Events, Seminars, Talks

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Abstract: Major technological advances have enabled the discovery of a small number of directly imaged exoplanets. These imaged worlds can be studied in far greater detail than exoplanets detected by indirect methods such as transit and radial velocity techniques. Next-generation telescopes such as the recently launched James Webb Space Telescope and the upcoming 30-m telescopes (e.g. ELT, TMT, GMT) will enable direct exoplanet characterisation. Based on the handful of exoplanets studied to date, it is clear that interpretation of future observational data hinges on a thorough understanding of their atmospheric processes. In this talk I will discuss our past, current and future efforts to investigate the atmospheres of imaged extrasolar worlds. In particular, I will discuss how a combination of observational and computational techniques will reveal three critical atmospheric processes: clouds, winds and aurorae. Each of these processes are well-studied in our own Solar System and we can now begin to study them on worlds beyond our own.

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Take a molecular cloud, collapse it to form a star and the leftover material will form planets. Sounds easy, right? But even our own solar system is riddled with clues that forming stars and planets is a bit. more. complicated. It turns out that accreting gas to form any small object is hard. Accreting gas at the rate needed to form the Sun in a few hundred thousand years is even harder. None of this is new. What is new is the observational revolution of the last 10 years, showing us the insides of protoplanetary discs, bringing fresh clues as to how both stars and exoplanets form [seemingly, together]. This has dramatic implications for our understanding of how accretion works. I will argue that the typical pathway to form stars and planets is a violent mess, imprinted in subtle and not-so-subtle ways on disc observations and also in the leftovers from our solar system’s formation. The story is misaligned flow, accretion streamers, infall, warps and variability. If you don’t care about stars or planets but the story sounds familiar, it’s because it’s not so different for making black holes or galaxies either... To arrange a visit with the speaker during the visit, please contact their host: Mike Lau

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In the era of `Precision Cosmology' remarkable advances have been made in the determination of cosmological parameters from the Cosmic Microwave Background and Big Bang Nucleosynthesis, with spectacular concordance between these two pillars of the Standard Cosmological Model. While much exposure has been given to the impressive results from the WMAP and Planck missions, perhaps less attention has been paid to the equally striking advances made in the last ten years in the measurements of the abundances of the light elements forged in the first few minutes of our Universe history. In this talk I shall focus in particular on the determination of the primordial abundance of deuterium, in an overview that spans almost 80 years, from the first seeds of the idea sown in the 1940s to the most recent results and forward look to the era of Extremely Large Telescopes and next generation Wide Field Surveys of the sky. To arrange a visit with the speaker during the visit, please contact their host: Eduardo Banados

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To arrange a visit with the speaker during the visit, please contact their host: Marcelo Alberto Cortes Vergara

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Over the next few years, Rubin/LSST, Euclid, Roman, SKA, and other instruments will produce petascale, information?rich datasets that trace stars, galaxies, and large-scale structure with unprecedented fidelity. Hidden in these data may be regularities that point to new and unexpected physical relationships. Can we build modelling frameworks that can discover such relationships accurately, efficiently, and in forms we can interpret? I will present two complementary directions we are developing to address this question. The first, PhySO, is a physics-aware symbolic regression engine which proposes compact mathematical equations using deep reinforcement learning with a dimensional-analysis grammar and imposable constraints. The second, NestyNet, assembles networks with analytic derivatives and trains them using second-order methods, yielding fast, high accuracy fits to datasets, solvers for ODEs/PDEs, action-angle transformations, Gaussian-mixture inference, and dynamical modelling, with exact gradients and Hessians throughout. I will demonstrate how symbolic search coupled with accurate derivatives and with PDE constraints can rediscover analytic solutions from textbook physics. This approach is a practical route toward explainable, robust models for the forthcoming data deluge-aimed less at "automating Kepler" than at accelerating analysis while keeping physical insight. To arrange a visit with the speaker during the visit, please contact their host: Morgan Fouesnau

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To arrange a visit with the speaker during the visit, please contact their host: Genevieve Parmentier

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The formation history of giant planets inside and outside the solar system remains unknown. I will present a new path for giant planet formation where runaway gas accretion is initiated only at a mass of ~100 M_Earth. This suggests that the transition to a gas giant planet, a planet whose composition is dominated by hydrogen and helium, occurs at ~Saturn’s mass. Delaying runaway accretion to later times (a few Myr) and higher masses is likely to be a result of an intermediate stage of efficient heavy-element accretion that provides sufficient energy to hinder rapid gas accretion. This implies that Saturn has never reached runaway gas accretion, and that it is a "failed giant planet". The transition to a gas giant planet above Saturn's mass naturally explains the differences between the bulk metallicities and internal structures of Jupiter and Saturn, and the characteristics of Uranus and Neptune. In terms of giant exoplanets, delaying runaway gas accretion to planets beyond Saturn's mass explains the transitions in the mass-radius (M-R) relations of observed exoplanets and the high metallicity of intermediate-mass exoplanets. To arrange a visit with the speaker during the visit, please contact their host: Saskia Hekker

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APEx Signature Speaker

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To arrange a visit with the speaker during the visit, please contact their host: Brian Reville

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Stellar feedback shapes the multi-phase interstellar medium in galaxies and the stellar initial mass function. Moreover, feedback impacts the large-scale evolution of galaxies by regulating star formation and by driving galactic fountain flows and outflows. Using modern high-performance computing simulations, we can study the relative importance of stellar winds, radiation, and supernovae in shaping the multi-phase interstellar medium. Using these simulations, we find that pre-supernova feedback is highly relevant for regulating star formation. In particular, the ionizing radiation of massive stars is dominating over the impact of non-ionizing radiation or stellar winds. On the other hand, supernovae drive hot bubbles and super-bubbles with substantially higher pressure than the typical midplane pressure of a disk galaxy, thereby pushing gas out into the circum-galactic medium. Additionally, cosmic rays, which are in this context most importantly accelerated by supernova shocks, help to sustain the galactic outflow via a vertical cosmic ray pressure gradient. In this talk, I will give an overview of the importance of stellar feedback for the evolution of galaxies, which we study using numerical simulations. To arrange a visit with the speaker during the visit, please contact their host: Cormac Larkin

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The High Energy Stereoscopic System (H.E.S.S.) is an array of imaging atmospheric Cherenkov telescopes (IACTs) that has been used to observe the sky in TeV gamma rays since 2004. Thanks to its unique location in the Southern Hemisphere and several upgrades to the system, the experiment continues to enable cutting-edge astrophysics despite its age. In my talk I will showcase the astrophysics that can be probed with IACTs, focusing on recent scientific highlights from H.E.S.S.

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The vast majority of known exoplanets orbits their host stars at quite close distances, compared to what we are used to from our own solar system. This proximity lets exoplanets and their host stars interact in significant ways through gravitational, magnetic, and radiation fields. The various flavours of those so-called Star-Planet Interactions manifest themselves as rotational and magnetic anomalies of the host stars' behavior, as well as in some cases as dramatic atmospheric evaporation of the exoplanets. Planet-induced alterations of stellar behaviour are hard to detect observationally, since cool stars display a number of stochastic magnetic phenomena of their own, which need to be distinguished from planetary effects. However, cleverly designed experiments and a wealth of space-based and ground-based data have allowed the field to make a lot of progress over the past decade. I will show exciting new results for observations of Star-Planet Interactions in this talk, and give an outlook on the possibilities for the field in the near future. To arrange a visit with the speaker during the visit, please contact their host: Joachim Wambsganss

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The discovery of billion-solar-mass black holes within the first Gigayear of cosmic history presents an intriguing puzzle: how did supermassive black holes (SMBHs) grow so rapidly in such a short amount of cosmic time? In this talk, I will introduce new approaches to probing the early growth of SMBHs. First, I will present the first measurement of the clustering strength of luminous quasars and their surrounding galaxies at z>6 using recent observations from the James Webb Space Telescope. These measurements allow us to infer the properties of the quasars’ host dark matter halos and their duty cycles, offering new insight into the environments that foster SMBH growth. I will then highlight new results from deep spectroscopic observations of background galaxies behind a luminous high-redshift quasar, which allow us to tomographically map the quasar’s ionized bubble, constraining the obscured fraction of quasars, their emission geometry, and the timescales of SMBH growth. To arrange a visit with the speaker during the visit, please contact their host: Nadine Neumayer

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Galaxy clusters, representing the peaks in the cosmic density field, serve as an independent and powerful tool for investigating the evolution of cosmic structures. The strategic identification of these clusters through multi-wavelength surveys is essential for advancing our understanding of gravitational theory, general relativity, and cosmological models. Launched in 2019 aboard the Spectrum-RG mission, eROSITA marked a major milestone in astronomy by enabling the construction of the largest pure sample of galaxy clusters and groups detected through their hot intra-cluster medium in the X-ray band. In this talk, I will present results from my group’s work on deriving cosmological constraints from the evolution of the cluster mass function, combining eROSITA data with optical surveys such as DESI Legacy, DES, HSC, and KIDS. These parameters are constrained at a percentage level through the evolution of the cluster mass function, representing a significant leap forward. Beyond cosmology, a central focus of my research is on AGN feedback and its role in shaping galaxy and structure formation. Leveraging the statistical power of the eROSITA sample, we have detected warm baryons within cosmic filaments and cluster outskirts, offering a first glimpse of baryons in the faint, diffuse cosmic web. To arrange a visit with the speaker during the visit, please contact their host: Matteo Maturi

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KoCo Signature Speaker (GC)

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