Vorträge, Seminare, Ereignisse

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The Milky Way consists of a large number of star clusters, with over 150 being labelled as old,metal poor globular clusters and thousands being labelled as young, metal-rich open clusters. However thesenumbers are small compared to the total number of clusters that have ever existed in the Milky Way, as mosthave fully dissolved between their time of formation and the present day. Most of the constraints that havebeen placed on the Milky Ways dissolved star cluster population have been made by extrapolating how thestar cluster initial mass function, initial size function, and formation rate have evolved over time. In thistalk, I will introduce some more direct ways of studying dissolved star clusters that make use of internal starcluster dynamics, orbital dynamics, chemical tagging, and high dimension analysis. More specifically I willintroduce a new method for constraining progenitor cluster properties through deep observations of stellarstreams, demonstrate how stellar siblings can be identified through the combined use of orbital dynamics,chemical tagging, and discuss a new particle spray code that can be used with high dimensional analysis tofind extra-tidal stars.

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Since their first discovery about 20 years ago, luminous quasars at cosmic dawn have proven to be exceptional tools to investigate how massive black holes formed, how the first massive galaxies and large-scale structures assembled, and what physical processes shaped the Universe at its last phase transition (the epoch of reionization). In this talk I will review how the search for a new quasar frontier has evolved in the last twenty years, and address some of the lessons we learned so far from the discovered quasars. I will discuss some of the new and old (but lingering) open puzzles. Finally, I will provide an outlook from a biased observer's perspective on what we should expect from the field in the coming years, as new facilities offer unprecedented, exciting opportunities to study massive black holes in the infancy of galaxy formation.

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Globular cluster formation is a major unsolved problem in astrophysics. The latest constraints tothe problem have come from puzzling abundance variations of light-elements among their stars. The pursuitto explain this longstanding problem using these chemical signatures has reinvigorated the study of globularclusters, and at the same time has challenged our understanding of nucleosynthesis and stellar evolution. Forthis talk, I will start with an overview of the challenges facing current models of globular cluster formation.Then I will present the steps being taken to build the next generation of globular cluster formation modelsand discuss how we can use the properties of globular cluster to trace the build-up of galaxies.

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Winds from massive stars have velocities of ~1000 km/s or more and produce hot, high-pressure gas when they shock. In the traditional, spherical model of these stellar winds, this high-pressure gas can act to quickly disperse the dense gas characteristic of regions where massive stars are born, acting to halt star formation. However, this classical theory is inconsistent with observations of wind-driven bubbles in the nearby universe and the observed high star formation efficiencies of super star clusters. I develop a new theoretical model for the expansion of stellar wind-driven bubbles that accounts for the turbulent structure of the surrounding gas. A key feature is the fractal nature of the hot bubble’s surface. The large area of this interface with surrounding denser gas strongly enhances energy losses from the hot interior, enabled by turbulent mixing and subsequent cooling at temperatures T ∼ 104–105 K, where radiation is maximally efficient. Due to this cooling, the solution is momentum-driven rather than energy driven, with resulting pressures in the shocked wind that are lower by up to a factor of 100. I explore the implications of such a theory and present a large suite of three-dimensional, hydrodynamical simulations that have been run to evaluate and test this theory. I also present simulations of self-consistently star-forming clouds where star formation is regulated solely by stellar wind feedback. These simulations allow us to test our theory in a more realistic context as well as track how wind material cools and collapses into subsequently formed stars.

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Cosmological models suggest that Milky Way-like galaxies are made up in part of stars that formed in situ and in part of stars that formed in other, smaller galaxies and that were subsequently accreted. Most of the more massive merger events should have occurred more than nine or ten billion years ago. In combination with massive ground-based photometric and spectroscopic surveys, Gaia is confirming and refining the cosmological picture. These data have uncovered numerous stellar tidal streams in our Galaxy, not all of which have known progenitors. Many come from disrupted dwarf galaxies, others from dissolving globular clusters - Gaia permits us to trace the detailed assembly history of our Galaxy, revealing the type of objects, their numbers, their properties, and the time of accretion. The most spectacular discovery is arguably that of the fairly massive dwarf galaxy Gaia-Enceladus or Gaia Sausage, which merged with the Milky Way about 10 Gyr ago. This event contributed many globular clusters and likely triggered the formation of the thick disk. In fact, Gaia data suggest that possibly half of our globular clusters come from merger events. Also, Gaia reveals the orbits of the surviving satellites, providing clues to their origins and future merger history.

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The ROME/REA project (2017-2020) aimed to discover extrasolar planets by regularly monitoringmillions of stars in the Galactic bulge and looking for ongoing microlensing events. From April to Septembereach year, when the Galactic bulge was visible from the Southern hemisphere, the robotic telescopes of theLas Cumbres Observatory were used to observe a total area of about 4 square degrees in the sky in threedifferent bands. An automated process assessed ongoing microlensing events in real time for their sensitivityto planetary signals and additional observations were requested to characterize signals of particular scientificinterest. Our final catalog of stars contains more than 4 million individual sources. As we prepare for our firstpublic data release, I will present some of the results, talk about the data products we will soon be releasingand describe our current work and plans for the future.

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High Mass X-ray binaries (HMXB) with black hole companions represent a key intermediate stepin the binary black hole formation channel. Detailed knowledge of the stellar and wind parameters of thedonor is essential to understand the complex behavior of such systems. Our current understanding of wind ofmassive stars in black hole binaries is mostly based on Cygnus X-1 in our Galaxy and was not analyzed bymeans of sophisticated stellar atmosphere models. In this study, we carried out a detailed analysis of windsof doner star in M33 X-7 as well as a re-analysis of Cygnus X-1. M33 X-7 is the only known eclipsing blackhole binary with a very massive O supergiant donor and one of the most massive black holes known in anHMXB. In this talk, I will present a detailed spectroscopic analysis (Xray+UV+optical) of the massive donorduring different orbital phases. This sheds light on the stellar and wind parameters of the metal-poor donorstar. The observed properties are compared with detailed binary-evolution tracks to constrain the possibleformation channel and evolutionary fate of the system.

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I present populations of satellite galaxies in a Lambda-CDM context using the IllustrisTNG suite of cosmological magneto-hydrodynamical simulations. Utilising the entire range of IllustrisTNG allows for an unprecedented combination of statistical sample size and numerical resolution, resulting in mass ranges that cover multiple orders of magnitude for both host and satellites samples, as well as the first statistical sample of 198 high-resolution Milky Way-/Andromeda-like (MW/M31) hosts. I discuss the galaxy-halo connection for satellite and central galaxies across the mass spectrum in the stellar-to-halo mass relation as the most fundamental relationship of galaxy evolution in the cosmological standard model. I analyse the abundance of past and present-day satellite and subhalo populations around MW/M31-like hosts, find a remarkable degree of diversity, and put them into context with both observational surveys and previous simulations of similar systems. Their satellites become increasingly quenched towards smaller stellar masses as they lose their gas reservoirs more easily after infall. Thus, I not only give a detailed view on the evolution of satellite galaxies after infall and the environmental effects they experience but overcome one of the remaining challenges to the Lambda-CDM model: there is no missing satellites problem according to IllustrisTNG.

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The semi-analytic Just-Jahreiss (JJ) model of the Galactic disk has been recently calibrated by us in the Solar neighborhood against the Gaia DR2 stars. We identified two star formation (SF) bursts that happened during the recent 4 Gyr. Now we present a global version of the JJ model applicable to a wide range of Galactocentric distances. This generalized JJ model includes exponential thin and thick disk and also atomic and molecular gas layers, as well as the flattened stellar halo and DM halo in the form of a cored isothermal sphere. The overall thickness of the thin disk is assumed constant at all radii, but flaring can be also modeled. The radial variation of the thin-disk star formation rate (SFR) reflects the expected inside-out disk growth scenario. Motivated by our findings for the Solar neighborhood, we allow the smooth power-law SFR to be modified by an arbitrary number of additional Gaussian peaks. Also, the vertical kinematics of the stellar populations associated with these episodes of the SF excess can differ from the kinematics prescribed by the age-velocity dispersion relation for the thin-disk populations of the same age. Using the observed metallicity distributions of the APOGEE Red Clump giants, we constrained the thin- and thick-disk age-metallicity relation for the distances 4 - 14 kpc. The public code of the JJ model is complemented by the two sets of isochrones, PARSEC and MIST. The generalized JJ model is a new stellar population synthesis tool that can be useful for a variety of tasks of Galactic archaeology, including the reconstruction of the Milky-Way disk formation history.

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The cycling of matter in galaxies between molecular clouds, stars and feedback is a major driverof galaxy evolution. However, it remains a major challenge to derive a theory of how galaxies turn their gasinto stars and how stellar feedback affects the subsequent star formation on the cloud scale, as a functionof the galactic environment. Star formation in galaxies is expected to be highly dependent on the galacticstructure and dynamics, because it results from a competition between mechanisms such as gravitationalcollapse, shear, spiral arm passages, cloud-cloud collisions, and feedback processes such as supernovae, stellarwinds, photoionization and radiation pressure. A statistically representative sample of galaxies is thereforeneeded to probe the wide range of conditions under which stars form. I will present the first systematiccharacterisation of the evolutionary timeline of the giant molecular cloud (GMC) lifecycle, star-formation andfeedback in the PHANGS sample of star-forming disc galaxies. I will show that GMC are short-lived (10-30Myr) and are dispersed after about one dynamical timescale by stellar feedback, between 1 and 5 Myr aftermassive stars emerge. Although the coupling efficiency of early feedback mechanisms such as photoionisationand stellar winds is limited to a few tens of percent, it is sufficient to disperse the parent molecular cloudprior to supernova explosions. This limits the integrated star formation efficiencies of GMCs to 2 to 10 percent. These findings reveal that star formation in galaxies is fast and inefficient, and is governed by cloud-scale, environmentally-dependent, dynamical processes. These measurements constitute a fundamental testfor numerical sub-grid recipes of star-formation and feedback in simulations of galaxy formation and evolution.

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Direct N-body computer simulations of the evolution of star clusters take into account all necessary - strong and soft - gravitational encounters between stars in the system. Since the times of von Hoerner and Wielen the ARI has been a place of cutting edge research in this topic. After a brief historic introduction I will introduce the current state of direct N-body simulation at the example of our mostly used code NBODY6++GPU, which is a research instrument having for our team the similar role as an observing instrument. Since a full simulation of a globular cluster over its lifetime of 12 billion years takes as many floating operations as the largest cosmological N-body runs, this is also partly a story of development of supercomputers and general purpose computing on graphics cards (GPU). The current record are models of star clusters with one million bodies (DRAGON simulations). Ideas are discussed how we will proceed in the future and why we need even more particles, e.g. to do proper modeling of nuclear star clusters. Depending on time one or two fields of current applications, done within our local and international team, will be shown and discussed: (i) evolution of black holes, their relativistic dynamics, and (ii) bound and free-floating planets in star clusters.

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Massive stars play a dominant role in the energetics of their host galaxies, primarily by their radiation-driven winds enriching their local stellar environments and by ionizing radiation. A subset of the massive stars, the Wolf-Rayet stars, which are direct progenitors of stellar-mass black holes, have particularly strong stellar winds. These winds are so powerful that they effectively push away the outer layers of the Wolf-Rayet star, obscuring it from sight. Hence, only the stellar wind can be observed from Earth. To infer stellar parameters, one needs to rely on a proper modelling of the winds of these stars. In this talk, I will show the deficiencies of the current wind modelling for Wolf-Rayet stars along with accompanying uncertainties on stellar parameters with solutions to construct more accurate models.

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The highly praised ESA Gaia satellite mission has already provided the astronomic communitywith high quality astrometric, photometric and other data for almost 2 billion stars, and will continue to do sofor the next years. As time goes by, the precision of the astrometry increases with the number of measurementsand the time-span during which these are obtained, growing. Thus the correction of systematic effects in thedata, such as aberration need to be corrected to a point, where the conventional means do not suffice anymore.To accomplish this, a programme was conceived, to track the satellite with highly precise (20 mas) groundbased astrometry to deliver the required data for the optimisation of Gaia’s accuracy, called Ground BasedOptical Tracking (GBOT). This programme has faced many challenges and uncertainties, as well as set backs,but finally GBOT has come to the point, where its data are being included in the processing of the Gaiaastrometry, since 2020.This presentation will give an overview of the history of GBOT, and the steps taken to ensure final success,after many years of challenges. I will also report on a project searching for asteroids on the existing GBOTdata, which has lead to observations of about 42,000 objects, of which about 18,000 were previously unknown.

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Gas and dust in inclined orbits around binaries experience precession induced by the binary gravitational torque. The difference in precession between gas and dust alters the radial drift of weakly coupled dust and leads to the formation of dust traffic jams where the radial drift is minimised. I explore this new phenomenon using 3D SPH simulations and investigate its dependence on disc initial inclination and binary eccentricity. I will then present a new dust evolution model that takes the mutual gas and dust inclination into account and reproduce the SPH results, which provides a straightforward way to understanding dust traffic jams as a consequence of the drag torque exerted by the gas on the dust. Finally, I will present the results of radiative transfer post-processing of the hydro simulations and discuss possible observational implications of these dust traffic jams.

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The processes of star formation and feedback take place on the cloud scale (~100pc) within galaxies and play a major role in governing galaxy evolution. The properties of the clouds in which stars form are set by the large scale environment of their host galaxies, directly linking the initial conditions of star formation to galactic-scale properties. In turn, the energy, momentum,and mass deposited by stellar feedback drive the continuous evolution of the interstellar medium at large. Characterising the physical mechanisms regulating this multi-scale cycle is therefore crucial to understand the evolution of galaxies. By applying a new statistical method to the high-resolution CO and narrowband-Halpha imaging from the PHANGS survey, we systematically measure the evolutionary timeline from molecular clouds to exposed young stellar regions on the scales of giant molecular clouds across an unprecedented sample of 54 main sequence galaxies. We find that clouds live for about one dynamical time (8-30 Myr) and are efficiently dispersed by stellar feedback within 1.2-5.1 Myr after the star-forming region has become partially exposed. These ranges do not indicate uncertainties, but reflect physical galaxy-to-galaxy variation, implying an important dependence of these timescales on the local conditions, shaped by the galactic environment. The statistically representative PHANGS sample covers a large range of galaxy properties and morphologies, which allows us, for the first time, to quantitatively link galactic-scale environmental properties to the small-scale evolutionary cycle of molecular clouds, star-formation, and feedback. I will present the first census of these multi-scale trends. These results enable the characterisation of the physical mechanisms regulating cloud assembly, star formation, and cloud disruption, which eventually participate in driving galaxy evolution, as a function of the galactic environment.

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High-mass stars are very important to many areas of Astronomy. These objects deeply impact their surroundings through their powerful winds and their deaths as supernovae. Therefore,understanding the behavior of such stars is essential to understand their impacts on their hostgalaxies' properties and history.The aim of this research project is to analyze the atmospheres of B supergiants (BSGs, evolvedmassive stars) using the CMFGEN (Hillier & Miller 1998), a 1D, non-LTE atmosphere code — which is one of the state-of-the art tools used to analyze hot stars. The focus of the project is to investigate whether more recent models (e.g., the inclusion of x-rays, clumping, more recent atomic data) can better explain the optical and UV observed spectra of these stars, since previous studies failed to model several important UV lines (Crowther et al. 2006; Searle et al. 2008).As results we obtained (i) an overall improved agreement between BSGs observed and model spectra at the UV considering the effects of clumping and x-rays in the wind. Also we noticed (ii)important differences in their properties between hot (B1 – B0) and warm (B2 - B5) BSGs were also found, and it is in agreement with recent hydrodynamical simulations, such as Driessen et al.(2019). Beyond that, (iii), we have found a general trend of the CNO abundances for BSGs compatible with previous works in the literature and to the current high-mass stellar evolutionpredictions. However, (iv) despite a decrease in terminal velocity at the Bi-Stability Jump, we found no increase in mass-loss, instead, we have found a slightly decreasing trend towards later spectral types.