Seminars and Colloquia at ESO Santiago

Bars are prominent features observed in most disc galaxies, having a crucial role in the secular evolution of their hosts. Indeed, they redistribute material within the galaxies, while rotating around the centre at a given bar rotation rate.
When formed in an isolated galaxy, a bar is expected to be born as fast rotating. During its evolution, the bar can be slowed through the exchange of angular momentum with the other components and/or when an efficient dynamical friction is exerted by the dark matter (DM) halo. In this case, the bar tends to rotate slowly, while the bar radius and strength increase. On the other hand, extremely fast rotating bars are unstable. Measuring the bar rotation rate becomes desirable both to investigate the secular evolution of barred galaxies and to test whether the measured DM distribution matches the prediction of LCDM cosmological simulations. 
The only model-independent way to recover the bar rotation rate is the Tremaine-Weinberg (TW) method: most of the analysed bars are compatible with the fast regime, while a non-negligible fraction are unstable, rotating extremely fast. I will explore the open questions related to the bar rotation rate derived with the TW method by
1. testing the reliability of the TW measurements which led to extremely fast rotating bars
2. pushing further the quest of slow bars applying the TW method to a sample of dwarf barred galaxies, the best candidates to host slowly-rotating bars, since they are commonly thought to host a massive and centrally-concentrated DM halo.

With the purpose of unveiling the optical counterpart of the first Black Hole Transient (BHT) ever detected, a 12 sq. degree field of view was surveyed with DECam around the estimated position of Cen X-2.  BHT are known to harvour a stellar black hole, making them the perfect field for hunting for this kind of objects. BHT show very little X-ray emission while they are in quiescences. When they go into outburst, they may outshine the brightest X-ray sources in the sky. Nevertheless, it is more efficient to look for the optical counterpart than waiting for new outbursts to come. This 12 sq. degree survey displayed initially more than 10**7 objects, were only a few hundred of them were known to be variable stars . Nevertheless, it was expected that thousands of variable stars would be found in the whole field. This project reviews the whole process that has been done to reduce the 10**7 initial objects to 10**4  confirmed variable stars. A brief analysis into statistical techniques to distinguish the variable stars from the rest followed by a light curve morphology study accompanied with color analysis to determine the nature of the object. Finally, the study concludes with the discovery of a dozen pulsant objects and a candidate list of objects to be the historical XRT Cen X-2

Magnetic fields influence a large range of processes in and around stars. For this reason it is important to classify the magnetic fields on stars of a wide range of masses and evolutionary stages. In this talk we will go through how stellar magnetic fields are obtained using high resolution spectroscopy by studying the influence magnetic fields on spectral line shapes in both polarised and non-polarised spectra. These results can also be used to understand how stars affect both themselves and their circumstellar environment. I will also talk about the CRIRES instrument, a spectropolarimeter at the VLT, that is a powerful instrument to study activity on cool stars.

Small Solar System bodies are among the oldest remnants of the building blocks that led to the formation of the planets. As they experienced little geological evolution since their formation, they are valuable tracers of the early time of the Solar System. Studying their physical properties is  thus essential. Recently, an important high-angular resolution imaging survey of large main belt asteroids with the VLT/SPHERE (Vernazza et al. 2021) provided shape models of these bodies with an unprecedented accuracy. The adaptive-optics images show that most asteroids with diameters larger than 100 km present irregular shapes with significant topographic features. Hence the main question is: what forces are responsible for sustaining the observed topography? To answer that, we have to explore more complex structural models by considering layered internal structures and solid elastic-plastic materials with cohesive and shear strength in addition to a weak gravity. After an overview of the small body population, a more specific and detailed study of (2) Pallas asteroid will be presented. Between December 2022 and March 2023 (ESO Program ID 110.23P6.001; PI: M. Marsset), new observations under an equator-on orientation have been conducted to complement previous observations oriented pole-on. Now that its external shape is better constrained, we attempt to determine if its surface is relaxed. Additionally, we assess the error on models coming from SPHERE imaging by comparing them with models from the Dawn mission for Ceres and Vesta.