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Institut d'Astrophysique et
de Géophysique (Bât. B5c)

Quartier Agora
Allée du 6 août, 19C
B-4000 Liège 1 (Sart-Tilman)
Belgique

Tel.: 04.366.9774
Fax: 04.366.9729
Recul rapide Année précédente Séminaires : Archives 2021 Année suivante  
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Mois précédent Mois suivant Janvier 2021
Séminaire suivant  1er séminaire : jeudi 21 janvier, 16h00
Stochastic GWSearch for a Stochastic Gravitational Waves Background using a Network of Detectors
Anirban Ain (INFN, Pisa)

The direct detection of Gravitational Waves (GW) by LIGO and Virgo is one of the most significant discoveries of modern times. The network of operational GW detectors is expected to observe a stochastic gravitational waves background (SGWB) in the near future. This all-sky source is made up of unresolved astrophysical sources and remnant GW from cosmological phenomena. Detection of the SGWB can be challenging because the signal is much smaller than the detector noise and being a stochastic signal there is no optimized filter to match-filter the data. Instead, an aperture synthesis technique is used to search for SGWB where data from different detectors are correlated to find signals and long stretches of data is added to build up significance. I have been working on the improvement of SGWB search pipelines. In recent years we implemented data folding and some new techniques to make the pipeline a few thousand times faster. I will be discussing SGWB mapping techniques and how they are optimized.
Séminaire précédant Séminaire suivant 2ème séminaire : jeudi 28 janvier, 16h00
Probing Early universe with Gravitational waves at nano-frequencies
Rome Samanta (CEICO, Institute of Physics, Czech Academy of Sciences, Prague.)

The discovery of Gravitational Waves (GWs) by LIGO and Virgo collaboration has unequivocally opened up a new cosmic frontier for the multi-frequency study of stochastic GW background (SGWB). There are several sources in the Early Universe (EU) that can produce detectable SGWB. Interestingly, recently the NANOGrav pulsar timing array (PTA) collaboration have reported strong evidence for a stochastic common-spectrum process over independent red-noises across 45 pulsars at a frequency f~1/yr. If this is interpreted as GWs, several interesting mechanisms such as models of inflation, primordial black hole dark matter, and baryogenesis can be probed. In this talk, I will briefly summarise some of the mechanisms/sources that can produce GW at nano-frequencies and therefore testable by the PTAs. I will particularly give emphasis on the models of baryogenesis and discuss how PTAs serve as novel probes of baryogenesis-one of the unresolved cosmological puzzles to date.
Mois précédent Mois suivant Février 2021
Séminaire précédant Séminaire suivant 3ème séminaire : jeudi 25 février, 16h00
Degeneracies in gravitational lens modeling: Can implicit assumptions about galaxy structure introduce problems for H_0?
Matt Gomer (STAR)

Gravitational lens modelling is subject to a number of degeneracies, meaning the same observables are reproducible with different lens characteristics. To make a decision between many possible solutions, a lens model is informed by assumptions about galaxy structure, such as that the radial profile being a power law or the lens having an ellipse shape. What if the lens does not correspond to these simple assumptions? This talk explores some possible ramifications for H_0 measurement when lens models are oversimplified.
Mois précédent Mois suivant Mars 2021
Séminaire précédant Séminaire suivant 4ème séminaire : jeudi 04 mars, 16h00
Seasonal cycle of Arctic Ocean circulation inferred from satellite altimetry
Francesca Doglioni (Alfred-Wegener Institute, Climate Sciences | Physical Oceanography of the Polar Seas)

Francesca Doglioni1, Benjamin Rabe1, Robert Ricker1, Torsten Kanzow1,2

1 Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany.
2 Department of Physics and Electrical Engineering, Universität Bremen Germany

In recent decades, the retreat of the Arctic sea ice has modified vertical momentum fluxes from the atmosphere to the ice and the ocean, in turn affecting the surface circulation. Satellite altimetry has contributed in the past ten years to understand these changes. Most oceanographic datasets are however to date limited either to open ocean or to ice-covered regions, given that different techniques are required to track sea surface height over these two surfaces. Hence, efforts to generate unified Arctic-wide datasets are still required to further basin-wide studies of the Arctic Ocean surface circulation.

We present here the assessment and the seasonality of a new Arctic-wide gridded dataset of the Sea Level Anomaly (SLA) and SLA-derived geostrophic velocities. This dataset is based on Cryosat-2 observations over ice-covered and open ocean areas in the Arctic during 2011 to 2018.

We compare the SLA and geostrophic currents derived hereof to in situ observations of ocean bottom pressure, steric height and near-surface ocean velocity, in three regions: the Fram Strait, the shelf break north of the Arctic Cape and the Laptev Sea continental slope. Good agreement in SLA is shown at seasonal time scales, with the dominant component of SLA variability being steric height both in Fram Strait and at the Arctic Cape. On the other hand, ocean bottom pressure dominates SLA changes at the Laptev Sea site. The comparison of velocity at two mooring transects, one in Fram Strait and the other at the Laptev Sea continental slope, reveals that the correlation is highest at the moorings closest to the shelf break, where currents are faster and the seasonal cycle is enhanced.

The seasonal cycle of SLA and geostrophic currents as derived from the altimetric product is in favourable agreement with previous results. A quasi-simultaneous occurrence of the SLA maximum happens between October and January; similar phase has been found in steric height seasonal cycle by studies using hydrographic profiles in several regions of the Arctic Ocean. We thereby find the highest SLA amplitude over the shelves, which other studies point to be possibly related to winter-enhanced shoreward water mass transport. Seasonal variability in the geostrophic currents is most pronounced along the shelf edges, representing a basin wide, coherent seasonal acceleration of the Arctic slope currents in winter and a deceleration in summer. This is consistent with the shelf-amplified SLA seasonal cycle described above. Density driven coastal currents near Siberia are strongest in spring/summer and weakest in winter, consistent with the cycle of river runoff. Enhanced south-western limb of the Beaufort Gyre in early winter is in agreement with a combination between the Beaufort High buildup and relatively thin sea ice.

In summary, we provide evidence that the altimetric data set has skills to reproduce the seasonal cycle of SLA and geostrophic currents consistently with in situ data and findings from other studies. We suggest that this dataset could be used not only for large scale studies but also to study Arctic boundary currents.
Séminaire précédant Séminaire suivant 5ème séminaire : jeudi 25 mars, 16h00
Maximising the resolution of large aperture telescopes to study planets close to their birth
Matt Willson (STAR)

Occupying the resolution space between coronagraphic imaging and long baseline interferometry, sparse aperture masking or non-redundant masking provides the ability to probe resolution spaces inaccessible to either. Using precise and self calibrating triangles of baselines, we are able to recover companions with contrasts of ~6-8mag on the lambda/D scale, and in some cases even closer. Additionally we can provide the missing short baselines for image restoration of long baseline interferometry data sets, adding in the necessary context to fully understand fine structure on the mas scale. I will describe results of a survey of young stars aimed at resolving the most promising regions for planet formation, discovering four potential protoplanets in their natal disc. I will then further present the results of our follow up efforts to reobserve and characterise these objects, including the potential discovery of a sub-stellar mass companion at an angular separation of 0.25lambda/D within the complex disc surrounding the Herbig star, V1247 Orionis.
Mois précédent Mois suivant Avril 2021
Séminaire précédant Séminaire suivant 6ème séminaire : jeudi 01 avril, 16h00
Deep learning searches for gravitational waves stochastic backgrounds
Andreï Utina (Nikhef)

The background of gravitational waves has long been studied and remains one of the most exciting aspects in the observation and analysis of gravitational radiation. The work focuses on the study of the background of gravitational waves using deep neural networks. An astrophysical background due to the presence of many binary black hole coalescences was simulated for Advanced LIGO O3 sensitivity and the Einstein Telescope design sensitivity. The detection pipeline targets signal data out of the noisy detector background. Its architecture comprises of simulated whitened data as input to three classes of deep neural networks algorithms: a 1D and a 2D convolutional neural network (CNN) and a Long Short Term Memory (LSTM) network. It was found that all three algorithms could distinguish signals from noise with high precision for the ET sensitivity, but the current sensitivity of LIGO is too low to permit the algorithms to learn signal features from the input vectors.
Séminaire précédant Séminaire suivant 7ème séminaire : jeudi 29 avril, 16h00
Seasonal Hydrography of the west Antarctic Peninsula: The Oceanic Forcing to Glacier Retreat
Borja Aguiar González (Physical Oceanography and Applied Geophysics, Universidad de Las Palmas de Gran Canaria)

Seasonal Hydrography of the west Antarctic Peninsula: The Oceanic Forcing to Glacier Retreat [to be completed]
Mois précédent Mois suivant Mai 2021
Séminaire précédant Séminaire suivant 8ème séminaire : jeudi 06 mai, 16h00
A bright future for cometary science
Cyrielle Opitom (The Royal Observatory, Edinburgh)

Comets are some of the most pristine relics of the early solar system. Our understanding of their formation, structure, and composition has made huge progresses in the last decade, in large part thanks to the Rosetta mission. However, many questions are still left to answer. In this talk, I will go through future mission, instrumentation, and areas of research that will allow us to tackle these questions in the future. I will first give a brief overview of the newly selected ESA Comet Interceptor mission, that will for the first time visit a comet coming directly from the Oort cloud. I then will talk about CUBES, the future near-UV spectrograph to be installed on the Very Large Telescope, which has a huge potential for the study of comets. Finally, in the last part of the talk, I will tell the story of interstellar comets. The first interstellar object was discovered only a few years ago, and they provide incredible opportunities to probe material that was formed around different stars and compare it to our own solar system comets.
Séminaire précédant Séminaire suivant 9ème séminaire : jeudi 20 mai, 16h00
Recent tests of gravity theories with pulsar timing
Paulo Freire (Max-Planck-Institut für Radioastronomie)

In this talk, I describe how the precise timing of pulsars in several binary systems and one triple system have in the last two years yielded far more stringent tests tests of fundamental properties of space-time and gravity. We concentrate on a few tests of the universality of free fall (UFF), which include tests of the fundamental nature of gravitational waves. In general relativity, the latter have a purely quadrupolar structure, but in alternative gravity theories they can have a large dipolar component. All experiments we have made confirm, within experimental precision, the purely quadrupolar structure of gravitational waves, and more broadly the UFF. These results can be seen as tests of specific gravity theories. In particular, the radiative test with the ``double pulsar'' system, J0737-3039, confirms the general relativistic quadrupole emission formula to a precision of 0.00013 (95 % C. L.), which is 25 times more precise than the previous best radiative test with the the original binary (Hulse-Taylor) pulsar. General relativity passes all the new pulsar tests, and several classes of alternative theories of gravity are either ruled out or severely constrained.
Mois précédent Mois suivant Octobre 2021
Séminaire précédant Séminaire suivant 10ème séminaire : jeudi 21 octobre, 16h00
Reading physics from stellar spectra
Maria Bergemann (New Horizons Solvay Lectures)

Observational stellar astrophysics is being revolutionised by large-scale stellar surveys and novel diagnostic techniques. I will focus on the frontiers in stellar spectroscopy.
First, I will describe the 3D non-equilibrium models that are poised to become workhorses of astronomy in the next decade.
Second, I will show how we overcome the challenge of combining of new models and complex noisy data. In particular, I will address the problem of the solar metallicity, and show new results for the solar Oxygen abundance obtained by our group.
Third, I will demonstrate how new data and new models drive progress in the areas that rely on stellar parameters and chemical composition of stars, focussing on precision tests of stellar structure and stellar evolution.
Séminaire précédant Séminaire suivant 11ème séminaire : jeudi 28 octobre, 12h30
Species diversity, ecomorphology and diversification of Iphimediidae (Crustacea, Amphipoda) on the Antarctic continental shelf
Marie Verheye (ULiège, Freshwater and OCeanic science Unit of reSearch (FOCUS))

From about 40 million years ago, while Antarctica geographically isolated from the rest of the world, the marine shelf fauna faced a dramatic decrease in water temperatures. Many lineages went extinct, while others adapted and flourished. The Antarctic clade of the amphipod family Iphimediidae was among the successful ones. However, the evolutionary processes which generated their exceptional diversity remain unknown. The first objective of this project is to assess species diversity within Antarctic iphimediids using a combination of DNA-based and 3D-geometric morphometric methods. Secondly, stable isotope (N, C) data and 3D shape data of the mouthparts were collected in order to explore the correlated evolution between those traits and the possible adaptive significance of morphological variation. And lastly, potential changes in the rate of lineage diversification are analyzed in parallel to the evolution of ecomorphological diversity along a time-calibrated molecular phylogeny, in order to test different radiation scenarios (single or multiple adaptive radiation(s) or a scenario of non-adaptive radiation). Applying such an integrative approach for the first time on Antarctic invertebrates, this study aims to improve our general understanding of the evolutionary processes which shaped the standing Antarctic shelf benthic biodiversity.
Séminaire précédant Séminaire suivant 12ème séminaire : jeudi 28 octobre, 16h00
E-TEST: A prototype of large cryogenic mirror isolated from low-frequency seismic noise
Christophe Collette (Precision mechatronics laboratory, ULiège & BEAMS department, ULB)

The objective of the E-TEST instrumentation program is to develop a prototype for validating the Einstein-telescope-low-frequency (ET-LF) technology, i.e. a ‘large’ silicon mirror, an excellent suspension system for isolating the mirror from seismic motion, and a cooling strategy that is compatible with the isolation system. An active isolation system will be used and combined with a quasi-radiative cooling system at 10 K. A new patterned heat exchanger working radiatively have to be designed and implemented. The prototype will be installed in the existing facilities at the Space Centre of Liège (CSL) to take advantages of the knowledge in cryogenic qualification of Space Instruments.
Mois précédent Mois suivant Novembre 2021
Séminaire précédant Séminaire suivant 13ème séminaire : jeudi 04 novembre, 16h30
Extreme astrophysics
Roger Blandford (KIPAC, Standford et titulaire de la chaire Jacques Solvay de Physique 2020)

The electromagnetic spectrum has been opened up from meter radio waves to ~ PeV photons and augmented with 10 - 300 Hz gravitational wave, MeV - PeV neutrinos and MeV - ZeV (160 J) cosmic ray messages. Consequently, there is a high rate of discovery and understanding of phenomena whose explanation invokes accepted physics - classical (including general relativity), atomic, nuclear and particle (including QED) processes - in extreme environments. The richness of the discovery space can be epitomized by describing some new observations and ideas pertaining to relativistic outflows formed by spinning black holes and neutron stars, Ultra High Energy Cosmic Rays accelerated by strong shock waves surrounding rich clusters of galaxies and Fast Radio Bursts, generated by neutron stars with 100 GT magnetic fields.
Séminaire précédant Séminaire suivant 14ème séminaire : jeudi 18 novembre, 16h00
Imaging giant planet formation
Valentin Christiaens (STAR)

The new generation of infrared high-contrast imagers and sub-mm interferometer has been revolutionizing our view of planet formation for almost a decade. Protoplanetary discs - the birth cradles of planets, have now been imaged at a high level of detail, revealing a wealth of structures including cavities, annular gaps, spiral arms, shadows and asymmetries. In order to connect these structures to forming planets and provide the most stringent constraints to planet formation theories, we need to directly image newborn planets in these discs. I will first highlight the challenges involved in this endeavour, and will then focus on the specific case of PDS 70, the only disc with confirmed directly imaged protoplanets thus far. In particular, I will summarize what we have learned from 3 years of follow-up and characterisation of this system. I will also present ongoing work on other promising sources, and briefly describe future prospects for the field, considering the advent of the James Webb Space Telescope and Extremely Large Telescopes.
Mois précédent Mois suivant Décembre 2021
Séminaire précédant Séminaire suivant 15ème séminaire : jeudi 02 décembre, 16h00
Control of the abyssal ocean overturning circulation by mixing-driven bottom boundary layers
Henri Drake (Princeton University and NOAA's Geophysical Fluid Dynamics Laboratory)

The abyssal ocean circulation is primarily driven by turbulent mixing, which is generally weak in the ocean interior but is enhanced above rough topography, such as mid-ocean ridges. Abyssal upwelling is the small residual of vigorous upwelling in thin sloping Bottom Boundary Layers (BBLs) and broad downwelling in the Stratified Mixing Layers (SML) above. A multi-scale hierarchy of models reveals these turbulent boundary layer dynamics and show the most complex of these models is consistent with observations. Sloping boundary layer theory predicts that SML downwelling largely compensates for BBL upwelling, resulting in a net overturning much weaker than observed; at equilibrium, however, the increase of stratification with height suppresses much of the SML downwelling and thus supports a realistic finite overturning circulation that scales roughly with the BBL upwelling. The magnitude of BBL upwelling is set by the near-boundary stratification, which is maintained by a variety of processes: a buoyancy-driven cross-slope circulation, restratification by submesoscale baroclinic eddies, topographic standing eddies, and the blocking of along-slope thermal wind shear by narrow canyon walls. Both mixing rates and the diapycnal velocity can be inferred using purposeful Tracer Release Experiments, although estimates of the mixing rates must account for additional diapycnal stretching effects which can be of leading order near rough topography.
Séminaire précédant 16ème séminaire : jeudi 09 décembre, 16h00 
The importance of hydrodynamics on coastal lagoons processes
Francisco López Castejón (Technical University of Cartagena)

When we think about a highly anthropogenic impacted area as the Mar Menor, we usually think about the biological, chemical and ecological aspects of it. Nevertheless the role of the hydrodynamic processes as a regulator of these processes usually is discarded. Recent studies have showed how coastal lagoons have a complex hydrodynamic structures: eddies, upwellings, countercurrents and the importance to know properly how the water exchange with the Mediterranean Sea is forced and how any change of the shape or bathymetry of the communication channels could affect to the lagunar ecosystem. Moreover, the hydrodynamic conditions could be a trigger of processes as eutrophication. In my presentation I will show the result of different hydrodynamic studies performed in the Mar Menor coastal lagoon and how different instrumentation (ADCP, pressure gauge) and methodologies (numerical model) are used for a better understanding of the role of the hydrodynamic processes on the lagoon.
Université de Liège > Faculté des Sciences > Département d'Astrophysique, Géophysique et Océanographie : CoWebAGO, Juin 2009.