Calendar

Déc
6
jeu
Tristan Cantat-Gaudin — Stellar clusters in the Gaia era @ salle 306
Déc 6 @ 14 h 00 min – 15 h 00 min
Séminaire

The ESA Gaia mission is revolutionising our understanding of the Milky Way by providing precise proper motions and parallaxes for over a billion stars, as well as excellent photometry. Among the many aspects of Galactic astronomy that Gaia can tackle, the unprecedented size, depth and quality of this dataset allows us to better characterise stellar clusters and to discover new objects. In this talk I will review some of the major results in Galactic cluster science recently obtained from Gaia data and discuss the ongoing and future work that Gaia makes possible.

 

Jan
15
mar
Maxime Ruaud : Physique et chimie des disques proto-planétaires @ Univers
Jan 15 @ 14 h 00 min – 15 h 00 min
Séminaire
Les observations à très haute résolution angulaire dans le domaine (sub)millimétrique révolutionnent l’étude des disques proto-planétaires. En résolvant spatialement l’émission des poussières et des molécules du gaz de ces disques, elles nous donnent accès à l’étude de leurs propriétés physiques et chimiques. Bien qu’un nombre limité de molécules aient été détectées dans ces objets, ces observations nous révèlent une grande diversité de structures et de compositions. Les observations conduites avec ALMA ont ainsi permis de mettre en évidence un certain nombre d’anneaux dans l’émission de certaines molécules telles que N2H+, CN, C2H, ou encore H2CO. Les observations avec ALMA ont aussi permis la détection récente de deux molécules organiques complexes, i.e. CH3CN et CH3OH, mettant en évidence, une chimie active à la surface des grains de ces objets. Ces observations nous donnent aussi accès à certaines contraintes sur la variation radiale des rapports isotopiques pour des éléments tels que D, 13C ou 15N ainsi que sur le rapport ortho/para de quelques molécules telles que H2CO ou encore H2O. Bien que ces observations nous donnent de précieuses informations sur la physique régissant ces objets, leur interprétation reste dans la plupart des cas difficile et des modèles détaillés sont nécessaires.
 
Je travail actuellement au développement d’un modèle permettant de suivre la composition du gaz et des grains au cours de l’évolution de ces disques. Durant cette présentation je présenterai le statut des observations dans ces objets ainsi qu’un certain nombre de contraintes obtenus à partir de ce modèle. Je discuterai de la présence des anneaux moléculaires de certaines des molécules détectées ainsi que de la formation des molécules organiques complexes dans les régions internes des ces disques.
Jan
16
mer
Laurent Chemin — The orbital anisotropy of velocity in spiral and irregular galaxies @ Univers 21
Jan 16 @ 11 h 00 min – 12 h 00 min
Séminaire

The ellipsoid of random motions is a fundamental ingredient of galaxy dynamics. In particular, it can be used to constrain the shape of orbits of stars or gas in galactic disks. First, I will present results from a recent study that constrained the structure of stellar orbits in nearby galactic disks. A new correlation between the stellar mass and the shape of the orbits has been evidenced: stellar orbits in more massive galaxies are radially biased, while less massive disks have more tangential to isotropic orbits. A discrepancy with expectations from the epicycle theory of orbits has also been evidenced. Then, results from ongoing studies of the velocity ellipsoid of HI and CO gas in other samples of galaxies will be shown. Contrary to the common assumption that random motions of gas are isotropic, it is shown a large diversity of orbits in nearby spiral galaxies. Disagreement with the epicyclic approximation are again observed in the atomic gas component.

Jan
23
mer
Benoit Commercon — Protostellar disk formation in low- to high-mass star formation @ Univers 21
Jan 23 @ 11 h 00 min – 12 h 00 min
Séminaire

The new generation of interferometers provide unprecedented constraints on the protostellar disk formation process. Observations indicate that most disks have a small extent at the Class 0 stage and that disks grow in size at latter stages. I will present the results of 3D protostellar collapse calculations that cover a wide range of initial mass (from 0.5 to 100 solar mass), as well as different initial rotation and/or turbulence support. The calculations are performed using the RAMSES code, including the effect of non-ideal MHD with the ambipolar diffusion and radiative transfer. I will show how ambipolar diffusion is regulating the disk and outflow formation at the early stages of the class 0 phase. I will discuss the disk properties: magnetisation level, magnetic field lines topology, stability. In a second part, I will present recent work done in the context of the protostar formation (second collapse) where the effects of non-ideal MHD (ambipolar and Ohmic diffusion) are taken into account. I will highlight the differences with previous results obtained with ideal MHD and show to what extent these kind of models can provide constraints on the protostellar evolution (disk, protostar). I will finally present preliminary results of protostellar collapse models which include coupled dust and gas dynamics.

Jan
24
jeu
David Barrado — Gods, heroes and constellations: cosmography and myths in Greek ceramics @ Univers
Jan 24 @ 14 h 00 min – 15 h 00 min
Séminaire

Eratosthenes of Cyrene (276-194 BCE) was one of the great scholars of the Hellenistic period. Responsible for the Library of Alexandria, 

he made fundamental contributions in both Astronomy and Geography. Among the legacy he left to us is the work « Catasterisms » 
(From Ancient Greek καταστερισμός (katasterismós, “star legend”, from καταστερίζω or “to place among the stars”) which describes the celestial
 constellations known at that time and are associated with a myth, by  the rise of the protagonist to the heavens. 
However, the process of defining the constellations began much earlier and some have  their origin in Mesopotamia or Phenicia.
The literature of the Archaic and Classic Periods allows us to reconstruct the creation of new constellations based on Hellenic
 myths and trace both the original Greek contributions and the heritage of older cultures. In this talk, I will present a summary of the process.
Jan
29
mar
Javier Olivares — Bayesian Modeling @ Univers
Jan 29 @ 11 h 00 min – 12 h 00 min
Séminaire

In this presentation I will talk about the creation of statistical models, Bayesian ones particularly.
I will give my personal interpretation of what models are and its construction process.
We will see examples of Bayesian models, and the results of these on the global properties (members, luminosity, mass and spatial distributions) of two open cluster, the Pleiades and Ruprecht 147.

Fév
6
mer
Panayotis Lavvas — Photochemical Hazes in Planetary Atmospheres From the Solar System to Exoplanets @ Univers 21
Fév 6 @ 11 h 00 min – 12 h 00 min
Séminaire

Photochemical hazes are ubiquitous components of planetary atmospheres with important ramifications for the atmospheric thermal structure, the chemical composition, and the circulation, as well as, for the planetary surface properties. In the solar system, observations with major space missions such as the Voyagers, the Cassini-Huygensand the New Horizons start to illuminate the mechanisms involved in the formation of hazes. Studies on the characteristic hazy atmospheres of Titan, Pluto, and Triton, reveal the similarities, but also the fundamental differences in the photochemical haze properties and their impact on their host atmospheres. Exoplanet atmosphere also appear to have hazes, which significantly affect the characterization of these environments. Although the composition of these components is currently a topic of great debate, we will see what the lessons we learn from the solar system can tell us about the hazes in these distant worlds.

Fév
13
mer
Cécile Engrand: The composition of interplanetary and cometary dust @ Univers 21
Fév 13 @ 11 h 00 min – 12 h 00 min
Séminaire

Small bodies have escaped planetary accretion and have best preserved the composition of the matter initially present in the solar nebula. Cosmic dust originates from these small bodies, asteroids and comets. Interplanetary and cometary dust are collected on Earth in places with a low accumulation rate of terrestrial dust, like the polar caps or the stratosphere. Interplanetary dust particles (IDPs) have been collected in the stratosphere by NASA for a few decades. A fraction of IDPs (at least) are proposed to be of cometary origin. Cosmic dust from the polar caps are larger than IDPs and are called micrometeorites. We collect micrometeorite at the Concordia Antarctic station at Dome C since 2000. The Concordia collection contains very pristine samples, including particles that are dominated by organic matter and that are very probably cometary. Spatial missions like Stardust (NASA), Hayabusa (JAXA) and Rosetta (ESA) also gave access to the structure and composition of asteroidal and cometary dust. Stardust brought back dust particles from comet 81P/Wild 2, but the collection occurred at high relative velocity (6 km/s) and the samples were altered during the collection. The Rosetta mission collected dust particles from comet 67P/Churyumov-Gerasimenko at much lower velocity (1-10 m/s), but the analyses had to be performed in situ onboard the Rosetta orbiter by the dust instruments (GIADA, COSIMA, MIDAS). The Hayabusa mission returned samples from asteroid Itokawa, which is an asteroid related to ordinary chondrites. At least two future spatial missions are bound to bring back samples from carbonaceous asteroids: Hayabusa 2 (JAXA, asteroid Ryugu) et OSIRIS-REx (NASA, asteroid Bennu). The CAESAR mission is also currently under study to bring back a sample from comet 67P/Churyumov-Gerasimenko.
The presentation will summarize the present knowledge on the composition of interplanetary and cometary dust, based on the results of laboratory analysis of dust particles collected on Earth, and of spatial missions.

Mar
12
mar
Nathalie Ysard — THEMIS: The Heterogeneous dust Evolution Model for Interstellar Solids @ Salle 306
Mar 12 @ 11 h 00 min – 12 h 00 min
Séminaire

Dust grains are ubiquitous in all astrophysical environments, from the Solar System and protoplanetary disks to interstellar and intergalatic clouds, and their influence on the radiative properties of all these very diverse media is always significant through the absorption, scattering, and (non-)thermal re-emission of starlight. They are also a major player in the determination of the interstellar gas temperature through photo-electric emission or gas-grain collisions. Similarly grains have a great influence on the chemical complexity in the interstellar medium: indeed, the role of grain-surface reactions is crucial to understand the formation of some very common molecules, such as H2, and of more complex molecules. The grain radiative properties and their catalytic efficiency are, at least, reliant on the grain size distribution, structure, and chemical composition, which vary throughout the dust lifecycle. Observations show that grain growth arises in dense molecular clouds and protoplanetary disks as traced by an enhancement of the dust far-IR emissivity, a change in the far-IR SED spectral index, and by the effects of cloud-/core-shine from the visible to the mid-IR. There are also more and more evidences for dust variations in the diffuse ISM both from cloud-to-cloud and within clouds. In the context of THEMIS (The Heterogeneous Evolution dust Model of Interstellar Solids), a core-mantle dust model, I will show how most of the variations in the observations of both diffuse and dense clouds are consistent with accretion and coagulation processes.

Mar
19
mar
Alvaro Hacar — Towards a unified model for low- and high-mass star-formation @ Univers 21
Mar 19 @ 14 h 00 min – 15 h 00 min
Séminaire

Recent Herschel observations have revealed that most of the stars in our Galaxy are formed inside filaments. Using single-dish and ALMA molecular observations, we have investigated the internal structure and dynamics of filaments along their entire mass spectrum, from the lowest mass filaments in Taurus to the massive Integral Shape Filament in Orion. In all cases, the analysis of different molecular line tracers indicates a high level of internal organization in which apparently single filaments are actually collections of small-scale fibers. In both low- and high-mass filaments, fibers are characterized by presenting transonic internal motions respect to their local sound speed and a mass per-unit-length close to hydrostatic equilibrium. Conversely, the fiber dimensions (width and length) appear to be self-regulated depending on their intrinsic gas density of their local environment. Combining observations in different star-forming regions, we identify a systematic increase of the surface density of fibers as a function of the total mass per-unit-length in filamentary clouds. Based on this empirical correlation, we propose a unified star-formation scenario where the observed differences between low- and high-mass clouds, and the origin of clusters, emerge naturally from the initial concentration of fibers.