Talks, Seminars, Events

Abstract
The giant molecular cloud (GMC) lifetime provides an upper bound on the local time-scale for star-formation, linking cloud-scale and sub cloud-scale physics to galaxy-scale trends in the star formation rate. Conversely, the galactic environment plays an important role in setting the cloud lifetime, leading to a complex interplay of physical mechanisms over a range of scales in the interstellar medium (ISM), from galactic dynamics to small-scale turbulence and feedback. Previous theories of GMC lifetimes have made predictions based on just one mechanism of cloud evolution, relevant only in a fraction of Galactic and extragalactic star-forming environments. That approach is inconsistent with recent observations, which show that a diverse range of entities are observationally-identifiable as clouds, and reveal environmentally-driven correlations between their gravitational boundedness and the galaxy-scale star formation rate. I present an analytic theory for GMC lifetimes, dependent on the large-scale dynamical environment of the ISM, including its local gravitational stability, cloud-cloud collisions, epicyclic perturbations, galactic shear, and interaction with galactic bars and spiral arms. Our analytic predictions depend on just five observable properties, accessible through measurements of the rotation curve, surface density and velocity dispersion of the host galaxy, and are applicable over a wide range of redshifts. In this contribution, I will present predicted cloud lifetimes and properties across a range of galactic dynamical environments. I will compare these results to hydrodynamic simulations performed using the moving-mesh code Arepo, where the influence of dynamics is combined with sub-cloud physics such as supernova feedback, HII-region feedback, and ISM chemistry. These theoretical and numerical results are consistent with pioneering observational results currently obtained with ALMA. Together, this combination of analytic, numerical and observational results show that the galactic dynamic environment plays a crucial role in determining GMC lifecycles and thus the star formation rates of their host galaxies.

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Massive black hole merger excite strong gravitational waves that travel throughout the Universe. Currently, about two events per week are being detected. Next to the associated amazing physics of merging black holes and gravitational waves, the question emerges: which astrophysical processes can lead to merging black holes, and which messages are encoded in the five fundamental properties measured in these events (the two masses, spins, and the distance)? An attempt to answer this leads to the physics of stars, supernovae and gamma-ray bursts, star clusters, and high redshift galaxies.

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ith about 4000 extrasolar planets orbiting about 3000 different host stars known today, we are now clearly in the realm of detailed statistical studies of planet and host star properties. Furthermore, rarer systems are found, which sometimes can constrain planet formation and evolution theories single-handedly. Here I will review the main results of two Doppler surveys: the Lick G and K giant survey, focusing on evolved and relatively massive host stars, as well as the Carmenes survey which targets M dwarfs, the least massive host stars. I will discuss both, ensemble properties as well as individual systems, and will summarize what we have learned so far.

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Star formation is an inefficient process and in general only a small fraction of the gas in a giant molecular cloud (GMC) is turned into stars. This is partly due to the negative effect of stellar feedback from young massive star clusters. Recently, we introduced a novel 1D numerical treatment of the effects of stellar feedback from young massive clusters on their natal clouds, which we named WARPFIELD. With this model we can show that the minimum star formation efficiency, i.e. the star formation efficiency above which the cloud is destroyed by feedback, is mainly set by the average cloud surface density and that a star formation efficiency of 1–6 per cent is generally sufficient to destroy a GMC. These results imply that feedback alone is sufficient to explain the low observed star formation efficiencies of GMCs. We have now coupled our feedback model to the plasma code Cloudy and the radiative transfer code Polaris, which enables us to predict more than 100 emission lines from young star-forming regions and to compare these synthetic observations to observational surveys. In this talk I will address how individual objects (out of our database of thousands of simulated star-forming regions) evolve in various diagnostic plots (e.g. standard nebular emission line diagrams, so-called BPT-diagrams). Furthermore, I will present the evolution of whole galaxies as seen in synthetic observations.

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Leading models of galaxy formation require AGN feedback to reproduce the properties of galaxies in the local Universe (i.e., colours; metallicities; galaxy stellar mass functions). Using extensive observations in the optical and Infra-Red of an X-ray selected sample of AGN taken from the SWIFT-BAT all-sky sample (z < 0.05), we have undertaken a multi-faceted comparison of the properties of local AGN to those predicted by the suite of EAGLE hydrodynamical simulations. We find that EAGLE can reproduce key aspects of the BAT AGN, including the distributions of their host masses, colours, star-formation rates and nuclear luminosities, as well as provide unique insight into the cosmic evolution of this population. Our studies help us identify the physics that make AGN special in both our local Universe and simulated ones.

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On the one hand, double stars are a nuisance for data reductions. On the other hand, they are scientifically interesting and important. After all, binaries probably constitute the majority of the overall stellar population. Thus they are crucial e.g. for our understanding of the formation of stars and planets in general. The Gaia mission sees, discovers, measures and parameterizes double stars - both optical pairs and physical binaries - in a surprising multitude of ways. Each of these ways poses an operational challenge as well as a scientific chance. Once fully exploited they will give a strongly revised picture of stellar binarity statistics. And they will remove all the disturbances caused by duplicity in the astrometric and photometric data of Gaia DR2. Gaia DR1 achieved an effective angular resolution (i.e.pair separations) of 2 arcsec, DR2 of 0.4 arcsec. But the actual optical resolution of the Gaia instrument is about 0.15 arcsec, and there are ways to detect and measure pairs down to the milli-arcsec level. All this can be done for hundreds of millions of stars, but it means a few more years of hard work by the Gaia data reduction consortium.

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Stars are formed by the gravitational collapse of dense, gaseous and dusty cores within magnetized molecular clouds. Understanding the complexity of the numerous physical processes involved in the very early stages of star formation requires detailed thermodynamical modeling in terms of radiation transport and phase transitions. I will discuss the outcome of our spherically symmetric radiation hydrodynamic simulations with which we investigate the collapse of molecular cloud cores including the stages of first and second hydrostatic core formation. We investigate the properties of Larson’s first and second cores and expand these collapse studies for the first time to span a wide range of initial cloud masses from 0.5 Msun to 100 Msun. I will highlight the strong dependence of a variety of first core properties on the initial cloud mass. Furthermore, based on our new 2D radiation hydrodynamic simulations, I will discuss the impact of different cloud properties on the formation of early disks.

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Theories of planet formation predict the existence of a population of free-floating planets that are not gravitationally tethered to any host star. Gravitational microlensing provides a unique tool for studying these objects. The first results of Sumi et al. (2011) claimed that Jupiter-mass free-floating planets are as common as main-sequence stars. However, these results disagree with censuses of substellar objects in young clusters and star-forming regions and with predictions of planet formation theories. I will present new results of the analysis of a ten times larger sample of microlensing events discovered by the OGLE-IV survey during the years 2010-2018, which shed new light on the population of free-floating planets. I will also discuss prospects for detecting free-floating planets with the future missions, like Euclid and WFIRST.

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In the course of their accretion phase, massive (proto)stars impact their natal environment in a variety of feedback effects such as thermal heating, MHD-driven protostellar jets and outflows, radiation forces, and photoionization / HII regions. Here, I present our most recent simulation results in terms of the relative strength of the feedback components and the size of the reservoir from which the forming stars gain their masses. For the first time, these simulations include all of the feedback effects mentioned above which allows us to shed light on the physical reason for the upper mass limit of present-day stars. Furthermore, we predict the fragmentation of massive circumstellar accretion disks as a viable road to the formation of spectroscopic massive binaries and the recently observed strong accretion bursts in high-mass star forming regions.

To advertise our latest code development, I will also overview the most recent results obtained in a variety of other astrophysical research fields from the formation of embedded Super-Earth planets' first atmospheres (Cimerman et al. 2017, MNRAS) to the formation of the progenitors of the first supermassive black holes in the early universe (Hirano et al. 2017, Science).

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