Colloquia

The massive globular cluster Omega Centauri is likely the stripped nucleus of an accreted dwarf galaxy and, therefore, provides a unique opportunity to study the central region of a galaxy, whose evolution halted billions of years ago.

In the last years we have created oMEGACat, the largest astrometric and spectroscopic dataset for any star cluster, with the goal to decipher both the formation history and the dynamics of Omega Centauri.

I will give an overview of this project and then focus on the exciting discovery of several fast-moving stars in the very center of the cluster. These stars provide the potentially best evidence for an intermediate-mass black hole (IMBH) we have to date.

 These elusive IMBHs have masses between the stellar mass black holes and supermassive black holes and may provide a missing link in our understanding of the formation of super-massive black holes

Massive stars are the cosmic engines of the universe, driving the chemical enrichment and mechanical evolution of galaxies. A large fraction of massive stars are found in binary systems, and interactions between these stars can fundamentally alter the evolutionary paths of both stars. One of the most critical phases in the evolution of massive binary stars is the contact phase, where both stars fill their Roche lobes and share a common envelope. The contact phase represents a crossroad in the evolution of massive binary stars.  Depending on the internal physics, the predicted end products can vary greatly including various exotic objects such as Be stars, magnetic massive stars, LBVs, peculiar Type-II supernovae, and gravitational wave sources. Nearly a quarter of all massive stars will evolve through a contact phase at some point during their lifetimes, however, despite its importance, large uncertainties exist in our understanding of the internal physics and the final evolutionary outcome of this phase. This is due to both the complex interaction physics and a lack of observational constraints: only 13 massive contact binaries are currently known.

Despite the small sample size, massive contact binaries can provide vital observational constraints to the various evolutionary pathways that involve binary mergers. In this talk, I will discuss the current state of the field of massive overcontact binaries, with a specific focus on the internal mixing processes during this phase.  I will discuss the theoretical predictions as well as what the observational data tells us, and how these compare and contrast with one another.  I will also describe a new spectroscopic analysis technique specifically designed to analyze these highly deformed systems and I will discuss how accounting for the 3D geometry can change our understanding of these objects. Finally I will discuss the future direction of the field and how we can attempt to bridge the gap between theory and observations in the coming years.

Interacting binary evolutionary products are ubiquitous in cluster environments. This talk presents an overview of recent observational progress on blue stragglers and related post-interaction systems, including blue lurkers, yellow stragglers, and extremely low-mass white dwarfs. These objects trace alternative evolutionary pathways and occupy regions of the Hertzsprung–Russell diagram that are inaccessible to single stars, reflecting a diversity of mass-transfer histories and evolutionary states in cluster environments. Multiwavelength observations, particularly in the ultraviolet, have proven to be powerful tools for identifying compact companions and constraining the present-day binarity and origins of these systems. In combination, time-series photometry and spectroscopic follow-up provide the most direct means of measuring their fundamental parameters and extending insights from cluster populations to analogous systems in the field.

Constraining dust optical properties is essential for interpreting remote sensing observations of planetary atmospheres and surfaces, as well as for understanding the radiative impact of aerosols in climate and circulation models. In this talk, I will present recent advances in the retrieval and experimental characterization of dust optical properties across the ultraviolet, visible, and near-infrared wavelengths. Our approach combines laboratory measurements with advanced light-scattering modeling to determine wavelength dependent complex refractive indices and other key parameters such as single-scattering albedos, cross sections, and efficiencies. Three Martian dust analogues were analyzed, each prepared with narrow particle size distributions representative of airborne dust in the Martian atmosphere. Particular attention was given to the effects of particle shape, composition, and size on the derived optical properties. The resulting validated optical property database covers wavelengths from 200 to 2000 nm and provides a physically consistent foundation for radiative transfer modeling. These results offer improved constraints for interpreting spacecraft and ground based observations of planetary materials and contribute to the broader understanding of dust scattering behavior in planetary environments.

The search for supermassive black hole binaries (SMBHBs) seemingly saw the dawn of exploration over the past twenty years with several hundred sub-pc candidates claimed from photometric and spectroscopic surveys monitoring active galactic nuclei (AGNs). While the existence of SMBH pairs have been detected at kpc separation, the observational evidence for sub-pc SMBHBs is however still inconclusive. Finding and
expanding the arsenal of SMBHB candidates is not only vital for understanding the co-evolution with their galactic hosts, but is complementary to pulsar timing arrays searching for low-frequency gravitational waves. With the advancements of upcoming high-precision, wide-field optical photometric surveys, like Vera Rubin’s LSST, robust electromagnetic detections may therefore happen in the near future.
Following the pursuit of confirming SMBHBs in the optical, we explore the possibility of using the ESA Plato space mission to detect the photometric signature of Doppler boosting and gravitational self-lensing events linked to their binarity. Although not designed for it, in this seminar we will discuss how Plato may play an essential role in future searches of SMBHBs and for AGN variability research in general. With a minimum 2-yr baseline per pointing field, our simulation study also serves as a benchmark for the upcoming Plato Guest Observer (GO) call in April 2026 designed for complementary sciences alike

HAYDN is one of the ten mission concepts proposed to ESA’s M-class call (M8) after its Step-1 selection. It is designed to revolutionise our understanding of stellar structure and evolution through high-precision, space-based asteroseismology in dense stellar fields, including clusters. By performing continuous photometric monitoring of stars in these dense stellar fields, HAYDN aims to map stellar interiors across a wide range of ages, masses, and chemical environments—providing unprecedented constraints on stellar physics, Galactic evolution, exoplanet’s formation, and the formation history of the Milky Way, for naming some of the HAYDN’s science cases.

Our work focuses on high-accuracy spectral modeling in NLTE, and the determination of chemical abundances for the oldest known stars, providing crucial insights into the early universe and nucleosynthesis processes. Utilizing state-of-the-art spectroscopic techniques, we have analyzed high-resolution observations of the hyper metal-poor star J0815+4729 to derive its precise elemental abundances, including the elements carbon and oxygen. We discuss the implications of our findings on our understanding of the formation and evolution of the oldest stars. In addition, we are constructing a comprehensive NLTE synthetic spectral library that spans from 0.2 to 3 μm in wavelength, which will be made accessible to the public. This spectral library will make a significant impact to large spectroscopy surveys such as APOGEE, DESI, LAMOST, 4MOST, and others.

The rapid progress of quantum technologies—highlighted by recent Nobel Prizes recognizing advances in controlling quantum matter—is reshaping how we investigate the fundamental laws of nature. In this colloquium, I will introduce the concept of quantum machines: platforms such as quantum computers, analog simulators, and engineered networks of photons or ultracold atoms that process information according to the principles of quantum mechanics. These systems are emerging as powerful tools to emulate physical phenomena that were previously inaccessible to theory or experiment. By bridging quantum information science, tensor network methods, and high-energy physics, this work exemplifies how quantum machines are becoming laboratories for exploring exotic phases of matter, the structure of gauge fields, and the processes that shaped the early cosmos. Together, these advances mark the beginning of a new era in which we can engineer, manipulate, and ultimately understand some aspects of the quantum fabric of the universe.

In the past decades, studies of the Milky Way have entered a golden era of large-scale surveys. The Gaia satellite provides precise positions, proper motions, and parallaxes for billions of stars, while ground-based spectroscopic surveys, such as LAMOST, deliver radial velocities and metallicities for nearly ten million stars. In addition, medium- and narrow-band photometric surveys, as well as Gaia’s slitless spectroscopic surveys, provide atmospheric parameters for hundreds of millions of stars. This talk will first summarize our efforts in measuring stellar parameters from these surveys, and then present our work on understanding the assembly history of the Milky Way—particularly its early phase, when the proto-Galaxy formed—and its dark matter distribution, based on the measured parameters of a very large number of stars.