Events, Seminars, Talks

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Understanding the formation of (exo)-planetary systems requires the combined effort of advanced computational models and high-resolution multi-wavelength observations. Multidimensional, multiphysics simulations using high-performance computing allow us to study the thermal and kinematical evolution of young circumstellar disks  and planets' birthplaces in detail.  Specially the inner disk regions, close to the silicate sublimation, are crucial for forming terrestrial planets. I will review our current understanding of the dynamic evolution of protoplanetary disks and show current results from the UFOS group at MPIA.

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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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Gas within and around galaxies is commonly multiphase, that is, vastly different temperates exists cospatially. This is for instance the case in the interstellar, circumgalactic or intracluster medium but also in galactic winds or in the solar corona. Due to the temperate and density contrasts as well as the different physical mechanisms at play the dynamics of such systems is difficult to model and understand (but nevertheless crucial for, e.g., the galactic ecosystem). In this talk, I will present some theoretical and numerical results which highlight under which conditions the phases can co-exist, what sets the mass transfer rate between them, and what are typical morphologies. In particular, I will show (and try to reason why) that cold clumps follow a Zipf's dN/dm ? m^-2 law which is also common in other astrophysical (IMF, dust mass, ...) and non-astrophysical contexts. Time provided, I want to switch gears and show how we can use the Lyman-alpha line of neutral hydrogen to constrain such systems and in particular what the emergent spectra can tell us about anisotropic gas distributions which is, e.g., relevant for the escape of ionizing photons.

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White dwarfs represent the final stage of the life cycle of more than 95% of all stars. These stellar remnants are essentially devoid of energy sources and are thus condemned to cool continuously over billions of years. Thanks to this property, white dwarfs act as "cosmic clocks" and hold a wealth of information on the history of the Galaxy. In recent years, the Gaia mission has increased the number of known white dwarfs tenfold and has provided an exceptionally detailed picture of the local population. In particular, unexpected features have been identified in the Gaia HR diagram of nearby white dwarfs, revealing significant gaps in our understanding of these objects. In this talk, I will present the latest modelling efforts aimed at filling these gaps, and I will show that the Gaia HR diagram can be elegantly explained by the transport of chemical elements in white dwarfs. On one hand, the bifurcation of the white dwarf sequence into two main branches can be attributed to the convective dredge-up of carbon in objects with helium-dominated envelopes. On the other hand, the accumulation of high-mass white dwarfs at a specific location in the HR diagram is the result of a distillation process triggered by the crystallisation of the carbon-oxygen core. Distillation gives rise to a very efficient downward transport of neutron-rich impurities (such as neon-22), which releases a large amount of gravitational energy and thus interrupts the cooling for billions of years. This phenomenon had been never observed in any type of stars before and challenges our very notion of white dwarfs as dead stars.

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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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A fundamental difference between terrestrial planet and giant planet atmospheres is the abundance of the light elements H and He. The abundance of He and the other inert gases are dominated by physical properties and hence serve as unique tracers of atmospheric evolution. When our solar system formed small grains and first condensates incorporated small amounts of noble gases from the surrounding gas of solar composition, resulting in - orders of magnitude - depletion of light He and Ne relative to heavy Ar, Kr, and Xe, leading to the “planetary type” abundance pattern. Further noble gas depletion occurred during flash heating of mm- to cm-sized objects (chondrules and calcium, aluminum-rich inclusions), and subsequently during heating—and occasionally differentiation—on small planetesimals, the precursors of planets. In contrast, the Sun and also gas giants like Jupiter attracted a much larger amount of gas from the protosolar nebula by gravitational capture. Radiogenic ingrowth of noble gas isotopes formed by radioactive decay processes (40Ar, 129Xe and fission Xe) allows insight into the chronology of the timing of mantle degassing and evolution of planetary atmospheres. In the case of the Earth, most of the mantle degassed within the first 200 Ma, but is still an ongoing process today. The highly energetic moon-forming Theia impact caused large losses of the primary atmosphere, while impacts of smaller planetesimals leftover from the accretionary phase added volatiles to the terrestrial inventory. On the early Earth, CO2 was likely was major constituent, as is still on Venus and Mars. However, most of the CO2 was bound in carbonates, removed by the carbonate-silicate-cycle, which acts as a stabilising factor on terrestrial climate, and can explain the compensation of the faint young sun by higher CO2 atmospheric levels in the past. With continuous CO2 removal, N2 became the main constituent of the terrestrial atmosphere, while enrichment of O2 lasted billions of years.

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Star clusters, once thought to be simple, coeval systems, often harbor multiple stellar populations with distinct chemical compositions and ages. In this talk, I will discuss the role of stellar rotation in driving this phenomenon, with a focus on how rotational mixing and mass loss can create chemical and evolutionary diversity within clusters. I will discuss how rapidly rotating Be stars - characterized by their hydrogen emission lines (Halpha and Hbeta), decretion gas disks, and high rotational velocities can contribute to the formation of multiple populations through mechanisms such as chemical enrichment, rotationally-induced evolutionary differences, and material ejection. We conducted a search for Be star candidates in the star clusters (SCs) (and the field) in the Small Magellanic Cloud (SMC) and the Bridge using the STEP survey, carried out with the VLT Survey Telescope (VST). With the help of STEP deep Halpha photometry, we retrieved numerous new Be star candidates in the 64 Young SCs and their field, compared to the literature-based observations. Serendipitously, during our Be star hunt, we confirmed some known Planetary Nebulae (PNe) (+some other emission stars like Herbig Ae/Be stars, C stars, Mira variables, etc.), and found some new PNe candidates with extremely high Halpha emission using STEP photometry.

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There is a considerable debate on how massive stars form, including whether a high-mass star must always form with a population of low-mass stars or whether massive stars can also form in relative isolation. High-mass stars found in the field are often considered to be runaways or walkaways from their parental star clusters or OB associations. However, there is evidence in the Milky Way and the Small Magellanic Cloud of massive stars that appear isolated without any clustering of low-mass stars around them and are not runaways from any known star cluster or OB association. In order to shed light onto this open question, we are undertaking a systematic survey of other star-forming galaxies in the Local Volume to address this question with better statistics, using high-resolution photometry from two UV-optical Hubble Space Telescope legacy surveys, GULP and LEGUS. In this talk, I will focus on the spiral galaxy NGC 4242 and compare our findings to the Local Group.

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The operations of Gaia, ESA's two billion star astrometric satellite mission, are now shortly before entering its final phase. Therefore it is a good time for a review by the local Gaia group, both looking back to many exciting and sometimes demanding years, and forward to the promises, the obtained Gaia data holds for the future. Over the last eleven years, one of the groups pivotal to ensure a consistant data quality, has been the Gaia First Look (FL), based at ARI. The FL is the first part of the Gaia consortium (DPAC) which gets to look into the newest data obtained by the satellite, albeit in the form of diagnostic data. Its duty is to access this data both in the short term, i.e. to identify problems, as also longer term, to identify trends, which might need to be addressed at some point. I will give a brief overall synopsis of how the FL works, then showing some examples of the issues which the FL-team has had to deal with. During the spring of this year, Gaia was first hit by a micro meteoroid impact, which caused a significant amount of periodic stray-light infall, followed by an electronic malfunction, which resulted in an important detector being permanently inoperable. This double blow and how the resulting issues have been addressed by several groups within DPAC, including the FL-team, will be a focal point of this presentation. This is a dramatic story, with the attempts to first analyse and understand the impact of these events, then to mitigate the really damaging effects, quite a few failures, and ultimatively, success. I will also point to the upcoming EoL phase, which will sound the knell only for the satellite itself, but by far not for the mission as such. Finally, I will also look ahead, at the two future releases, i.e. the best part of the Gaia dataset, which is yet to come.

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To a great extent, our knowledge of Astronomy hinges upon our understanding of how massive stars (M > 8 Msun) behave and evolve across different eras of the Universe. Despite having short lives and being far outnumbered by their low-lass analogs, high-mass stars deeply impact their surroundings due to their strong winds (high mass-loss rates and wind speed), ionizing fluxes, and usually violent death as supernovae. Moreover, they are progenitors of neutron stars and black holes, which produce gravitational waves. Massive stars spend most of their main-sequence lifetime as O- and B-type objects. However, most of them are expected to evolve towards cooler temperatures and high luminosity, becoming blue supergiants/hypergiants. The full evolutionary picture, however, remains unclear as many factors play important roles in their evolution, such as internal mixing, rotation, and mass-loss rates. For instance, the status and origins of B supergiants (BSGs) and B hypergiants (BHG) are still under heavy debate. Additionally, important aspects of the nature of their atmospheres/winds are still not well understood. Even less is the connection between their stellar and wind properties -- for instance, the behavior of their mass-loss rates with stellar temperatures. To address this problem and deepen our understanding of the atmospheric/wind properties of BSG/BHGs, we analyze their spectra using state-of-the-art comoving-frame stellar atmosphere codes. In the first study we present in the colloquium, we use CMFGEN (Hillier et al. 1998) to produce the largest multi-wavelength spectral analysis of BSGs in the Small Magellanic Clouds in the context of the ULLYSES/XShootU collaboration -- dedicated to studying hot stars in low metallicity environments. The properties of the late BSGs are compatible with H-shell burning objects whereas the early BSG have a more unclear status. Concerning the wind, we find a sharp decrease of the wind terminal velocity at B1 spectral type, but no corresponding increase in mass-loss rates towards cooler temperatures is present. This aligns better with recent theoretical "mass loss recipes" which challenge the current scenario, that predicts an increase in mass-loss rates at low temperatures due to the recombination of Fe IV to Fe III at inner layers. In the second study, we use PoWR^HD (Sander et al. 2017, 2018) to produce the first hydrodynamically consistent model of BHGs. Through that, we investigate the conditions and mechanisms behind the mass loss and driving of the wind. We find Fe III is the ion responsible for the wind acceleration in these stars (even at lower metalicities), with very little contribution of other metals to the velocity field, which reveals a very shallow acceleration. Additionally, we find evidence for a clumped atmosphere already from sub-photospheric layers. These models also allow us to investigate the impact of different properties (e.g. clumping, turbulence, mass) on the wind properties. Our current findings reveal that higher clumping in the inner layers increases the wind density, producing spectra more similar to those of luminous blue variables.

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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]lambda5007 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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Globular clusters are not the simple stellar populations we used to think they were. The vast majority of them consists of two main populations, dubbed the 1P (pristine) and 2P (polluted) populations, with distinct light-element chemical abundances. How multiple stellar populations unfold remains a riddle. A decade of observations has shown unambiguously that the fraction of 1P stars in clusters, F_1P, is a decreasing function of their present-day mass. That is, the multiple-stellar-population phenomenon is exacerbated in massive clusters. The present-day distribution of Galactic globular clusters in the (mass, F_1P) space must therefore hold clues regarding the formation of their multiple stellar populations. In this talk, I will decipher this distribution, detailing the processes and parameters shaping it.

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