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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)

Tel.: 04.366.9774
Fax: 04.366.9729
Recul rapide Année précédente Séminaires : Archives 2020 Année suivante  
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Mois précédent Mois suivant Janvier 2020
Séminaire suivant  1er séminaire : jeudi 09 janvier, 16h00
In pursuit of a reliable chemical classification of comets to unveil our Solar System origins and evolution
Manuela Lippi (NASA/GSFC)

Comets are cryogenically preserved relics from the early Solar System and their compositional properties are directly relevant to understanding the processes affecting material in protoplanetary system formation; moreover, they could test the hypothesis that small icy bodies have delivered prebiotic matter to early Earth. In the last 20 years, about 70 comets have been observed through ground-based high-resolution spectroscopy, targeting the 3 to 5 μm infrared spectral region with various high-resolution spectrometers (NIRSPEC, CSHELL, CRIRES, iSHELL), to quantify primary volatiles released directly from the nucleus. However, these systematic studies may suffer from inaccuracies introduced by the continuous evolution of the algorithms and molecular models used for interpreting fluorescence in comets, considering also different approaches that diverse groups could have used to analyze the data. Using the latest procedures and fluorescence models we can obtain updated chemical abundances of each comet, and acquire information about their origins and their role in Solar System formation and evolution.
Mois précédent Mois suivant Février 2020
Séminaire précédant Séminaire suivant 2ème séminaire : jeudi 20 février, 16h00
Transient continuous gravitational wave searches using machine learning and the Hough transform
Andrew Miller (UCLouvain & Virgo)

We present two new methods to search for gravitational waves from isolated neutron stars lasting O(hours-days), and the results of two searches of real LIGO/Virgo data for a remnant of the first binary neutron star merger GW170817. One method looks for signals that follow a specific power-law model; the other uses convolutional neural networks to detect signals that deviate from this model. No significant candidates were found in either search, therefore we place upper limits on the possible gravitational wave emission from the remnant.
Mois précédent Mois suivant Mars 2020
Séminaire précédant Séminaire suivant 3ème séminaire : jeudi 05 mars, 16h00
Exoplanet imaging by interferometry: a new paradigm
Denis Defrère (STAR)

The spectral characterisation and understanding of terrestrial exoplanets is currently one of the most ambitious and challenging long-term goals of astrophysics. All observing techniques with the potential to tackle this challenge face the same limitations: the overwhelmingly dominant flux of the host star and/or the lack of angular resolution. A very promising technical solution around these issues is nulling interferometry, which combines the advantages of stellar interferometry (high angular resolution) and coronagraphy (starlight rejection). Thanks to decades of research and investments by ESO, interferometry has now become sufficiently mature to enable the direct characterisation of exoplanets as showcased by new results from the GRAVITY instrument installed on the Very Large Telescope Interferometer (VLTI). In this talk, I will present a new project recently-funded by the European Research Council (ERC) to build and install the first VLTI instrument with nulling capabilities. By leveraging its state-of-the-art infrastructure, long baselines, and strategic position in the Southern hemisphere, the new VLTI instrument will be able to carry out several high-impact exoplanet programmes to characterise the chemical composition of Jupiter-like exoplanets at the most relevant angular separations (i.e., close to the snow line) and better understand how planets form and evolve. I will end this presentation with long-term prospects to search for biosignatures on the surface of rocky exoplanets with a similar instrument launched into space.
Séminaire précédant Séminaire suivant 4ème séminaire : jeudi 12 mars, 16h00
Gravity waves in stellar interiors: from dynamical effects to seismic probing potential
Charly Pinçon (STAR)

Gravity waves, or buoyancy waves, are well observed in the Earth’s atmosphere and oceans. Similarly, their presence is ubiquitous in stellar interiors. In low-mass stars, convective motions in the external envelope generate gravity waves that tunnel toward the underlying stably-stratified core in which they can propagate. The spectral characteristics of these waves depend on the properties of their propagation cavity and their observation is thus expected to provide precious seismic constraints on stellar cores. In turn, we know that these waves can interact with the medium and modify the global dynamics, although they are still neglected in current stellar evolution codes. Gravity waves therefore stand for both a dynamical actor and an information carrier. In the last decades, the advent of the space-borne missions CoRoT and Kepler provided exquisite seismic data for thousands of low-mass stars. Among other successes, the detection of gravity modes in red giant stars gave us an unprecedented insight on the inner regions of stars and deeply questioned our understanding of stellar evolution. In particular, several studies clearly showed that the current modeling of the internal rotation evolution is insufficient, leading to huge uncertainties in the description of transport processes (i.e., of angular momentum, heat, or chemical species), as well as in the observational estimates of stellar parameters (i.e., mass, radius, or age). In this talk, I will demonstrate that the dynamical influence of low-frequency internal gravity waves on the rotation of stars is a serious option to reconcile the theoretical predictions with the observations. Furthermore, I will show that the probing potential of gravity modes in red giant stars (or more exactly mixed modes, owing to their coupling with acoustic modes at surface) has not been fully exploited yet, and that a detailed seismic analysis of their spectrum can provide us with even more constraints on the internal properties of these stars.
Mois précédent Mois suivant Avril 2020
Séminaire précédant Séminaire suivant 5ème séminaire : jeudi 30 avril, 16h00
The ultraviolet aurorae at Jupiter: Juno's perspective (with a little help from Hubble and Hisaki)
Dr. Bertrand Bonfond (University of Liège, STAR institute, LPAP)

It is always challenging to understand a complex system from fragmented pieces of information only. This is particularly true for the magnetosphere and aurorae at Jupiter, since we only had access to single point measurements and a few snapshots of the aurora and of the plasma torus. Furthermore, the strength of the magnetic field, the larger distance to the Sun, the presence of an internal plasma source and the rapid rotation of Jupiter make these systems fundamentally different from what we know on Earth, making the direct importation of concepts from one planet to another perilous. The Hisaki and Juno missions, together with the large Hubble Space Telescope observations campaigns supporting them, considerably helped to fill critical gaps in the datasets by giving us access to the history of events. In particular, the spatial and/or temporal continuity that characterize these new observations has allowed us to better disentangle the internal and external factors ruling the way matter and energy circulate into these systems. Moreover, high resolution images and comparisons between in-situ and remote sensing measurements also proved to be particularly powerful tools to test theories. During this seminar, I will focus on a few key results from the Juno era, including the satellite footprints, plasma injection signatures, the general auroral morphology, dawn storms and the polar emissions. I will discuss how some of them essentially confirmed our understanding of the auroral processes while others challenged them considerably.
Mois précédent Mois suivant Octobre 2020
Séminaire précédant Séminaire suivant 6ème séminaire : jeudi 08 octobre, 16h00
Using continuous gravitational waves to detect neutron stars, black holes, and dark matter
Andrew Miller (UClouvain)

Continuous gravitational waves are quasi-infinite, quasi-monochromatic signals that are expected from a variety of interesting sources, namely asymmetrically rotating neutron stars, inspiraling primordial black hole binaries, and depleting axion clouds around black holes. Each of these sources results from different physics, and yet could leave remarkably similar (but still distinct) imprints on the LIGO-Virgo interferometers. In this talk we will explain how gravitational waves are emitted by these sources, and what the implications of a detection would be for each of them. We will also describe how the same data analysis methods can be tuned specifically to each of these sources. Additionally, the most recent upper limits on the gravitational wave emission from these sources will be presented.
Séminaire précédant Séminaire suivant 7ème séminaire : mardi 13 octobre, 16h00
Lagrangian modelling of a person lost at sea during the Adriatic scirocco storm of 29 October 2018
Matjaž Ličer (National Institute of Biology, Slovenia)

Lagrangian particle tracking of objects lost at sea is an important branch of ocean forecasting. Maritime search and rescue (SAR) or other types of civil service responses depend on timely and reliable estimates of the most probable areas which contain the drifting object. On 29 October 2018 a windsurfer's mast broke about 1 km offshore from Istria during a severe scirocco storm in the northern Adriatic Sea. He drifted in severe marine conditions until he eventually beached alive and well in Sistiana (Italy) 24 h later. In my talk i will present our efforts to create a modelling system which will assist authorities in the future. In this case we had a rare opportunity to conduct an interview with the survivor to reconstruct his trajectory and to gain insight into his swimming and paddling strategy. Part of survivor's trajectory was verified using high-frequency radar surface current observations as inputs for Lagrangian temporal back-propagation from the beaching site. Back-propagation simulations were found to be largely consistent with the survivor's reconstruction. We then attempted a Lagrangian forward-propagation simulation of his trajectory by performing a leeway simulation for various drifter types. I will briefly discuss HF radar observational systems, the physics of marine drift problems and Lagrangian approach to their modeling.
Séminaire précédant Séminaire suivant 8ème séminaire : jeudi 15 octobre, 16h00
Dust and stellar property estimates via machine learning techniques
Wouter Dobbels (U. Gent)

Large surveys have been performed from the ultraviolet (UV) to the far-infrared (FIR). Some galaxies are observed over this whole wavelength range, and through SED fitting we can estimate the stellar and dust properties. Unfortunately, most galaxies are only detected at a limited part of this spectrum.

With machine learning techniques, we can use the UV-FIR galaxies as a blueprint: we learn the mapping from a subset of their fluxes to their properties. For example, a mapping from UV-NIR (i.e. stellar emission) to dust mass can be established, and then applied to galaxies that lack FIR data. Whereas traditional SED fitting methods can only estimate dust mass using FIR data, our method is accurate (RMSE = 0.3 dex) and unbiased in the absence of FIR. Besides what can be directly estimated from the SEDs alone, this technique implicitly uses relations that follow from galaxy evolution. To avoid a black box, we take special care to estimate uncertainties on our predictions and to interpret the model. The main idea can be generalised: a sample with a broad set of observations can be used as a blueprint, to improve estimates of samples with limited observations.
Séminaire précédant Séminaire suivant 9ème séminaire : mardi 27 octobre, 16h00
Sentinel-3: the Blue Sentinel
Dr Hayley Evers-King (EUMETSAT)

Over the last 50 years, developments in satellite Earth Observation have provided new perspectives on our planets oceans. A growing legacy of sensors have been able to capturing the vast range of processes that occur in the ocean over smaller/shorter, and larger/longer spatio-temporal scales. In this seminar I will provide an overview of the Sentinel-3 mission – a constellation of satellites providing ocean data as part of the European Commission Copernicus programme. These satellites carry a suite of instruments, representing the interdisciplinary nature of oceanography, measuring biological and physical ocean parameters through ocean colour and temperature radiometry, and radar altimetry. The data is being used for a wide variety of scientific studies, as well as in downstream services for activities in the marine domain including shipping, fisheries, aquaculture, maritime safety, marine spatial planning and policy development. I will share some examples of these applications, and provide attendees with further information on how to start using Sentinel-3 data for their own work.
Mois précédent Mois suivant Novembre 2020
Séminaire précédant Séminaire suivant 10ème séminaire : mardi 24 novembre, 17h00
Coastal deoxygenation related to a poleward shift of the Gulf Stream
Mariona Claret (University of Washington)

Deoxygenation is widely regarded as one of the ‘big three’ threats of climate change to the ocean, along with acidification and sea level rise. Global observations show that the ocean lost approximately 2% of its oxygen inventory over the past five decades. Because of their higher oxygen requirements, deoxygenation is of particular concern for large fish and invertebrates, the majority of which are found in highly productive coastal regions. But we currently have a very blurry vision of how deoxygenation of these coastal regions will progress in future, given that historical records of past oxygen changes are sparse, and coastal regions are very poorly simulated by standard global climate models.

This talk will focus on the Gulf of St. Lawrence, the largest estuary in the world, where a thriving marine life supports an environment and an economy of high value. The sustenance of this rich ecosystem is, however, threatened by a dramatic deoxygenation as oxygen levels of bottom waters dropped by 50% between 1930 and 2003. A global climate model at unprecedented high resolution shows that this severe oxygen decline occurred throughout the northwest Atlantic and will likely continue with further global warming. We found that the undergoing local deoxygenation is largely driven by a remote shift in the large-scale currents in response to anthropogenic CO2 emissions in the atmosphere. Specifically, by a poleward retreat of the oxygen-rich Labrador Current that results in a rising proportion of oxygen-poor Gulf Stream waters on the shelf. This shift is also observed using centennial-scale hydrographic observations in the open ocean and the model suggests that it is probably linked to a historical weakening of the Atlantic Meridional Overturning Circulation. Our results show that coastal deoxygenation in the northwest Atlantic is a canary in a coal mine, in the sense that it is a sensitive indicator of a large-scale dynamical shift that is underway as a result of climate change.
Séminaire précédant Séminaire suivant 11ème séminaire : jeudi 26 novembre, 16h00
Pulsar Glitches: A typical timing irregularity to probe superfluid inside the neutron stars.
A. Basu (University of Manchester)

Pulsars are rapidly rotating neutron stars, observed to emit beamed radio emission along the open magnetic field lines. Their extremely stable rotation rate along with pulsed emission makes them perfect cosmic clocks. Though they are known to have stable rotation, often the younger population exhibits sudden deviation in such properties. Such irregularities are called as glitches and timing noise. Such variations are studied via the technique of pulsar timing. Pulsar glitches are excellent probes on the superfluid inside the neutron star. In particular, the post glitch evolution of the rotation rate is closely connected to the inhomogeneous matter inside the neutron star crust and the properties of the superfluids. In this talk, I will present a pedagogical introduction to the physics of pulsar glitches and pulsar timing. I will also talk about our recent results obtained from the theoretical and observational studies using the Ooty Radio Telescope (ORT) and Upgraded Giant Metrewave Radio Telescope (uGMRT)
Mois précédent Mois suivant Décembre 2020
Séminaire précédant Séminaire suivant 12ème séminaire : jeudi 03 décembre, 16h00
Dense Matter inside Neutron Stars: Constraints from Astrophysical Observations
Prasanta Char (ULiège)

Neutron stars are the densest object found in the observable universe. They are proven to be excellent laboratories to study the physics of matter at extreme conditions which are beyond the scope of any terrestrial experiments. Recent multimessenger observations of neutron stars such as the measurements of the tidal deformability from the gravitational wave event GW170817, the X-ray observations of the mass and radius of the object J0030+0451 by the Neutron Star Interior Composition Explorer (NICER) instrument aborad the International Space Station along with the discoveries of massive radio pulsars with ∼ 2M⊙ in the last decade have significant implications towards the understanding the equation of state of dense nuclear matter. In this talk, I will discuss how these information are combined with the low energy nuclear experimental data to constrain the properties and the composition of the interior of neutron stars. I will also review the correlation between certain nuclear empirical parameters such as the symmetry energy, incompressibility with the macroscopic structure parameters such as the radius, tidal deformability of the star.
Séminaire précédant 13ème séminaire : mardi 15 décembre, 16h00 
Marine diazotrophic cyanobacteria
Mar Benavides (Mediterranean Institute of Oceanography (France))

Marine diazotrophic cyanobacteria provide ~50% of the bioavailable nitrogen in the ocean. They are diverse in size, shape, life strategies and metabolism, and their biogeography spans from the Equator to poles. In this lecture we will explore the role of diazotrophs in marine biogeochemistry and their expected responses in the light of climate change. Some current research topics will be discussed, including their aggregation dynamics, their sub/mesoscale variability and their potential as prey for higher trophic levels.
Université de Liège > Faculté des Sciences > Département d'Astrophysique, Géophysique et Océanographie : CoWebAGO, Juin 2009.