Vorträge, Seminare, Ereignisse

Abstract
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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Abstract: The nature and composition of small planets' interiors remain uncertain. Catastrophically evaporating rocky planets provide a unique opportunity to study the composition of small planets. The surface composition of these planets can be constrained via modelling their comet-like tails of dust. In this work we present a new self-consistent model of the dusty tails. We model two catastrophically evaporating planets: KIC 1255 b and K2-22 b. For both planets we find the dust is likely composed of magnesium-iron silicates (olivine and pyroxene), consistent with an Earth-like composition.

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The Universe began with a simple chemical composition of hydrogen, helium, and a small amount of lithium. Since then, this composition has evolved thanks to nucleosynthesis events. Low-mass stars serve as fossil records of these chemical enrichments, as their surface composition remains largely unchanged throughout their long lifetimes. In this talk, I aim to demonstrate how stellar chemical abundances can enhance our understanding of stellar populations in the Milky Way and the origin of elements. I will focus on the stellar halo of the Milky Way, where we can observe stellar populations that formed outside the Milky Way and those that formed in the early Universe. I will first review recent progress in this field, particularly on the discoveries of spatial and kinematic substructures from Gaia data. Next, I will discuss the astrophysical implications derived from the chemical abundances of stars in these substructures, including insights into the host of a 33 solar-mass black hole, disrupted globular clusters, and the Milky Way's galaxy accretion history. I will emphasize that high-precision chemical abundance measurements are crucial in obtaining these insights. Finally, while characterizing stellar populations through chemical abundance relies on our knowledge of nucleosynthesis, I will illustrate that observational constraints on chemical enrichment histories of stellar populations provide valuable constraints on the origin of elements. Those unable to attend the colloquium in person are invited to participate online through Zoom (Meeting ID: 942 0262 2849, passcode 792771) using the link: https://eu02web.zoom-x.de/j/94202622849?pwd=dGlPQXBiUytzY1M2UE5oUDRhbzNOZz09

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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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Planets and planetesimals form ubiquitously in extra-Solar planetary systems and are built from circumstellar material, present as leftover dust and gas from star-formation. 
The formation of these large planet/planetesimal bodies occurs during two main phases. Either these form in gas-rich protoplanetary disks during the first few Myr of a star's life, or later, from rocky planetesimal collisions over 10s to 100s of Myr. Planetesimal collisions also form dusty debris disks, structures that can survive over Gyr timescales. In both cases, planet-disk interactions can shape the morphologies of disks and give rise to distinct disk sub-structures dependent on planetary architectures and system-wide evolutionary processes. JWST, ALMA and other high angular resolution instruments are now resolving the morphologies and sub-structures of disks, and are thus providing critical data to understand how and where planets form, and how planetary systems evolve over their complete life cycles. Nevertheless, a number of core open questions remain: which types of sub-structures are present in protoplanetary and mature debris disks, on what timescales do these evolve, and under what conditions are planets uniquely responsible for these sub-structures? In this talk, I will give an outline of the recent history and findings in this topic, present new results that highlight progress answering these questions, and discuss future trends to explore with upcoming observations and facilities.

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The gaseous atmospheres of extrasolar planets and those of their birth environments, the protoplanetary discs, may hold the key to understanding the observed diversity of these distant worlds and might provide important insights on fundamental questions, including habitability. In this talk I will review the results of recent efforts to connect the protoplanetary disc evolution, driven by their central star, to the formation of planets. Special attention will be given to outflows and what can be/ has been learnt from them. Some of the unanswered questions, rely on the understanding of the chemical composition of atmospheric gas, particularly with regards to important species like (polycyclic aromatic) hydrocarbons, that control the thermodynamics in the far ultra-violet regime and play an important role in the coupling of the atmospheric gas to magnetic fields. The same molecules may play a very important role in the evolution of planetary atmospheres. Current and future efforts to constrain their abundances in discs and planets will also be reviewed. Those unable to attend the colloquium in person are invited to participate online through Zoom (Meeting ID: 942 0262 2849, passcode 792771) using the link: https://eu02web.zoom-x.de/j/94202622849?pwd=dGlPQXBiUytzY1M2UE5oUDRhbzNOZz09 During her visit to Heidelberg, Prof. Ercolano will be available for meetings by arrangement with her host, Kees Dullemond (dullemond@uni-heidelberg.de).

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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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Fast inward migration of planetary cores embedded in gaseous disks is a common problem in the current planet formation paradigm. Even though dust is ubiquitous in protoplanetary disks, its dynamical role in the migration history of planetary embryos has not been considered until recently. In this talk, I will show that a planetesimal embedded in a dusty disk leads to an asymmetric dust-density distribution that can exert a net torque under conditions relevant to planetary embryos up to several Earth masses. Building on the results or a large suite of numerical simulations for measuring this dust torque under a wide range of conditions, I will present the first study showing that dust torques can have a significant impact on the migration and formation history of planetary embryos. Those unable to attend the colloquium in person are invited to participate online through Zoom (Meeting ID: 942 0262 2849, passcode 792771) using the link: https://eu02web.zoom-x.de/j/94202622849?pwd=dGlPQXBiUytzY1M2UE5oUDRhbzNOZz09 During his visit to Heidelberg, Prof. Pessah will be available for meetings by arrangement with his host, Maria Bergemann (bergemann@mpia.de).

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Even small-aperture (amateur) telescopes are indespensible for astronomical research. This way, deep exposures of galaxies in the local universe reveal a complexity of substructures. Here I will show selected highlights from a dedicated campaign, the HERON survey, that allowed us to investigate the formation channels of galaxies with some peculiar morphologies such as boxy halos.

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The interstellar medium plays a central role in the galaxy evolution process; it is the reservoir that fuels galaxy growth via star formation, the repository of material formed by these stars, and a sensitive tracer of internal and external processes that affect entire galaxies (e.g. accretion and feedback). In this overview talk, I will discuss how observations of the interstellar medium are shedding light on the vast range of physics and scales at play in the star formation and galaxy evolution processes, using results from recent observing campaigns with (sub)mm/radio facilities (IRAM, ALMA, JCMT, APEX) as well as large optical spectroscopic surveys (DESI). By connecting these observations with theory and simulations, a picture emerges where galaxy evolution is driven by gas availability on galactic- and molecular cloud-scales and the efficiency of the star formation process out of this gas, depending on local conditions in the interstellar medium. These results highlight the multi-scale nature of star formation and galaxy evolution, and help draw a path forward to understand mass assembly in the Universe. Those unable to attend the colloquium in person are invited to participate online through Zoom (Meeting ID: 942 0262 2849, passcode 792771) using the link: https://eu02web.zoom-x.de/j/94202622849?pwd=dGlPQXBiUytzY1M2UE5oUDRhbzNOZz09 During her visit to Heidelberg, Prof. Saintonge will be available for meetings by arrangement with her host, Dominika Wylezalek (dominika.wylezalek@uni-heidelberg.de).

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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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