Vorträge, Seminare, Ereignisse

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Over the last years significant advances have been made in our understanding of how and where stars and planets form and how they evolve during their earliest stages, both from a physical and chemical point of view. Deep observations of the gas and ice in the environments in which young stars form demonstrate that these regions are characterised by rich and varied chemistry with high abundances of complex organic molecules – some perhaps even of prebiotic relevance. At the same time, a picture has emerged where the first seeds for planets are planted in protoplanetary disks already during the first few 100,000 years after stars form. But, what is the link between this complex chemistry and the structure of the newly formed protoplanetary disks – and does it have any implications for the origin and composition of planets outside of our own Solar System? In the colloquium I will discuss how our understanding of the earliest stages of star and planet formation has evolved over recent years. In particular, I will focus on how ALMA has helped shedding new light on how the properties of emerging protoplanetary disks may reflect the evolution of protostars and conditions in their natal environments.

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One of the most outstanding problem in today's understanding of the Universe is the nature of the elusive dark-matter (DM) that dominates the matter content. Rotation curves have been used to put forward the concept of DM in the 70s, and have since revealed a potential problem, for the standard CDM model, on small scales (<1kpc) where DM profiles are too flat (cored) compared to predictions. Alternative DM models have been proposed to produce cores naturally. Hence, rotation curves are dark-matter laboratory given that  the shape of RC on intermediate scales (1-5kpc) is a measure of the shape of the inner DM profiles. Measuring the shape of DM profiles in distant marginally resolved galaxies (with z>0) was deemed impossible, but recently, thanks to recent innovations, it has become possible to study the shape of RCs of hundreds of distant galaxies. I will present the most recent innovations, including our recent results from the MUSE 3D spectrograph.

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Millimeter (mm) emission has been observed as an excess in the SED of RQ AGN. Observations with ALMA have confirmed that mm emission originates from the central, very compact nuclear region (? 1 pc) and remains unresolved even at 0.1". While the origin of this emission is still debated, the observed mm spectra and the tight correlation between X-ray and mm emissions suggest that it is a self-absorbed synchrotron emission coming from the accretion disk X-ray corona. Although this mechanism is the most preferable, the absence of correlated variability between high-resolution ALMA mm observations (100 GHz) and X-ray bands (2–10 keV), as recently found in observations of IC 4329A, a nearby unobscured RQ AGN, raises the question about the origin of compact mm emission again. In this talk, I will present the latest results of the investigation of compact mm emission in RQ AGN, including the surprisingly high mm variability, which exceeds that in X-rays. I will also discuss the possible mechanisms for variability in the compact, corona-size region where the mm emission originates, as well as the very first attempts to define the mm origin using ALMA mm polarimetry.

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Common envelope evolution is a phase in the life of binary stellar systems during which one of the components, a giant star, expands and initiates a dynamically unstable mass transfer onto its more compact companion, causing the latter to be swallowed up. This process is a key step in the formation of various observed tight binary systems, such as cataclysmic variables, X-ray binaries, or type Ia supernova progenitors. In addition, common envelope evolution is at the origin of a significant fraction of gravitational wave progenitors. Despite being arguably one of the most crucial major processes in binary star evolution, common envelope evolution is also the least-well-constrained and more generally one of the most important unsolved challenge in stellar evolution. Despite being numerically challenging and subject to major uncertainties, 3D-hydrodynamic simulations have provided a comprehensive understanding of the initial phase consisting of the rapid inspiral of the two cores inside the shared envelope. However, because of the wide range of temporal and spatial scales that need to be resolved and the associated high numerical cost, such simulations are often halted soon after the end of this first phase, when the inspiral of the two cores has slowed considerably. In this talk, I will present recent results from the first 3D-magnetohydrodynamic simulations focusing on the late phases of common envelope evolution by means of an original setup mimicking the preceding rapid inspiral, with the adaptive mesh refinement code Athena++. I will discuss the impact of mass and angular momentum accretion on the orbital contraction timescale of the binary, and the short-term variability of accretion and its remarkable similarity with that in circumbinary disks (Gagnier & Pejcha 2023). Finally, I will discuss the mechanisms behind magnetic energy amplification, and the impact of magnetic fields on binary separation evolution and angular momentum transport (Gagnier & Pejcha 2024).

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Making an informed choice between competing physical models becomes increasingly important in the era of precision cosmology. Central to model selection is a trade-off between performing a good fit and low model complexity: A model of higher complexity should only be favoured over a simpler model if it provides significantly better fits. In Bayesian terms, this can be achieved by considering the evidence ratio, enabling choices between two competing models. We generalise this concept by constructing Markovian random walks in model space governed by the logarithmic evidence ratio. This is in analogy to the logarithmic likelihood ratio in parameter estimation problems. We apply our methodology to selecting a polynomial for the dark energy equation of state function based on data for the supernova distance-redshift relation.

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Studying the AGN feedback effect on the cold molecular gas of their host galaxies is key to understanding its impact on the local star formation. I will present a study of the CO(2-1) emission line distribution and kinematics in a sample of four local Seyfert galaxies with luminosities L_AGN ? 10^44 erg/s. They were observed with ALMA, using a spatial resolution of ?100 – 400 pc, and covering up to ?10 kpc radii. Comparing the CO(2-1) observations with imaging data of [O III]?5007 emission lines from HST, we find that the ionized gas is generally observed in regions deficient in molecular gas, which we interpret to be caused by the AGN radiation partially destroying it. Although the kinematics of the cold molecular gas is dominated by rotation, all Seyfert galaxies present regions with double peaks in CO(2-1), which trace clouds with more complex motions. In particular, for NGC 3281 and NGC 6860, the cold molecular gas outflows were detected at the edges of their bipolar [O III] emission, surrounding it. I will also discuss my ongoing project to analyze the complex kinematics of the ionized gas in high-redshift radio galaxies (z ~ 3) obtained with JWST.

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The need for understanding the winds of Wolf-Rayet (WR) stars cannot be understated: the light of these stars, their mass-loss rates, ionization capabilities and ultimately their further evolution is all greatly affected by the behaviour of their wind. Despite WR-star winds being notoriously difficult to model, advancements on this matter have been made. One approach is using non-LTE, co-moving frame computations with the Potsdam Wolf-Rayet (PoWR) code where now hydrodynamic consistency throughout the wind domain is enforced. While already applied multiple times for the regime of hot, hydrogen-free WR stars, we now present their first wide-range application in the regime of nitrogen-rich late-type WN stars that still contain hydrogen in their spectra (WNLh type). A newly generated temperature sequence of these WNLh-star models reveals a sudden change in the wind regimes: Below 30 kK, the mass-loss rates increase significantly, while the terminal wind velocity drops strongly, accompanied with large changes in the emergent model spectra. This discontinuous behaviour greatly resembles the well-known bi-stability jump in B-supergiants. Examining the models, we discover that our obtained regime change does not correspond to the switch from Fe IV to Fe III as expected, but is linked to the higher ionization switch of Fe V to Fe IV, therefore also coinciding with higher stellar temperatures. Hence, this bi-stable behaviour occurs both due to a different cause and in a different temperature regime as the "classical" case for B-supergiants, making it a different phenomenon altogether; a new bi-stability jump for Wolf-Rayet stars.

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We aim to trace the evolution of eight edge-on star-forming disc galaxies through the analysis of stellar population properties of their (thin and thick) discs. We use Multi-Unit Spectroscopic Explorer (MUSE) observations and full-spectrum fitting to produce spatially resolved maps of ages, metallicities and [Mg/Fe] abundances and extract the star formation histories of stellar discs. Our maps show thick discs that are on average older, more metal-poor and more ?-enhanced than thin discs. However, age differences between thin and thick discs are small (around 2 Gyr) and the thick discs are younger than previously observed in more massive and more quiescent galaxies. Both thin and thick discs show mostly sub-solar metallicities, and the vertical metallicity gradient is milder than previously observed in similar studies. [Mg/Fe] differences between thick and thin discs are not sharp. The star formation histories of thick discs are extended down to recent times, although most of the mass in young stars was formed in the thin discs. Our findings show thick discs that are different from old thick discs previously observed in more massive galaxies or more quiescent galaxies. We propose that thick discs in these galaxies did not form quickly at high redshift, but slowly in an extended time. The thin discs were formed also slowly, but with a larger mass fraction at very recent times.

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