the details of these processes can impact the overall life of galaxies, and how we can use new observational and statistical methods to constrain these details.

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The diverse architectures of discovered exoplanetary systems have provoked an equally diverse range of explanations of the processes governing their formation and evolution. However, the star formation environment is a factor that is frequently overlooked in studies that aim to synthesise the observed planet properties. I review the growing evidence suggesting that environmental feedback plays a significant role during and after planet formation. Two mechanisms that may be particularly influential are external photoevaporation due to irradiation of the protoplanetary disc by neighbouring OB stars, and subsequently star-star encounters that can generate instability in the mature planetary system. I discuss observational constraints and examples of these mechanisms caught in the act, and how they might alter a system from that which forms ?in isolation'. Finally, I contextualise the Solar System in terms of possible environmental sculpting.

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Modern wide-field surveys will provide us with alert streams generating millions of alerts every night and by that providing a comprehensive picture of the variable sky. Traditionally, microlensing events have been found in the most crowded place in the Milky Way. Thanks to the footprint, microlensing events in the disk will play an increasingly important role. The opportunities opening up by these surveys will lead to more discoveries and the characterization of extrasolar planets (bound and unbound), black holes and insights into the binarity of stars and brown dwarfs. The most audacious surveys will certainly provide the best alert stream but some of the aforementioned science cases still require automated follow-up observations. The "fire-hose of alerts" demands cloud-based tools to trigger heterogeneous follow-up observations with a homogeneous software package - the TOM (Target and Observation Manager) Toolkit. We will show early results from the most recent observing programs running at Las Cumbres Observatory (LCO) global network using a TOM and how that can be applied to the surveys of the future.

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Direct N-body simulations are the only simulation method capable of properly resolving dense star clusters from the scales between close interacting stars in a binary all the way up to their interaction with distant halo stars. Nevertheless, they are frequently complemented with approximate Monte-Carlo methods, because these are much quicker computationally. Recently, there have been a number of code advances concerning the stellar evolution. This is crucial, as the evolution of individual stars has a huge effect on the overall dynamical evolution of a star cluster and thus amongst others the formation of gravitational wave emitting sources that may be detectable with LIGO and VIRGO. In this talk, I will highlight the state of stellar evolution routines in the direct N-body code NBODY6++GPU and the Henon-type Monte-Carlo code MOCCA by comparing the results of small star cluster simulations performed with both. The results of these are important groundwork to ensure that the upcoming million-body simulations of globular and nuclear star clusters are astrophysically up-to-date.

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The source of about half of the heaviest elements in the Universe has been a mystery for a long time. Although the general picture of element formation is well understood, many questions about the astrophysical details remain to be answered. Here I focus on recent advances in our understanding of the origin of the heaviest and rarest elements in the Universe.

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On September 2nd 2020, the LIGO-Virgo collaboration announced the detection of GW190521, a gravitational wave source associated with the merger of two black holes (BHs), 66 and 85 Mo masses, which left behind an intermediate-mass black hole (IMBH) with a mass of 142 Mo. This binary merger is peculiar because its primary mass falls in the so-called upper mass-gap, a region of stellar masses where modern stellar evolution predicts the absence of remnants, and the final remnant represents the first specimen of a "light" IMBH. In this seminar, I will describe a novel channel suitable to explain the properties of GW190521, namely a sequence of three mergers among stellar mass black holes. We discovered serendipitously such a process in a high-resolution N-body model of a dense star cluster. We combine these simulations with an analysis based on numerical relativity fitting formulae and on observed properties of globular, young, and nuclear clusters, to show that if GW190521 originated via such a mechanism its observation gives us insights on the distribution of stellar BH natal spins and on the environment that harboured such a system, most likely a dense and young star cluster.

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Supermassive black holes of million solar mass and above are commonly hosted by massive galaxies, but are also present in local dwarf galaxies. Black holes are a fundamental component of galaxies and galaxy evolution, but their origin is still far from being understood. Large-scale cosmological simulations are crucial to understand BH growth and their interplay with their host galaxies. We recently compared the black hole population of six of these large-scale cosmological simulations (Illustris, TNG100, TNG300, Horizon-AGN, EAGLE, and SIMBA) and I will review how the simulation sub-grid models affect the build-up of the BH population and their correlations with galaxies properties. The next two decades will be dedicated to the exploration of the high-redshift Universe with upcoming space missions such as LynX, Athena, JWST, WFIRST, and LISA. I will present how we can use cosmological simulations to prepare these missions and maximize their scientific return.