Events, Seminars, Talks

Abstract
The Milky Way contains of order 100 million stellar-mass black holes. Yet, fewer than 100 black hole candidates are known in the Milky Way, and only about 25 are dynamically confirmed. For the last 50 years, our view of the Milky Way's black hole population has been shaped almost entirely by observations of X-ray binaries, which shine brightly but represent an extremely rare outcome of binary evolution. The supremacy of X-ray binaries is about to end: the 4th data release of the Gaia mission -- this December! -- is expected to uncover a population of non-interacting black holes that is larger than the X-ray binary population, has a better-understood selection function, and probes a different regime of stellar and binary evolution. I will discuss the tip of the iceberg that has already been revealed by astrometry from Gaia DR3, upcoming opportunities from DR4, and our evolving view of the compact object population. I will highlight constraints on the stability and efficiency of mass transfer from au-scale binaries, which astrometry is particularly sensitive to. To arrange a visit with the speaker during the visit, please contact their host: Hans-Walter Rix

Abstract
Stellar counter-rotation (CR), where a large fraction of stars rotate opposite to the old stellar disk, is a striking dynamical signature of past galaxy interactions. Such kinematically decoupled structures are thought to form after the accretion of gas or during fine-tuned collisions of galaxies that bring in material with opposite angular momentum. Using MaNGA survey data, we have identified over one hundred CR galaxies, but one system stands out: NGC 5717, the most massive known CR galaxy. This galaxy is a brightest cluster galaxy, morphologically resembling an elliptical with an embedded, massive disk with young stellar population and ongoing star formation. The scale and persistence of this structure in a dense environment challenge current views on how disks survive and reform after merging. In this talk, I will present the sample of such galaxies, the properties of NGC 5717, and discuss its implications for understanding disk assembly and the role of galaxy collisions in shaping CR-galaxies.

Abstract
The stellar initial mass function (IMF) is one of the most important empirical distributions in astrophysics, yet its physical origin remains a central question in astronomy. Nearly three decades ago, the observed resemblance between the core mass function (CMF) and the IMF inspired the idea that the stellar mass spectrum might be imprinted early in molecular-cloud cores. It has since become increasingly clear, however, that the stellar mass spectrum is not only a statistical distribution, but also a record of how gas fragments, accretes, and assembles stars across scales.

In this talk, I will discuss the origin of the stellar mass spectrum from the observer perspective of star formation. I will first present the largest ALMA mosaicked observations of a Galactic filament containing multiple evolutionary stages in situ. This dataset provides a dynamic view of the core mass spectrum as an evolving, cloud-scale population rather than a fixed initial condition. I will then move to the highest-mass end of the IMF, where the formation of the most massive stars is expected to be regulated by the available gas reservoir, accretion flows, feedback, and cluster-scale dynamics. Through a detailed case study of a proto-Trapezium cluster of four forming OB stars, we resolve gas rotation, inflow motions, and the gravitational potential, revealing how orbital energy dissipation sets the initial architecture of clustered massive-star formation, and may even determine the cut-off at the high-mass end. I will conclude by discussing future observational and theoretical steps toward a formation-based understanding of the IMF.

Abstract
To arrange a visit with the speaker during the visit, please contact their host: Hans-Walter Rix

Abstract
I will review the recent progress that we have made in understanding nucleosynthesis in the Universe and the accretion history of the Milky Way through chemodynamical analysis of the stellar halo. Over the last five years, we have been complementing the discoveries from the Gaia mission with high-resolution spectroscopic follow-ups of halo stars. These studies have provided detailed chemical abundances of stars in stellar streams, revealing the origins of these streams. There have also been serendipitous discoveries during these follow-ups leading to new insights into broader topics in astrophysics, such as the discovery of Li-rich stars in stellar streams and the association of a 33 Msun solar mass black hole with a stellar stream. After reviewing these discoveries, I will discuss the future prospects of this field in light of the recently started spectroscopic surveys, namely 4MOST and WEAVE, hopefully with initial results from them by the time of the colloquium. Finally, I will briefly introduce the next generation spectroscopic facilities that are currently being discussed. In addition to these scientific topics, I will also share my personal experience of being a Gliese fellow at ARI for three years and how it has impacted my career.

Abstract
TBD

Abstract
The first macroscopic interstellar object (ISO) passing through our Solar System, `Oumuamua, was discovered in 2017 and caused a lot of excitement, due to both its novelty and its unexpected properties. The subsequent two discoveries, 2I/Borisov and 3I/ATLAS, appear more like comets. 3I/ATLAS has been observable for most of the last year, and as the first ISO seen in the JWST era has been well studied, and appears to be an ancient object that is significantly older than our Solar System. I will discuss what we have learned about the ISOs seen so far, what the prospects are for this field in the era of Rubin/LSST, and how we could potentially send a spacecraft to see a future visitor up close. To arrange a visit with the speaker during the visit, please contact their host: Markus Hundertmark

Abstract
Westerlund 1 (Wd 1) is the most massive young stellar cluster in the Galaxy, and hosts a uniquely numerous, diverse and nearby population of evolved massive stars including 24 Wolf-Rayet stars, several yellow and red supergiants, a luminous blue variable and over 100 OB supergiants. In this talk I will report on the supergiant B[e] star Wd1-9, which we now know to be a recently stripped massive binary system deeply embedded in a dense rotating circumstellar medium and bipolar photoevaporating outflow reminiscent of a protoplanetary disk wind. I will also detail how hydrodynamic simulations of outflows from massive stars coupled with EWOCS data can constrain supergiant evolutionary pathways in clusters. I will show how a recent (~10 kyr ago) non-conservative mass transfer event can explain the unusual nebulosity around the WR+O binary system Wd1-72. I will also demonstrate how the mysterious "pillar/rat" nebula in Wd 1 could have been produced by the yellow supergiant Wd1-4 transitioning from a red supergiant in the past ~15 kyr, and how similar models of cool supergiants in clusters have the potential to robustly measure their uncertain mass-loss rates.

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TBD

Abstract
Hot, massive stars are astrophysical keystones which shape their environment via mechanical and radiative feedback and chemically enrich their cosmic neighborhood since the first generation of (very) massive stars. However, turning this general textbook picture into quantitative prediction remains an ongoing challenge. Recent discoveries, such the surprisingly high metallicity and early nitrogen enrichment in high-redshift galaxies discovered by JWST, put current descriptions and modelling approaches into question, illustrating that the complex puzzle of massive stars and their interplay is everything but complete. To get a robust, quantitative understanding, decoding and predicting the light of hot massive stars marks an astrophysical key technique. Revolving around via the development and application of expanding stellar atmosphere models for hot stars, I will present a selection of recent research results from my group. A particular focus will be on star with strong stellar winds, their enigmatic cosmic role, and the challenge to get a coherent structural and evolutionary understanding of these objects. Moreover, I will present our recent discovery of an unexpected direct transition in the Wolf-Rayet regime from WN to WO subtype occuring at subsolar metallicity as well as ongoing and forthcoming efforts in further developing atmosphere modelling and quantitative spectroscopy.

Abstract
TBA

Abstract
Our unique perspective from within the Milky Way gives us the ability to take images and spectra of individual stars throughout most of the Galaxy. By analyzing this data we have attained a detailed understanding of our Galaxy's stellar populations, gas and dust content, internal dynamics, and star formation, chemical enrichment, and assembly histories. Unfortunately, this level of detail is somewhat unattainable in external galaxies due to stellar crowding, so we must get creative with our analysis methods to account for this. I will present results from two projects. The first involves fitting panchromatic SEDs of resolved stars in two nearby galaxies that have archival observations from HST and JWST, which combine to create the highest resolution broad wavelength coverage possible. This provides an incredible opportunity to study dusty star forming galaxies in high resolution, unveiling why these galaxies are so UV bright for their dust content, and how they form stars despite their low molecular gas mass. The second project involves full-spectrum-fitting of Local Group star clusters from the APOGEE survey. From these high-resolution, integrated-light spectra we can measure the clusters' kinematics, metallicities, and α abundances, which trace the chemical enrichment and assembly history of the galaxy. These two approaches can constrain the assembly and chemical enrichment history of nearby galaxies while providing insight into the fundamental physics governing galaxy evolution in the nearby universe.

Abstract
TBD