• Jeudi 20 février 2020 à 16h00 (Salle de conférence du bâtiment 17)

    Discovery of an azimuthal density gradient in a gas-rich debris disk possibly related to a massive collision

    Garima Singh (LESIA)

    The gas-rich debris disk of HD 141569, first discovered in 1999 with the HST in near-IR and later in visible revealed structures such as multiple rings and outer spirals extended as far out as 410 AU. More recently, the direct imaging SPHERE instrument has resolved several non-uniform concentric rings inside the inner cavity (<100 AU) in near-IR. A North-South asymmetry was discovered in the brightest and innermost ringlets at 40 AU, which is aligned with the disk projected major axis. This observed asymmetry cannot be explained by the light scattering properties of dust. We proposed instead an azimuthal variation of the dust density. However, this interpretation is complicated by the post-processing techniques commonly used for scattered light disk observations in total intensity, which particularly impact the shape and local photometry of extended objects. In 2017, we have acquired polarimetric data using the dual polarimetric imaging mode of SPHERE/IRDIS. We discovered that the Lorentzian azimuthal distribution of the intensity reported in the innermost ring in Perrot et al. (2016) is significantly different in total intensity and polarized intensity. Assuming a model based on the massive collisions between planet embryos, we found that both images can be described as a combination of a phase function and an azimuthal density variation which takes a Lorentzian profile peaking to the south-west of the ring. The complementarity of polarimetric and total intensity images has allowed us to constrain the actual dust density distribution and to relate this morphology to a potential massive collision. In this talk, I will introduce HD141569 followed by the imaging modes of SPHERE and the disk structure observed within 100 AU both in total and polarimetric intensity. I will then present the hypothesis that observed asymmetries are produced by an azimuthal variation of the dust density, possibly connected to massive collisions.


  • Lundi 27 janvier 2020 à 11h00 (Salle de conférence du bâtiment 17)

    The characterisation of exoplanetary systems - From stellar to planetary parameters

    Roxanne Ligi (Osservatorio Astronomico di Brera, Italy)

    The harvest of transiting exoplanets discoveries has led to the quest of their characterisation. However, the radius, mass and density of exoplanets totally rely on the host star parameters. But interferometry allows to measure the stellar radius and the transit light curves give the stellar density. This directly yields the stellar mass, a parameter that cannot be directly measured otherwise.

    We applied this method to the systems of 55 Cnc and HD219134 which host one and two transiting exoplanets, respectively. Thanks to the newly measured stellar and exoplanets parameters, we derived the exoplanets internal compositions using an inference scheme. We show that, contrary to what previous studies show, the transiting exoplanet 55 Cnc e may only have a thin atmosphere and its interior structure might not be dominated by carbon. Our new radii for HD219134 b and c remove them from the "Fulton gap" and thus suggest a rocky nature. The lower density of the inner more massive planet could then be explained by a molten interior possibly induced by tidal heating caused by a high eccentricity during its formation.

    Transiting exoplanets around bright stars constitute benchmark systems to investigate exoplanet properties and calibrate stellar models. With the on-sky TESS satellite and the forthcoming mission PLATO, we will be able to extensively apply this method, and thus revisit exoplanets populations and deepen our knowledge of stellar evolution.


  • Vendredi 24 janvier 2020 à 11h00 (Salle de conférence du bâtiment 17)

    Wave turbulence and dissipation in solar and stellar winds

    Victor Réville (IRAP)

    Solar-like stars, and particularly the Sun, exhibit hot coronae that can reach temperatures of several million Kelvin, consequently driving expanding winds. Mechanisms for the heating and the acceleration of these winds are still largely debated, however, the thorough study of the solar wind for the past 60 years has motivated an important focus on wave turbulence. In this talk, I will review and discuss models for Alfvén wave propagation and turbulent cascade, which eventually leads to dissipation. In the light of a recent study, we will see how compressible effects -namely the parametric decay instability- can be involved in the generation of a turbulent power spectrum from a narrow band emission of Alfvén waves at the chromosphere. I will then discuss how to implement turbulent transport and dissipation in global models and compare numerical simulations with the first results of the Parker Solar Probe. I will show how these global simulations are useful to make the remote sensing / in situ connection in the context of PSP and Solar Orbiter. Finally, zooming out to a more stellar context, I will discuss the important constraints that need to be taken into account to transpose solar models to other solar-like stars.


  • Jeudi 23 janvier 2020 à 16h00 (Salle de conférence du bâtiment 17)

    Sensing differential pistons between petals of the ELT

    Arielle Bertrou (LESIA)

    The presence of a six legged 50 cm-wide spider supporting the secondary mirror of the ELT breaks the spatial continuity of the incoming wave-front. Atmospheric turbulence, low wind effect, thermo-mechanic drift of the deformable mirror are all contributors to misphasing of the six petals of the telescope. It is therefore necessary to measure these differential pistons in order to reconstruct the full wave-front.

    The pyramid wave-front sensor is currently the preferred design for adaptive optics systems. However, it was shown to be a poor petal sensor in the visible and under partial turbulence correction. This talk will present results that highlight the inability to sense petal pistons given the actual design of MICADO SCAO instrument. Based on this, we investigate the possibility of a petalometer : a sensor specifically designed to answer the need for petal pistons sensing.


  • Jeudi 12 décembre 2019 à 16h00 (Salle de conférence du bâtiment 17)

    Telescope wavefront control at ELT

    Henri Bonnet (ESO)

    The ESO Extremely Large Telescope will be exposed to large amplitude dynamic perturbations in a broad range of spatial and temporal scales.

    The task of the Telescope Control System is to bring the quality of the beam delivered to the instruments within the capture range of their post-focal Adaptive Optics systems. The Phasing and Diagnostic Station, located at the folded Nasmyth focus, below the optical axis, hosts the sky metrologies used in the telescope engineering mode to validate the wavefront performance at the interface to the instruments. The talk will present the baseline scenario for the phasing of the primary mirror and the telescope adaptive optics mode, based on a pyramid wavefront sensor operated in the near infrared. This mode encompasses background algorithms in charge of maintaining the alignments and the connection between the sectors of the pupil.


  • Mardi 10 décembre 2019 à 11h00 (Salle de réunion du bâtiment 14)

    Electric current evolution at the footpoints of solar eruptions

    Krzysztof Barczynski (LESIA)

    Electric currents play a critical role in the triggering and dynamics of solar eruptions. Characterizing their location and evolution can contribute in fine-tuning the standard flare model in 3D, in deriving the acceleration of coronal mass ejections, and in addressing the long-standing debate between the circuit and MHD approach of flare physics. Using a constrained selection of X-class eruptive flares as observed by SDO, complemented by a generic MHD simulation, we analyse the time-evolution of photospheric currents at the footpoints of erupting flux ropes. The latter are believed to be located within the area surrounded by the hook of current and/or EUV flare ribbons. We focus on footpoints of field lines that remain within the erupting flux-rope during the main phase of the events considered, so as to discard flare-related reconnecting loops from the analysis. In the observations, for each case where a flux-rope fooptoint is identifiable and the currents can be measured with HMI, we identify that both the mean electric-current density and the total current are dominated by direct-currents, and that they are strongly diminishing in the early phases of the flare. We also find the same trend in the MHD simulation. There we show that the current decreases at the line-tied photospheric boundary is caused by the lengthening of coronal field lines, in which the twist per unit-length also diminishes. The coupled analysis between the model and the observations leads to several conclusions. Firstly, the photospheric electric current should neither be considered as a physical source nor as a boundary condition in solar eruption models. Secondly during eruptions these surface currents evolve as a response to the coronal dynamics, in line with the MHD paradigm. Thirdly, measuring their time-evolution may offer a new window for measuring the rate of expansion of coronal flux rope in the early stages of solar eruptions, in the region where they are typically difficult to observe far above the limb yet below the edge of the coronograph.


  • Jeudi 28 novembre 2019 à 14h00 (Salle de réunion du bâtiment 14)

    Exploring impulsive energy release in the solar atmosphere using focusing optics X-ray observations

    Sophie Musset (University of Minnesota, USA)

    The Focusing Optics X-ray Solar Imager (FOXSI) is a sounding rocket experiment designed to demonstrate focusing imaging and spectroscopy of the solar hard X-ray emission, in order to study the fundamental processes of energy release in the solar corona. Previous solar-dedicated hard X-ray instruments have used indirect, Fourier-based imaging techniques with limited dynamic range and sensitivity. Due to recent technological advances, focusing optics for hard X-rays are now available and have been optimized and demonstrated for solar observations with the FOXSI sounding rocket experiment. This has made it possible to observe faint coronal sources of hard X-ray emission near sources of energy release. The FOXSI rockets flew three times in 2012, 2014 and 2018. I will present the FOXSI sounding rocket design and the technological achievements and scientific observations realized during the past three flights. I will then present the future missions proposing focusing optics for solar X-ray observations with a particular focus on the fourth flight of the FOXSI sounding rocket and the FIERCE Mid-Explorer mission which have been proposed to NASA this year, and the scientific return expected from these observations.


  • Jeudi 14 novembre 2019 à 16h00 (Salle de conférence du bâtiment 17)

    High angular resolution study of the Super Stellar Cluster population in IRAS 17138-1017

    Lâm Nguyen (LESIA)

    Using GeMS-Gemini high angular AO-aided imaging in the near-IR, together with a radiative transfer code, we study the population of Super Stellar Clusters (SSCs) in terms of age, extinction, mass and luminosity. We detect with a fair degree of confidence 71 SSCs of mKs between 14 mag and 22 mag with a median photometric accuracy of 0.14 mag. When plotted on a color-color diagram and a color-magnitude diagram, it appears that most of the sources are much extinct with respect to an unreddenned theoretical evolutionary track. The result points unambiguously to two distinct and very recent starburst episodes, at 2.2 and 4.5 Myr. While the SSCs in the 4.5 Myr starburst are distributed along the spiral arms, the 2.2 Myr SSCs are concentrated in the central region, south of the true nucleus. The luminosity function presents a classical power-law behaviour, with however a slope which is shallow compared to other LIRGs. Comparison with radiative transfer simulations shows that especially for the youngest SSCs, the thermal emission by dust is not negligible and could explain the few very red SSCs that could not be dereddened safely. This effect could lead to an misevaluation of the age of the starburst by at most one or two Myr.


  • Jeudi 7 novembre 2019 à 14h00 (Salle de conférence du bâtiment 17)

    Magnetism in Massive Stars - from the core to the surface

    Kyle Auguston (AIM-CEA)

    Massive stars as with all stars possess convective regions, from the PMS to their explosive demise. These regions likely participate in vigorous dynamo action, leading to the construction of magnetic fields that can be in superequipartition with respect to the convection when in a rapidly rotating regime. Moreover, near surface regions of sufficiently massive stars can be convectively unstable, producing the so-called iron-bump convection zone. This region too will likely be running a dynamo. However, if it is threaded with fossil fields extant in the radiative regions of these stars which are of sufficient strength, it may be possible to quench the convection or shift it to very small scales, leading to weaker excitation of magneto-gravito-inertial waves in the radiative photospheric regions of these stars that manifest as macroturbulence. Yet establishing such strong fossil fields remains a largely murky process, where the complexities of the early evolutionary history of the star comes into play. Thus, this talk aims to cover the evolutionary history of massive stars in terms of their convective dynamos, their fossil field development, and the observational implications of such fields.


  • Jeudi 24 octobre 2019 à 14h00 (Salle de conférence du bâtiment 17)

    Cours d’évolution stellaire II

    Arlette Noels, Professeure de l’Université de Liège

    Arlette Noels nous avait fait une première partie de ce cours l’année dernière, concernant principalement les stades précoces et la séquence principale. Cette année, nous nous concentrerons sur les stades postérieurs à la séquence principale, les stades évolués. Pour cette occasion, nous ne regarderons pas nos montres, la durée de ce séminaire sera laissée libre.


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