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.
Eratosthenes of Cyrene (276-194 BCE) was one of the great scholars of the Hellenistic period. Responsible for the Library of Alexandria,
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.
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.
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.
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.
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.
Despite the existence of co-orbital bodies in the solar system, and the prediction of the formation of co-orbital planets by planetary system formation models, no co-orbital exoplanets (also called trojans) have been detected thus far.
I will present my latest results regarding the stability of co-orbitals exoplanets under dissipation and mass change (accretion). An analytical model is developed to extract a stability criterion as function of the planetary masses and the dissipative forces. This criterion is then compared to both the evolution of co-orbital exoplanets in protoplanetary 1-D disc models, and hydrodynamics simulations. This study is a step toward understanding which should be the preferred configuration and environment of co-orbital exoplanets.
Nous essayerons de donner quelques éléments de contexte concernant ce résultat spectaculaire, à la fois sur les observations VLBI et sur la physique des trous noirs et des radiogalaxies. La présentation se voudra d’un niveau très accessible.