Seminars and Colloquia at ESO Garching and on the campus

For decades, the standard paradigm has held that internal photoevaporation efficiently carves clean, empty gas cavities in protoplanetary discs, directly producing the transition discs we observe. By self-consistently coupling the evolving disc structure with photoevaporative flows in 2D radiation-hydrodynamical simulations, we revealed a very different dynamical reality. Once a density depression begins to form, the local mass-loss rate sharply drops, effectively choking off the wind. Simultaneously, viscous inflow and radial surface flows partially refill the gap. The result is not a clean cavity, but a persistent, shallow gas depression. Nevertheless, a pressure maxima forms at the outer edge of the gas depression acting as a dust trap, creating the observational illusion of a cleared transition disc. Finally, we provide a new, dynamic 1D mass-loss prescription that captures this stalling behavior, offering an updated tool for planet population synthesis models.

We explore the formation history of the Milky Way using RR Lyrae variable stars (RRLs) as fossil tracers of ancient stellar populations. The investigation focuses specifically on the Oosterhoff dichotomy, a phenomenon characterized by the separation of RRLs into two distinct groups in the Bailey diagram. The primary objective of this research is to confirm whether this dichotomy is an intrinsic feature of the Galaxy or if it was "imported" through merging events with dwarf galaxies, such as Gaia-Enceladus and Sagittarius. To test this hypothesis, chemo-dynamic classifications from existing literature were applied, analyzing their distribution in the integrals of motion space to distinguish between in situ and accreted populations. A new Period-Wesenheit-Metallicity (PWZ) relation was derived to ensure the accuracy of the distances required for the dynamical analysis. The adopted methodology relies on a robust statistical approach based on: the use of photometric parallaxes combined with Gaia astrometric data, a Bayesian MCMC (Markov Chain Monte Carlo) algorithm for parameter inference. Preliminary results indicate that the PWZ relation coefficients are consistent with current literature. Future work will involve a detailed characterization of the pulsational properties (including Fourier parameters) for each identified dynamical component, aiming to reconstruct the early evolutionary stages of our Galaxy with greater precision.

By harnessing the unique capabilities afforded by optical interferometry, VLTI/GRAVITY+ has recently become a powerful new tool for the direct detection and characterisation of exoplanets. Its extreme astrometric precision allows us to accurately constrain orbital geometries and dynamical masses just as its direct K-band spectra permit peering into the atmospheres of these objects. On top of this, its unmatched inner working angle enables exoplanet detections at separations of a few AU from the stellar hosts, a parameter space currently inaccessible to classical imaging instruments. Combined with the plethora of astrometric data contained in Gaia DR4, we will soon be able to efficiently build a population-level sample of benchmark exoplanets with direct mass measurements on orbits comparable to those found in our own Solar System. Besides building an extensive target sample for future follow-up with ELT/METIS, exploiting the profound synergies between Gaia DR4 and GRAVITY+ in a concerted large-scale follow-up effort will shed light on long-standing questions as to the giant planet occurrence rate around the water ice line, preferred formation channels, the validity of initial entropy models, and cloud properties around the L-T transition. In this talk, we make the case for focussing these endeavours in a coordinated and collaborative ESO Large Programme.

Artificial Intelligence (AI) has changed our way of assessing, processing, and distributing information, and Large Language Models (LLMs) like Chat GPT, Gemini, Claude and others could be seen as a replacement of intellectual performance previously only available through human intelligence. In the scientific community, many of us have been using AI and LLMs to accelerate their scientific productivity. However, it is unclear whether LLMs are actually needed to do high-quality science or simply yield higher productivity without substantially extending knowledge? This informal discussion was triggered by a discussion I had with Jason at the ESO guest house in Chile in January. I would like to discuss whether science actually profits from LLMs or if these tools rather undermine novel, quality research by design.

Accretion disks in quasars cannot be directly resolved in most systems, so indirect methods are needed to study their structure. One of these methods is chromatic microlensing in gravitationally lensed quasars, where stars in the lens galaxy produce different magnifications for emission regions of different size. This effect can be detected by comparing the flux ratios of the continuum and the emission line cores between the lensed images, with the emission line cores defining the baseline for no microlensing and the wavelength dependence of the continuum flux ratios constrains the size of the accretion disk. Then, we can use simulations to estimate both the disk size and the temperature profile.

In this talk, I will present a spectroscopic analysis of four lensed quasars observed with VLT/X-shooter and FORS2. We detect chromatic microlensing in all four systems and use it to estimate the accretion disk size and the slope of the temperature profile. We find accretion disk sizes larger than those expected from the standard thin-disk model, while the inferred temperature profiles are in better agreement with the thin-disk prediction than in many previous microlensing estimates. I will also discuss the differences among the systems and their implications for the interpretation of single-epoch microlensing measurements.

Early high-energy emission provides a direct window into the physics of relativistic jets, the properties of the central engine, and the underlying radiation mechanisms. This is particularly important in the era of multi-messenger astronomy with gravitational waves, as binary neutron star mergers are possible progenitors of short gamma-ray bursts (GRBs). 

In the first part of my talk, I will present a systematic study of the early X-ray emission of merger-driven GRB candidates. Using 20 years of data from the Neil Gehrels Swift Observatory, we performed a time-resolved spectral analysis of their bright steep decay phase, covering the 0.3-150 keV energy band. We adopted both a physical synchrotron model and an empirical smoothly broken power-law model, allowing us to track the evolution of the spectral peak energy and bolometric flux during the steep decay. We find a tight correlation between rest-frame peak energy and isotropic luminosity, providing new insight into the intrinsic properties of short GRBs. We also assess the detectability of these sources with wide-field X-ray cameras such as Einstein Probe.

In the second part of my talk, I will discuss GRB 250702B, detected by Fermi Gamma-ray Burst Monitor and followed up by Swift and the Nuclear Spectroscopic Telescope Array. Its gamma-ray emission lasted over three hours, making it the longest MeV transient ever observed, and was followed by a rapidly decaying soft X-ray counterpart over the subsequent ten days. We performed a time-resolved spectral analysis of both the gamma-ray and X-ray emission. The extreme duration, spectral properties, X-ray evolution, and energetics suggest the emission originates from a relativistic jet launched during the tidal disruption of a star by a compact object.

The tidal breakup of binary stars by a massive black hole (MBH), known as the Hills mechanism, occurs when a binary approaches sufficiently close to the MBH. This well-known process produces a hyper-velocity star that escapes the galaxy and a captured star that is tightly bound to the black hole on a highly eccentric orbit.

In this talk, I explore what happens both before and after the Hills breakup. Prior to disruption, perturbations from the MBH over numerous pericenter passages can excite the inner binary orbit to high eccentricities, triggering strong tidal interactions between the two stars. We find that a significant fraction of initially wide binaries are driven into close configurations through tidal interactions in the chaotic regime, and subsequently undergo Hills breakup as close binaries.

After the breakup, the long-term evolution of the captured stars can produce a variety of nuclear transients. These include repeating partial tidal disruption events (TDEs) on timescales of years or longer and quasi-periodic eruptions (QPEs) with periods ranging from hours to months. We further show that collisions between Hills-captured stars and accretion disks formed by unrelated TDEs can efficiently circularize their orbits, providing a natural pathway for producing the shortest-period (sub-day) QPEs.

tbd