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.
Active galactic nuclei (AGN) are the most powerful long standing phenomena in the universe. Among them, the most extreme sources display ultra relativistic particle jets which radiate over the full electromagnetic spectrum, from radio to very high energies (E > 100 GeV). The cosmological distances of these sources make very difficult to decipher the location and origin of their high energy emission, which remains one of the major not (fully) answered question of this research field.
I will show how the parsec-scale imaging from radio very-long-baseline-interferometry (VLBI) observations coupled to broadband spectral models and hydrodymamic jet simulations lead us toward and updated unification scheme of the jetted AGN phenomenon.
The properties of coronal mass ejections (CME) in the heliosphere is determined by a complex chain of processes. This presentation highlights this fact by reviewing CME’s (1) intrinsic properties set at the Sun (e.g., orientation, velocity), (2) processes that may occur during eruption and propagation (e.g., shocks, confinement or magnetic erosion), and (3) in the specific interaction with the planet (e.g., magnetic properties, preconditioning mechanisms), and which together determine the CME’s actual impact. The relative importance of these processes is discussed, as well as implications at planets other than the Earth, including exoplanetary systems.
Benoit Lavraud
IRAP, CNRS, CNES, Université de Toulouse, France
Most stars form in clusters within giant molecular clouds (GMCs). However, the processes that induce the collapse and fragmentation of GMCs into star-forming clumps and cores are poorly understood. While the effects of turbulence and gravity have long been studied, the role of magnetic fields in the star formation process is only now becoming clear. In this talk, I present simulations and observations of forming star clusters to shed light on connections with their environments. First, I introduce magnetohydrodynamics simulations of GMCs evolving quiescently vs. those embedded in converging flows. The filamentary gas structures and star formation properties resulting from each scenario are quantified with particular attention given to the role of magnetic fields. These results are then compared with polarization studies as well as recent ALMA observations of massive star-forming clumps. Finally, I discuss work being done in the ongoing ALMA-IMF large program towards determining the origin of the stellar initial mass function (IMF).
Star formation is a multi-physics, multi-scale process. the physical scales that are involved vary by 10 orders of magnitude, from the size of entire galaxies down to the size of the Solar system. The physical processes that are involved include gravity, turbulence, magnetic fields, radiation, chemical reactions, and cooling and heating processes. This multiplicity of processes and scales can generate a significant amount of variation in the outcome of star formation from galaxy to galaxy and from region to region within galaxies, in particular in terms of key quantities such as the stellar initial mass function (IMF), the star formation rate (SFR), and the star formation efficiency (SFE). I will present a brief overview of the current status of observations for the IMF and the SFR in the Milky Way and in nearby galaxies and discuss theoretical ideas and numerical simulations that attempt to reproduce these observations.
In contrast to the water-poor planets of the inner Solar System,
stochasticity during planetary formation and order of-magnitude deviations
in exoplanet volatile contents suggest that rocky worlds engulfed in thick
volatile ice layers are the dominant family of terrestrial analogues among
the extrasolar planet population.
Here we use numerical models of planet formation, evolution and interior
structure to show that a planet’s bulk water fraction and radius are
anticorrelated with initial 26Al levels in the planetesimal-based
accretion framework. The heat generated by this short-lived radionuclide
rapidly dehydrates planetesimals before their accretion onto larger
protoplanets and yields a system-wide correlation of planetary bulk water
abundances, which, for instance, can explain the lack of a clear orbital
trend in the water budgets of the TRAPPIST-1 planets.
Qualitatively, our models suggest two main scenarios for the formation of
planetary systems: high-26Al systems, like our Solar System, form small,
water-depleted planets, whereas those devoid of 26Al predominantly form
ocean worlds. For planets of similar mass, the mean planetary transit
radii of the ocean planet population can be up to about 10% larger than
for planets from the 26Al-rich formation scenario.