Talks, Seminars, Events

Abstract
The theoretical description of stars is plagued with severe scale problems: the many physical processes at play act on vastly different spatial and temporal scales. The classical approach to deal with this problem has been to formulate the underlying equations assuming spherical symmetry and hydrostatic equilibrium thus allowing for (numerical) solutions that explain the main phases of stellar evolution and reproduce many observables. This success, however, came at a price: casting inherently multidimensional physical processes in a one-dimensional framework required strong parametrization and sometimes even tinkering with the underlying physics. This diminishes the predictive power of stellar models and improvements are required to interpret and guide current and future observations. The next generation of stellar models clearly has to be build on multidimensional simulations. Are current (super-)computational resources sufficient to meet this challenge? What numerical techniques are required to enable simulations of challenging multi-physics, multi-scale stellar models? I will discuss three examples where a one-dimensional treatment clearly fails: convection and hydrodynamical instabilities ins stellar interiors, stellar explosions, and binary stellar evolution. These cases illustrate how the rapid evolution of computational astrophysics enables progress in stellar modeling.

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The Galactic archeology is trying to understand the formation history of the Milky Way. To study the early Milky Way history one can use old population stars that carry information about their past in their current dynamical and chemical properties, which makes them invaluable in studies of the Milky Way formation history. In my research, I use old (< 10 Gyr) variable stars of the RR Lyrae class, which serve as standard candles and tracers of metallicity within the Local Group. In my talk, I will discuss their use for studying the northern stellar overdensity near the Small Magellanic Cloud (SMCNOD) together with their spatial and kinematical distribution in the Galactic bulge and Galactic disk. I will also address some of the open questions related to RR Lyrae variables, like the Oosterhoff dichotomy and the uncertainty in their mass due to the lack of RR Lyrae stars in binary systems.

Abstract
Massive stars play a crucial role in the Universe. They shape their surroundings by injecting large amounts of energy and momentum and they produce new, heavy elements that are the building blocks of new stars, planets, and life. They are usually observed in close binaries. Due to the lack of observations covering the earliest stages of their lives, the formation process of massive (binary) stars is poorly understood. I will present observational studies of the outcome of massive star formation. I will show the first spectroscopically confirmed population of massive pre-main sequence stars in the giant HII region M17 where we measured their temperature, luminosity, radius, and projected radial velocity. I will discuss their multiplicity properties and show that the young stars in M17 have a very low radial velocity dispersion in comparison to somewhat older stellar clusters of similar mass. I will present evidence for the hypothesis that massive stars are formed in binaries with wide orbits that shrink in the first few million years of evolution. Finally, I will present ongoing projects to test the wide binary hypothesis.

Abstract
The ``Lambda cold dark matter'' (LCDM) cosmological model is one of the great achievements in Physics of the past thirty years. Theoretical predictions formulated in the 1980s turned out to agree remarkably well with measurements, performed decades later, of the galaxy distribution and the temperature structure of the cosmic microwave background radiation. Yet, these successes do not inform us directly about the nature of the dark matter. Indeed, there are competing (and controversial) claims that the dark matter may have already been discovered, either through the annihilation of cold, or the decay of warm, dark matter particles. In astrophysics the identity of the dark matter manifests itself clearly on subgalactic scales, including the dwarf satellite galaxies of the Milky Way and especially less massive dark matter halos, too small to have made a galaxy. I will discuss predictions from cosmological simulations assuming cold and warm (in the form of sterile neutrinos) dark matter and show how forthcoming astronomical observations can conclusively distinguish between the two.

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Dwarf elliptical galaxies are commonly presumed to be simple systems: non star-forming and with a smooth spatial distribution of light. In this talk, we present a sample of 6 dwarf galaxies from the Virgo cluster which, at first sight, seem to be stereotypically passive and elliptical. However, through the application of a newly developed method, we are able to reveal spectacular spiral features which lay buried within the much brighter diffuse component of these galaxies. As a result, we find that for our sample of passive dwarf galaxies the spiral arms contribute ~2-14% of the total galaxy light within 2 effective radii. Next, we construct an hypothesis that explains the presence of these hidden features by performing high-resolution simulations of dwarf galaxies being thrown into a cluster potential. It is possible to reproduce the observed spiral features through the tidal-shocking of a thin, cold, and highly rotationally-supported stellar disk that plunges deep into the cluster core. This result implies that some passive dwarf galaxies may have a faint thin disk buried within a more luminous thick disk, the former only revealing its presence by forming spiral features after being subjected to tidal shocks.

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Giant molecular clouds (GMCs) are the home of the most extreme conditions and the most dramatic events found in the interstellar medium (ISM). As hosts of the densest, coldest portion of the ISM’s gas, gravitational collapse is inevitable, and leads to the formation of star clusters. These young star clusters, in turn, host massive and luminous stars that profoundly alter — and ultimately destroy — their birth clouds, by a combination of photoevaporation, radiation forces on dust, and strong shocks from winds and supernovae. Because GMCs are porous, the energy injected by massive stars also escapes to power the surrounding ISM. Given the complex array of processes involved, numerical simulations are essential to developing quantitative models of the lives and deaths of star-forming GMCs. In this talk, I will describe results from recent radiation (magneto-) hydrodynamic simulations that have helped us to understand how star-forming GMCs self-regulate, while simultaneously regulating the thermal, ionization, and turbulent states of the distant diffuse ISM.

Abstract
The presence of young stars can have a profound effect on the ongoing process of star formation in a molecular cloud. In particular, the light emitted by them can heat up or even ionise regions of the cloud and alter their dynamics and the future production of stars. Therefore, the numerical study of star formation could not be complete without accounting for the effects of radiative feedback. One of the accurate ways of modelling the propagation of light through a cloud is by using Monte Carlo radiative transfer (MCRT). I will present a new numerical scheme coupling MCRT and particle-based hydrodynamics to model ionising stellar feedback, and the expansion of the resulting H II region. The scheme has been thoroughly tested and further applied to a simulation of a star-forming cloud in the Central Molecular Zone (CMZ) of the Milky Way. Additionally, I will present synthetic emission maps of the simulated CMZ cloud prior to the predicted ionisation event, and compare them to observations.

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The accretion of Earth is a mysterious era in geologic history without any surviving rock record. However, this is when Earth's bulk geochemistry and geophysical structure was established. Using sophisticated astrophysical-geological modeling, we can reconstruct this era, date important events such as the Moon-forming event, and determine basic characteristics of the nascent protoplanetary disk. By considering the consequences for Earth, we can better understand the stark contrast of Venus--a planet without a planetary magnetic field, without plate tectonics, and without a Moon. This earliest eon which lies at the intersection of astrophysics and geology had profound consequences for life on Earth that are just beginning to be understood.

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KoCo Signature Speaker

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KoCo Signature Speaker

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