Sauf exception, les séminaires ont lieu sur le site de Meudon, dans la salle de conférence du bâtiment 17.
S’abonner aux mises à jour de cette page :
Lundi 13 mai 2024 à 16h00 (Salle de conférence du bâtiment 17)
Antranik SEFILIAN (Friedrich-Schiller University in Jena, Germany)
Main-sequence stars are commonly surrounded by debris discs analogous to the Solar System’s asteroid and Kuiper belts. High-resolution observations of debris discs frequently reveal a variety of structures such as gaps, spirals, and warps. Most existing models for explaining such structures focus on the role of planets, ignoring the gravitational effects of the disc itself. This assumption, however, may not always be justified, especially since debris discs could contain tens of Earth masses in planetesimals. In this talk, I will present results showing the importance of disc self-gravity in two regimes. First, I will demonstrate that the secular interactions between a single planet and an external debris disc can sculpt a wide gap within the disc. This happens due to secular apsidal resonances, which, somewhat contrary to naive expectations, occurs when the disc is less massive than the planet. I will also show that the same mechanism may lead to the launching of a long-lived spiral arm beyond the gap as well as the circularization of the planetary orbit. Second, I will demonstrate that when the disc is more massive than the planet, the disc gravity can hinder secular stirring by planets, resulting in strong suppression of planetesimal eccentricities and collisional velocities throughout the disc. Finally, observational implications of these effects will be discussed, both for inferring yet-unseen planets and for indirectly measuring the total masses of debris discs.
Vendredi 3 mai 2024 à 11h00 (Salle 103a du bâtiment 14 - Site de Meudon)
David PAIPA ( LESIA)
Energetic electrons accelerated by solar flares in the corona may propagate downward, produce X-rays in the chromosphere, and upward, producing coherent type III radio bursts in interplanetary space. Previous statistical studies of radio and X-ray flare observations have found a good temporal link between the two wavelengths but only a weak correlation between the intensities, in part due to the different emission mechanisms. Assuming both electron populations share properties from a common acceleration region, theory has predicted a link between the speed of the electron beams travelling outwards (deduced from radio) and the energy density of the electrons travelling downwards (deduced from X-rays). The Solar Orbiter mission is equipped with the STIX and RPW instruments, allowing for simultaneous observations of both X-ray and Radio emissions that can test this theory. We present results derived from the comparison of 35 flares observed by STIX in the 4-150 keV range associated in time with radio type III bursts detected by RPW (<10 MHz). From X-ray spectroscopy we obtained the electron spectrum of the associated HXR peak, from which the power can be estimated. We derived the Type III exciter speed using the rise and peak times of the time-profiles (V_r an V_p, respectively) in the 0.4-4 MHz range. We find the observed ratio V_r/V_p is 0.78 +- 0.06, complementing previous similar studies with observations at higher frequencies (30 – 70 MHz) and a ratio of 0.8+-0.06. We report a correlation between the power delivered by electrons with energies above 25 keV and V_r (cc=0.47), whilst a weaker one is obtained when comparing it with V_p (cc=0.2). There is an anticorrelation of the velocities V_p and V_r with the electron spectral index as expected, however the anticorrelation coefficients are weak. A weak correlation is also seen between the power (E>26keV) and the peak Radio intensities, the latter having strong correlations with electron spectral index (cc=0.71). Our results suggest that, whilst the electron acceleration maybe temporally correlated, the energy distribution of escaping and confined electrons for some events may depend on other parameters like the geometry of the reconnecting magnetic field.
Lundi 29 avril 2024 à 16h00 (Salle de conférence du bâtiment 17)
Aurélien FALCO (LMD)
With the new observational capabilities of space telescopes, it should be possible to better characterize the atmospheres of exoplanets, and provide constraints on interiors. Ultra hot rocky exoplanets, for which the stellar irradiation may maintain a magma ocean at the surface for a long period of time, are candidates for such observations. It has been suggested that the primary hydrogen envelope that is captured during the formation of a planet could be kept in the magma ocean, and therefore we could observe planets with a silicate atmosphere mixed with hydrogen. Our model relies on a Gibbs free-energy minimization to find the vapor composition in equilibrium with the magma ocean (a modified version of the CEA/NASA code (Gordon & McBride (1996)). The vapor composition is then used in an atmospheric model, ATMO (Amundsen et al. 2014), which solves for the pressure-temperature profile by finding the energy flux balance in each layer of the model. Synthetic observations are generated via ATMO.
We confirm the thermal inversion of silicate atmospheres and the associated emission features of SiO (Ito et al. 2015, Zilinskas et al. 2022), as well as MgO, Na, K, Fe, which are the strongest candidates for detection. We show that hydrogen will water down the other species, and the thermal inversion is reduced or removed, depending on the temperature of the planet. Cases with a lot of hydrogen will be linked to absorption features of H2O. Surface temperatures will also be affected, and increase for higher content of hydrogen. We investigate potential candidates for observation.
Lundi 22 avril 2024 à 16h00 (Salle de conférence du bâtiment 17)
Robin SULTANA (LESIA)
Titan’s surface thermal emission at 2.2-cm has been recorded for almost 13 years by the RADAR onboard the Cassini spacecraft operated as a radiometer (passive mode) (Janssen et al.,2009, 2016). We used this large brightness temperature data-set to investigate the seasonal evolution of the surface temperature probed by the microwave radiometer in two frequently observed regions in the northern polar region of the moon : the sea Ligeia Mare and its nearby solid terrains.
Over the sea, the low loss tangent of liquid methane and ethane let us to observe the seafloor (Mastrogiuseppe et al., 2014) and to probe the complete column of liquid at 2.2cm. Hence in order to estimate the sea effective temperature, we used a 2 layers emissivity model over the sea - developed in (Le Gall et al., 2016) - taking into account the bathymetry (Hayes, 2016), the sea composition (Mitchell et al., 2015) and the geometry of observation. Despite the arrival of the boreal summer at the end of the mission, we report a decrease of temperature over Ligeia Mare of about 0.7±0.2 K. In contrast, the nearby solid terrains slowly warm by about 1.4±0.3 K through the boreal spring over the course of the Cassini mission. The slow summer warming of solids terrains retrieved at 2.2 cm is in very good agreement with the observation by Cassini’s Composite Infrared Spectrometer (Jennings et al., 2019) and support the idea that evaporation of liquid at the surface takes place after the vernal equinox, possibly after precipitation around the equinox (Turtle et al., 2018 ; Dhingra et al., 2019, 2021), delaying the increase of temperature even if the summer approaches. Moreover, the evolution of temperature in Ligeia Mare observed by the radiometer suggests that evaporative cooling takes place at the sea surface after the equinox.
Comparing the seasonal sea temperature variation to prediction from an ocean circulation model (Tokano & Lorenz, 2016) indicate the that the onset of convection in the sea is likely to happen in Ligeia Mare, the temperature at depth decreases as cool liquid from the surface sinks, lowering the temperature sensed by the radiometer in the early boreal summer. Overall, this work highlights the key role of methane hydrology in controlling the surface and submarine temperatures in the boreal pole of Titan.
Lundi 15 avril 2024 à 16h00 (Salle de conférence du bâtiment 17)
Sophia Stasevic (LESIA)
Dynamical interaction between debris disks and planets can induce several features in the disk, such as brightness asymmetries, warps, gaps, and spirals. Detections of such features in high-contrast imaging observations can therefore be used to infer the existence of yet-undetected companions. The edge-on debris disk detected around the young, nearby A0V star HD 110058 shows warped features resembling those seen in the disk of beta Pictoris, which could indicate the presence of a perturbing planetary-mass companion in the system. We investigated new and archival scattered light images of the disk in order to characterise its morphology and spectrum. Our work uses data from two VLT/SPHERE observations and archival data from HST/STIS. We analysed vertical profiles along the length of the disk to extract the centroid spine position and vertical height, and extracted the surface brightness and reflectance spectrum of the disk. We detect the disk between 20 au (with SPHERE) and 150 au (with STIS), at a position angle of 159.6° ± 0.6°. The disk is marginally vertically resolved in scattered light with SPHERE, with a vertical aspect ratio of 9.3 ± 0.7% at 45 au. Analysis of the spine shows an asymmetry between the two sides of the disk, with a 3.4° ± 0.9° warp between 20 au and 60 au. The outer parts of the disk are also asymmetric with a tilt between the two sides, compatible with a disk made of forward-scattering particles and an inclination of <84°. Dynamical models suggest an undetected inner planetary-mass companion on a mutually inclined orbit with the disk could explain the warp.
Vendredi 12 avril 2024 de 11:00 à 12:00 (Salle de conférence du bâtiment 17)
Prof. Di LI (Chief Scientist of FAST, National Astronomical Observatories of Chinese Academy of Sciences)
With FAST, the largest single-dish telescope ever built , we have designed the Commensal Radio Astronomy FAST Survey (CRAFTS), which realizes, for the first time at any major facility, simultaneous data recording of pulsar search, HI imaging, HI galaxies, and transients (FRB and SETI). CRAFTS has discovered 200 pulsars, 10 FRBs including the only persistently active repeater FRB 20190520B, and 5000 d^2 HI images with 1% calibration consistency, 5-10 times better than what is available from Arecibo. Based on CRAFTS, we derived a FRB event rate 120K per day per 4pi. We find universal frequency-dependent depolarization among repeating FRBs, which can be well fitted by multi-path scattering and a single free parameter sigma_RM that described the complexity of the magnetized environments of FRBs. We have published in 2021 the first complete burst rate energy distribution toward any FRB, which is clearly bimodal. Such bimodality was later borne out in the subsequent monitoring of all the active repeaters. Recently, 10% drop of FRB 121102’s DM on a decade time scale, is being robustly detected. I am proposing an evolutionary picture of FRBs, which aims to unify not only repeating FRBs, but most if not all non-repeaters.
Lundi 25 mars 2024 à 16h00 (Salle de conférence du bâtiment 17)
Sylvain Douté (IPAG) and Audrey Delsanti (LAM)
The “Extremely Large Telescope” (ELT), currently built by ESO in Chile, will be decisive for unprecedented observations of asteroids, trans-Neptunian objects, and Centaurs as it will offer very high spatial resolution and sensitivity. The MICADO and HARMONI instruments in particular will allow in-depth studies of rocky and icy dwarf planets. They will also bring constrains on the physical properties (size, shape, rotation, density, mass, internal structure) of hundreds of asteroids and about twenty TNOs by imaging and high precision astrometry. Their composition will be mapped by narrowband imaging and integral field spectroscopy. MICADO will be served by MORFEO, a multi-conjugate adaptive optics with several laser stars and several deformable mirrors. The use of MORFEO will be crucial to observe faint and sometimes fast objects against the sky with the best possible resolution. Members of the MORFEO and HARMONI teams, we aim at carrying out the scientific preparation of the instrument by (i) defining strategies for performing the observations of the minor bodies of the Solar System (ii) simulating images and spectra with a complete set of numerical tools and laboratory reflectance data. We will present this approach and the expected performances after a brief description of the MICADO/MORFEO instruments.
Lundi 4 mars 2024 à 16h00 (Salle de conférence du bâtiment 17)
Dr. Shanhong Liu, Associate Professor, Key Laboratory of Science and Technology on Aerospace Flight Dynamics (BACC) in Beijing (China)
China’s planetary exploration mission will be shortly introduced. Some typically planetary radio science results will be reviewed. Then, this report will introduce the Tianwen-2 mission, China’s follow-up mission to the Tianwen-1 Mars exploration. The Tianwen-2 mission focuses on near-earth asteroid and main-belt comet research. Targeting asteroid 2016HO3 and main-belt comet 311P, the report will delve into their characteristics, scientific goals, and exploration strategies. It aims to analyze orbital dynamics, gravitational fields, and material composition, which can unravel the solar system’s formation and evolution. Additionally, the mission’s outline and payload details will be briefly outlined.
Lundi 4 mars 2024 à 14h00 (Salle de conférence du bâtiment 17)
Pr KASABA (PPARC and Graduate School of Science de Tohoku Univervity, Japan)
In plasma and radio waves sensing, we are now running Plasma Wave Experiment (PWE) aboard Arase spacecraft which are investigating Geospace. This spacecraft have provided continuous observations of the inner magnetosphere, which extension until FY2032 has been approved. With good particle measurements, we are revealing electron and ion waves which can contribute to the acceleration and loss of energetic particles in Geospace.
Associated with this mission, we are also running two major collaborations with Europe including LESIA colleagues. The first is Plasma Wave Investigation (PWI) aboard the Mio spacecraft of BepiColombo, ESA-JAXA joint mission to Mercury, which will observe Mercury on the orbit from the end of 2025. The second is Radio and Plasma Wave Investigations (RPWI) aboard ESA JUpiter ICy moons Explorer (JUICE) mission to Jupiter, which will observe Jovian system on the orbit from 2031. In both Japan and Europe, main players are overlapped and collaborating strongly in both missions and beyond.
In this presentation, we show (1) Recent progress by Arase, (2) recent Mercury flyby studies of BepiColombo/Mio PWI, and (3) recent status of JUICE RPWI, focusing to Lunar-Earth flyby plans in this Aug which is linked to the feasibility studies on Jovian orbit.
Mardi 27 février 2024 à 16h00 (Salle de conférence du bâtiment 17)
Vincent HUE (Laboratoire d’Astrophysique de Marseille)
The polar atmosphere of Jupiter is a complex region were chemistry, dynamics and magnetospheric-coupling are intertwined. Several decades of ground-based observations and spacecraft measurements (Voyager, Cassini, Juno) are progressively revealing how rich and complex these regions are. Species such as HCN, CO, H2O were brought in large amount during the Shoemaker-Levy 9 impact in 1994 and provide important dynamical tracers, as they have spread across the atmosphere over the last 30 years. Hydrocarbons originate from methane-photolysis and are affected by auroral precipitations, leading to the formation of aerosols across the polar cap. Magnetosphere-ionosphere coupling generates powerful electrojets that drag the neutral stratosphere underneath. I will review our current understanding of these regions, based on observations from Cassini, Juno, ALMA, Gemini, and IRTF.
