Talks, Seminars, Events

Abstract
In cosmological simulations, it is found that the degree of fragmentation of the Population III (Pop III) protostellar disk varies strongly from halo to halo. This is to be expected if turbulence within the cloud plays a role in determining when and where fragmentation actually occurs. It is also reasonable to expect that the amount of angular momentum present at small-scales will have a large impact on Population III protostellar disk formation and evolution. We conducted a numerical study with the Voronoi moving-mesh AREPO in which we examined how different levels of initial subsonic turbulence and rotation within the star-forming cloud affect the outcome of Pop III star formation in terms of the total number of objects formed, the overall mass spectrum and the individual accretion history. We used controlled initial conditions which examine the collapse of a gas cloud with pre-defined levels of turbulence or rotation within a small computational box. This allowed us to draw conclusions on the specific role, strength and importance of the particular physical parameters applied here. In this talk, I will present some of our results and discuss the relevance of our numerical approach in which we have performed several realizations per combination of initial conditions and have averaged over the realizations instead of just considering single, individual runs.

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Over the course of the past few decades, it has become clear that the class of metal-poor stars known as carbon-enhanced metal-poor (CEMP) stars are powerful probes of a number of areas of interest to contemporary astrophysics. I review the multiple lines of evidence that demonstrate the association of CEMP-no stars (which do not exhibit neutron-capture element enhancements) with the nucleosynthesis products of the very first stars, their likely birth place in low-mass mini-halos, and (once accreted by the halo) their role as tracers of the outer-halo population of the Galaxy. The CEMP-s stars (which exhibit enhancements of the heavy s-process elements), by contrast, are likely to have been born in more massive mini-halos, and serve as tracers of the inner-halo population. The well-known increasing frequency of CEMP-no stars (and newly recognized relative constancy of CEMP-s stars) with declining metallicity, and the identification of the primary groups in the Yoon-Beers diagram of A(C) vs. [Fe/H], provide the means to explore these associations in more detail, and to constrain numerical models of the formation of the Milky Way.

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This talk gives a brief overview on compact object mergers. The electromagnetic counterpart of the first unambiguously observed neutron star merger provides very strong evidence that compact object mergers play a major role for the Galactic enrichment by heavy elements formed through the rapid neutron-capture process. I will describe the different channels of mass ejection and nucleosynthesis in compact object mergers and sketch the goals of the corresponding SFB project.

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In 2021, ESO's 4m Vista Telescope will be converted into a spectroscopic survey facility, called 4MOST. With 2436 fibers, 4MOST will be able to obtain 812 high-resolution spectra (R=20,000) and 1624 low-resolution spectra (R=6,000) simultaneously in a field-of-view with 2.5 degrees diameter. In 2018, the project passed its critical design review; it has now entered the construction phase. The LSW is contributing two major sub-systems to 4MOST: the high-resolution spectrograph and the control software. The 4MOST Consortium is preparing a set of large surveys. One of them will obtain 1.5 million high-resolution spectra of Milky Way halo stars (PI: N. Christlieb, LSW). The ESO community will be invited in 2019 to propose additional projects for 4MOST. I will summarize the status of 4MOST, and provide an overview of the further path towards first light and scientific exploitation of the instrument.

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Chemical elements are produced by stars, making stellar generations always more metal-rich with time. Hence, stellar generations pass information from one to the other one via their chemical compositions. This information provides a fundament for the motivation to create a spectroscopic survey of stars in the Milky Way. The chemical composition of thousands of stars, combined with their dynamical properties and their ages, serve to constrain the models of formation and evolution of our Galaxy. In this talk, I will first review the state-of-the art of the precision and accuracy of such industrial stellar abundance measurements. Then I will present a novel method to visualise and interpret these datasets, which is based on phylogenetic trees, and has been so far tested in a dataset of higher precision and accuracy.

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Star formation is a hierarchical process where fragmentation and gas dynamical processes start on the largest galactic scales and continue down to the formation and accretion processes around the protostars. Starting with bar-spiral arm interfaces, then going step-wise to smaller scales of molecular clouds, star-forming regions, cores and potential (massive)accretion disks, I will present recent observational results related to the dynamical processes during cloud and star formation.

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During this seminar I will describe my research program aiming at a better understanding of the physics of supernovae and of the origin of nuclei by increasing the quality and predictive power of numerical models as well as nucleosynthesis calculations. Supernovae play essential roles in the frameworks of many branches of astrophysics: star formation, galaxy dynamics, high-energy astrophysics, galactic chemical evolution, and cosmology. In spite of their ubiquitous presence in astrophysics, there are many uncertainties related to progenitor systems, treatment of the explosions, cross section determinations at such high temperatures, and comparisons with spectra. Most popular results in the field of nucleosynthesis during explosions are still mostly based on one-spatial dimension calculations. The pioneering and very innovative aspect today is the possibility of coupling nucleosynthesis to multidimensional simulations of different type of supernovae. I will show recent results and future perspectives in multi-dimensional calculations of thermonuclear as well as core-collapse supernovae, using tracer particle method for nucleosynthesis. I will illustrate detailed comparison of 1D and 3D supernova models with nucleosynthesis calculations and discussing the needs of multi-D (and where it is needed). Despite the huge investments in nuclear physics experiments, theoretical studies establishing priority lists of reactions to be measured and precision required for astrophysics are currently very limited. During this seminar I will also discuss a priority list for future experiments and improvements in predictions of key nuclear reactions for explosive nucleosynthesis. My expertise in Galactic Chemical Evolution modelling lead to the possibility to study a dependence of the SNe yields on metallicity and their contribution over the galactic age up to reproducing the Solar System composition. During my talk I will refer different times to result of chemical evolution studies with the need of a more clear understanding of the impact of supernovae at the earliest stages of the evolution of galaxies, and their contribution to the Solar System composition. The wealth of information from galactic surveys makes this the ideal time for a theorist to understand the formation and evolution of galaxies with a new generation of chemical evolution models. To this goal, at the end of my talk, I will describe a novel project to model chemo-dynamical evolution of the cosmos, based on a N-body SPH RAMSES code making use of the framework on a moving mesh, adjusting automatically spatial resolution but using a large number of isotopes.

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Galactic globular cluster stars typically show light element abundance variations interpreted as signatures of multiple stellar populations. If multiple populations do indeed form exclusively in sufficiently massive clusters as our current knowledge suggests, they may be used to trace the globular cluster (GC) contribution to the field. Building on our earlier work, we analyzed approximately half a million stellar spectra from SDSS DR14 in order to identify halo field giants showing light element abundance anomalies traced by CN and CH. These chemical signatures are revealing candidates that were likely stripped from GCs. In fact, our search has more than quadrupled earlier samples, yielding the largest data set of candidate former globular cluster stars available so far. These data allow us to constrain the contribution of GCs to halo field stars. We find that the GC contribution is pronounced in the inner halo, but minor in the outer halo, which presumably formed mainly through the accretion of dwarf galaxies. We are now in the process of constraining the orbits and tracing the origin of our candidates using Gaia data.

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Globular clusters are the relics of extreme star formation in high-redshift galaxies. Their enormous potential as tracers of high-redshift galaxy formation is broadly recognised, but concrete applications of this link have remained out of reach. The key missing ingredient has been to construct an end-to-end model for star cluster formation and evolution in a cosmological context. I will review recent efforts towards formulating models for globular cluster formation and evolution during galaxy formation, showcasing the variety of techniques used, as well as their strengths and weaknesses. I will then present results from the E-MOSAICS project, in which we carry out fully self-consistent, cosmological zoom-in hydrodynamics simulations of the co-formation and evolution of globular clusters and their host galaxies. This work has led to two crucial insights. The first is that the formation of young massive clusters and old globular clusters can be described by a single modelling framework, showing that globular clusters are the relics of regular star formation in high-redshift environments. The second is that the high-pressure formation environment of globular clusters has shaped a wide range of their present-day properties, enabling their direct use as tracers of high-redshift galaxy growth. We demonstrate how globular cluster metallicities, masses, ages, kinematics, and spatial distributions provide a new and exciting window for reconstructing the host galaxy merger history, distinguishing between in-situ and ex-situ galaxy growth, and probing the conditions of cloud-scale star formation and feedback at high redshift. Specifically, I will demonstrate the power of unifying cluster formation and destruction processes across cosmic time by using the E-MOSAICS simulations to derive the formation and assembly history of the Milky Way, culminating in the reconstruction of its merger tree.

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The High Altitude Water Cherenkov (HAWC) observatory in Mexico (4100 m .a.s.l.) has just finished its first year of data taking and produced some interesting results. Because of its wide field of view it is currently a unique detector to study extended sources (>2 degree) at energies above a few TeV. One of the largest structures in the gamma-ray sky are the so-called Fermi bubbles, extending to the North and South from the milky way centre. At energies around 1TeV, and above, the HAWC observatory is in a unique position to make observations (or constrain the flux) of the bubbles. From the results of HAWC, modelling of the bubbles with a focus on the energy range above 500 GeV and improving the HAWC event reconstruction, both energy accuracy and flux sensitivity are expected to improve.

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Strong gravitational lenses can map an extended background source to several highly distorted and magnified images. Analysing the properties of those images yields important information about the distribution of the deflecting mass and the background source. Common approaches to reconstruct the source or the deflecting mass distribution model the global properties of the source and the lens. They obtain a consistent description of the entire configuration by refining the model until it matches the observation to a predefined precision. We develop a new approach to infer local properties of the gravitational lens and to reconstruct the source only using the properties of the multiple images without assuming a lens or a source model. In the talk, I will introduce the method and its applications in comparison to standard lens modelling methods. Since our leading principle to separate data-based information from model assumptions can also be applied to a broader range of research questions, I will conclude with an outlook how this ansatz can be transferred to other topics, based on my former experience searching for open star clusters in the HSOY catalogue.