SPHERE visits the asteroid main belt
mardi 20 novembre 2018, 10h30, NEF - Mont Gros - CION - talk in english - retransmis à Valrose (Salle O. Chesneau)
Pierre VERNAZZA - LAM, France
Asteroids in our solar system are metallic, rocky and/or icy objects, ranging in size from a few meters to a few hundreds of kilometers. Whereas we now possess constraints for the
surface composition of most D>100 km primordial main-belt asteroids, little is known regarding their internal structure. Yet, this is a fundamental property whose characteristics result directly from (a) their formation location, (b) their time of formation, and (c) their collisional history. Characterizing the internal structure of the main compositional classes of asteroids would therefore allow us to address entirely new questions regarding the earliest stages of planetesimal formation and their subsequent collisional and dynamical evolution. To achieve this goal, we carry out - via an ESO Large Program (LP) that was awarded 152h on VLT/SPHERE (the observations are spread over 4 semesters from April 1st, 2017 till March 30, 2019 in service mode) - disk-resolved observations of a substantial fraction of all D>100 km main-belt asteroids (sampling the four main compositional classes) at high angular resolution with VLT/SPHERE throughout their rotation. These observations enable us to derive their volume (via their 3-D shape) which combined with already existing mass estimates allow us to determine their bulk density and hence to characterize their internal structure. The high-resolution images also allow us to detect craters larger than 30 km and use their morphology (crater diameter and depth) to characterize the density of the outer shell. The knowledge of both the bulk density and the density of the outer shell of our targets allows a characterization of their internal structure. This information, in turn, allows us to determine: (a) the nature of their initial building blocks (rock only, or a mixture of ice and rock) and (b) which compositional classes experienced differentiation. These constraints then serve as direct inputs to thermal evolution models for determining the time of formation and the formation location (inward or outward of the snowline) of the main compositional classes of objects in the asteroid belt.
Directly imaged exoplanets with mid- to high-resolution spectroscopy
mardi 27 novembre 2018, 10h30, NEF - Mont Gros - CION - talk in English - retransmis à Valrose (Salle O. Chesneau)
Georges KORDOPATIS & Mamadou N'DIAYE - Lagrange
Since the mid-1990s, nearly 4000 planets have been detected outside our solar system, providing us hint on the demography, the formation and evolution of substellar mass companions. A few exoplanets were
detected using high-contrast imaging methods to retrieve the signal of faint companions within the glare of their parent star. Further spectral analysis sheds light on the physical nature and the atmospheric chemical composition of the observed planets. Low-resolution spectroscopy (R<400) has enabled the study of young gas giant planets with the recently deployed exoplanet imagers (VLT/SPHERE, Gemini/GPI, Subaru/SCExAO). Higher spectral dispersion is currently investigated to detect molecular features in the atmosphere of these planetary companions. We will review the first results and provide future prospects on directly imaged planets with medium to high dispersion spectroscopy.
The metal-poor galactic disc seen with the eyes of Gaia
mardi 27 novembre 2018, 10h30, NEF - Mont Gros - CION - talk in english - retransmis à Valrose (Salle O. Chesneau)
Georges KORDOPATIS & Mamadou N'DIAYE - Lagrange
The lower metallicity value down to which one can find Milky Way's disc stars has a direct impact on the formation scenarios of the thin and thick discs. In particular, the
individual abundance patterns as well as the kinematics of these metal-weaker disc stars highlight the conditions under which the thin and the thick disc were assembled. With the advent of the second data-release of Gaia, the signature of a major impact that would have happened roughly 10 Gyr ago has strongly been suggested by several authors. This stellar population, dubbed in the literature either as the "Blob", "Enceladus" or the "Gaia Sausage", seem all to point towards the same event. In this talk, I will show preliminary results regarding the analysis of the properties of the metal-weak tails of the disc as seen by the combination of the RAVE spectroscopic survey with Gaia DR2, and how they possibly relate to Enceladus.
mardi 11 décembre 2018, 10h30, NEF - Mont Gros - CION - talk in English - retransmis à Valrose (Salle O. Chesneau)
Orlagh CREEVEY & Benoit CARRY - Lagrange
The formation and early evolution of planet-forming disks
mardi 8 janvier 2019, 10h00, NEF - Mont Gros - CION - talk in english - retransmis à Valrose (Salle O. Chesneau)
Patrick HENNEBELLE (CEA)
It is now well understood that low mass stars form through the collapse of prestellar dense cores. It is believed that due to the conservation of angular momentum, a centrifugally supported
disk, which eventually gives birth to planets, forms around the protostar. The conditions under which this exactly happens remains however a subject of active research. Observationally it is difficult to disentangle the collapsing envelope and the young disk and it is only recently that accurate constraints have been obtained, revealing that most disks appear to be relatively small in size and significantly smaller that would be obtained if angular momentum was conserved. In parallel, much progresses have been accomplished from the theory side. In particular it has been realized that magnetic field is most likely playing a fundamental role through magnetic braking. This latter is so efficient that it may even lead to the complete disappearance of the disk, an effect also know as catastrophic magnetic braking. During the talk, I will describe the various simulations that have been performed to study this process, stressing the key role played by non-ideal MHD phenomena, as well as a simple analytical model to estimate the disk size. I will then present recent works aiming at bridging the gap between the early disk formation and the later evolution.
The Antikythera Mechanism: Decoding an astonishing 2000 years old astronomical computer
mardi 15 janvier 2019, 10h30, NEF - Mont Gros - CION - talk in english - retransmis à Valrose (Salle O. Chesneau)
John H. SEIRADAKIS - Aristotle University, Thessalonique, Greece
A lecture that attempts to explain the functional details, the operation and the purpose of use of an ancient astronomical mechanism, built about 2000 ago. The Antikythera Mechanism was found by
chance, in a shipwreck, close to the small Greek island of Antikythera, in April 1900, by sponge divers. The shipwreck was dated between 86 and 67 BCE (coins from Pergamon). Later the Mechanism was stylistically dated, around the second half of the 2nd century B.C. (200 – 100 BCE). It was a portable (laptop-size), geared mechanism which calculated and displayed, with good precision, the movement of the Sun and the Moon on the sky and the phase of the Moon for a given epoch. It could also predict eclipses and calculate the dates of the four-year cycle of the Olympic Games! Its 39, precisely cut, gears were driven by a manifold, with which the user could select, with the help of a pointer, any particular epoch. While doing so, several pointers were synchronously driven by the gears, to show the above mentioned celestial phenomena on several accurately marked spiral dials. It contained an extensive user’s manual. The exact function of the gears has finally been decoded and a large portion of the manual has been read after 2000 years by a major new investigation, using state of the art equipment. New astronomical results will be discussed and the ability of ancient Greeks to use hard metals and cutting tools will be examined.
mardi 22 janvier 2019, NEF - Mont Gros - CION - talk in French with slides in English - retransmis à Valrose
Hervé Le Treut
mardi 29 janvier 2019, 10h30, NEF - Mont Gros - CION - talk in english - retransmis à Valrose (Attention: Salle M. Dugué)
Jenny SORCE - CRAL, Lyon
To unveil the nature of 95% of the Universe, missions such as Euclid aim at reaching a few percent precision. In this quest for precision, tensions between the standard cosmological model
and observations already arise: local and global H0 measurements are incompatible at more than 3 sigma, anomalies emerge within the CMB, etc. These tensions suggest that we should perhaps not be so quickly inclined to disregard our observational site as a bias factor: Accuracy is not Precision. Few percent precision and local biases are of the same order of magnitude. A precise mapping of the local distribution of matter is essential to properly account for these biases. Simulations constrained to resemble the local Universe constitute the tool of choice for such a mapping. I will summarize the genesis of the initial conditions of such simulations as well as present a few results that promise to tremendously impact our understanding of the local biases that will matter in futur e analyses. I will also present prospects for significant progress (the GMO-Constrained LOcal & Nesting Environment Simulations) to reach an Accurate Precision Cosmology.
Improved line formation models for more accurate stellar chemical abundances
mardi 5 février 2019, NEF - Mont Gros - CION - talk in english - retransmis à Valrose (Salle O. Chesneau)
Anish Amarsi, MPIA Heidelberg
Stars leave their fingerprints on the light that they emit, in the form of absorption and emission features. Decoding this information to infer stellar parameters and chemical compositions is of key
interest to astronomers. However, such measurements are heavily model dependent, and for late-type stars such as the Sun their accuracy is often limited by the use of one-dimensional (1D) hydrostatic model atmospheres and the assumption of local thermodynamic equilibrium (LTE); it is not uncommon for 1D LTE abundance errors to reach 0.4 dex. Recently however it has become feasible to relax both assumptions simultaneously, i.e. to perform 3D non-LTE spectroscopic analyses. In this talk I shall review the physics of this more accurate method, present our latest results for carbon, oxygen, and iron abundances in the Sun as well as in disk and halo stars across the Milky Way, and discuss our efforts going forw ard to improve the accuracy of large stellar surveys like Gaia, APOGEE, GALAH, WEAVE, and 4MOST.
mardi 26 février 2019, 10h30, NEF - Mont Gros - CION - talk in english - retransmis à Valrose (Salle O. Chesneau)
Francesca FRAGKOUDI - MPIA Garching, Germany
mardi 2 avril 2019, 10h30, NEF - Mont Gros - CION - talk in english - retransmis à Valrose (Salle O. Chesneau)
Faustine CANTALLOUBE - MPIA Heidelberg, Germany
Recherche dans l'Historique