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.
Galaxy history is marked by a peak of star formation ten billion years ago and a subsequent drop of the star formation rate (SFR) by an order of magnitude. To understand this evolution, it is crucial to probe the gas reservoirs from which stars are formed. With programs observing the molecular gas phase in typical star-forming galaxies at different epochs, I will present how the cosmic evolution of the SFR is mainly driven by that of the molecular gas fraction. The depletion time associated to star formation indeed only weakly changes with redshift, both at galactic and at sub-galactic scales. I will show that the molecular gas content during the winding-down of star formation does not seem to correlate with morphology, suggesting an ongoing supply of molecular gas to compensate for star formation while bulges grow. In contrast, molecular gas reservoirs can be dramatically depleted in extreme environments such as cluster centres. While structure formation is primarily driven by dark matter (DM) dynamics in ΛCDM cosmology, gas processes can in turn affect the DM distribution at galactic scales. Using theoretical modelling and simulations, I will discuss how outflow episodes and gas density fluctuations induced by stellar feedback can expand both the DM and the stellar distributions and hence provide a simple understanding of the formation of DM halo cores and ultra-diffuse galaxies.
Kora Muzic (CENTRA, University of Lisbon, Portugal)
With masses between those of stars and planets, brown dwarfs represent a critical link between the two classes of objects. Understanding their origin has been a major motivation for recent deep studies of star-forming regions and clusters as well as a driver for the development of state-of-the-art simulations. Deep surveys show that brown dwarfs are a ubiquitous outcome of star formation, with about 0.2-0.5 sub-stellar objects formed for each star. One of the big questions in brown dwarf studies is whether the birth environment affects their formation efficiency, as predicted in several formation theories. The expectation is that high gas or stellar densities, as well as the presence of massive OB stars, may be factors that boost the incidence of newly formed brown dwarfs with respect to stars. To test these scenarios, we compare the findings of our decade-long deep survey SONYC, in which we characterized sub-stellar populations of several nearby star-forming regions, with the results of our new investigations of sub-stellar objects in the massive young clusters RCW 38 and NGC 2244, characterized by drastically different star-forming environments. Here I will present the current status of young brown dwarf studies, compare the low-mass Initial Mass Functions in a variety of Milky Way environments, and outline the implications of these results for our understanding of sub-stellar formation processes.