Seminars and Colloquia at ESO Santiago

Stellar flares cannot be spatially resolved, meaning we have to extract complex three-dimensional behavior from a one-dimensional disk-integrated spectral timeseries. Due to their proximity to Earth, solar flares can serve as a stepping stone for understanding their stellar counterparts, especially when using a Sun-as-a-star instrument such as HARPS(-N) in combination with spatially resolved observations. In this talk, I will discuss how high-resolution observations with a limited field-of-view can be converted into approximations of disk-integrated spectra using the newly developed Numerical Sun-as-a-Star Integrator (NESSI). Our findings suggest common patterns in the disk-integrated spectra between flares of different strengths and locations that can be used to better interpret stellar flares without resolved context.

Less than 30 years ago, we did not know whether planets exist outside our solar system. Fast forward to 2024, astronomers have discovered well over 7000 planets orbiting other stars similar to our Sun, including some that may have the right conditions to host life. As we learned that the formation of planets seems to go hand-in-hand with the birth of stars, we begin to wonder:

• What happens to planetary systems when their host stars run out of fuel, and turn into Earth-sized white dwarfs?
• Are those systems, if they exist, detectable?
• What will happen to our solar system, and to the Earth?
• And what are the possible implications for life?

I will discuss the final fate of planetary systems, the observational fingerprints of planets and their debris orbiting white dwarfs, and how studying these exotic systems help us to improve our general understanding of the formation of planets.

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Fundamental questions remain about the accretion and outflow physics of cataclysmic variable stars, and their counterparts, AM CVn systems. In an AM CVn system, both the accretor and donor are white dwarf stars. To date, only one AM CVn system has been detected in the radio spectrum and the mechanism for this is yet to be determined. Using observations from the Karl J. Jansky Very Large Array (VLA), I have obtained the deepest radio constraints to date, for two AM CVn systems: AM CVn and HP Lib. I have cross-matched these data and catalogue data, with large scale surveys such as LOFAR, RACS and VLASS to constrain the radio properties of the population of AM CVns for the first time ever. With this, we can investigate the radio emission mechanisms in accreting white dwarf binaries. Additionally, this study aims to contribute to ongoing efforts in understanding whether the radio emissions observed in CVs are due to flares in the atmosphere of the donor star, versus transient jets originating from the accretion disk. In this talk, I will discuss how understanding outflows and radio emissions from accreting white dwarfs such as these may help constrain radio emission mechanisms in CVs and similar compact binaries undergoing accretion.

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The most compelling evidence for existence of cosmic black holes enshrouded by
their event horizons is presently provided by the LIGO/VIRGO detections of
gravitational waves, the GRAVITY tracking of relativistic stellar and
accretion disk motions in the Galactic Centre, and the EHT imaging of the
strong gravitational lensing on scales down to several gravitational radii in
M87 and Sgr A*.  All these observations however can be still reconciled also
with a range of hypothetical horizonless objects such as wormholes,
gravastars, and naked singularities. This poses a problem of identifying the
most efficient means and ways for discerning between the canonical black holes
and their exotic cousins. While this problem is currently being addressed from
a number of different directions, it can be argued that measurements of the
total strength and three-dimensional structure of magnetic field on scales
below about ten thousand of gravitational radii may turn out to be the most
effective tool for that. Exploration of this possibility has been the prime
focus of the M2FINDERS project which employs several approaches for making
magnet field estimates down to the event horizon scale. These include studies
of the synchrotron spectrum and opacity, brightness temperature, and Faraday
rotation made with the EHT, GMVA, VLBA, JVLA and, prospectively, also with
ALMA. A review of the M2FINDERS methodology, observations, and early results
will be presented here and further discussed in context of perspective
developments of this line of research.

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The Pyramid Wavefront Sensor (PWFS) is a leading candidate for current and next-generation adaptive optics (AO) systems due to its high sensitivity. However, it has non-linearities and a limited dynamic range. The classical solution of using modulation to increase the dynamic range comes at the cost of reduced sensitivity and also limits the speed at which the AO system can operate to less than 1 kHz. Therefore, we propose using an unmodulated PWFS in the near-infrared, as the dynamic range increases linearly with wavelength. This eliminates the need for modulation and removes the speed limitation. The classical PWFS design employs a 4-sided pyramid (4PWFS) which suffers from a roof defect at the tip – an important issue in the absence of modulation, where precision at the tip is essential. The 3-sided PWFS (3PWFS) does not have this defect, making it a preferable  alternative. Additionally, it requires fewer pixels, making it less sensitive to readout noise.

We have developed a prototype of a 3PWFS and set up an AO bench testbed at the Geneva Observatory to evaluate its performance. We conducted laboratory and simulation tests under simulated OHP/PAPYRUS observing conditions. The results from the laboratory tests are in close agreement with our simulations - the 3PWFS successfully closes the AO loop under varying turbulence conditions, achieving a Strehl ratio of ~70% for seeing up to 4 arcsecs. I will present the design, operation, and results from the AO testbed. I will also present results from the on-sky tests conducted at the PAPYRUS bench. The results obtained at PAPYRUS bench are representative of expected performance at the VLT, as both sites have similar D/r₀ values. These results highlight the potential of the 3PWFS-based AO system as a prototype for integration into future high-contrast VLT and ELT instruments like RISTRETTO and ANDES.

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