Lundi 1er juillet 2024 à 16h00 (Salle de confĂ©rence du bĂątiment 17)
Julia Lienert (MPIA, Allemagne)
The chemical evolution of protoplanetary discs is not fully understood. One factor influencing the distribution of disc material is the inward-drift and evaporation of pebbles that enriches the inner disc with vapour. In particular, it is first enriched with water vapour, resulting in a low C/O ratio, before carbon-rich gas from the outer disc elevates the C/O ratio again. However, it is unclear how internal photoevaporation, which carries away gas and opens gaps that block inward-drifting pebbles, affects the chemical composition of the disc.
To study these effects, we use a semi-analytical 1D disc model. The code chemcomp includes viscous evolution and heating, pebble growth, drift, evaporation and condensation, and a simple chemical partitioning model.
We show that internal photoevaporation plays a major role for the (chemical) evolution of protoplanetary discs : As it opens a gap, inward-drifting pebbles are stopped and cannot contribute to the volatile content any more. Additionally, gas from the outer disc is carried away by photoevaporative winds. Consequently, the C/O ratio is low. In contrast, gaps opened by giant planets allow the gas to pass, resulting in an elevated C/O ratio, similar to viscous discs without internal photoevaporation. This allows observational differentiation between these two scenarios when measuring the C/O ratio, implying that the cause of gap structures can be inferred. In the case of a photoevaporative disc, we additionally find an elevated water content in the inner disc as the water vapour and ice undergo a cycle of evaporation/re-condensation, preventing its inward accretion onto the star.
Lundi 24 juin 2024 à 16h00 (Salle de confĂ©rence du bĂątiment 17)
Roger STABBINS (Rikkyo University & IASA-JAXA (Japan) and Natural History Museum of London (UK))
The JAXA Martian Moons eXploration (MMX) mission will address the question of the origin of Phobos and Deimos by launching a spacecraft to the Mars system in 2026, performing dedicated surveys of the moons, and by collecting a sample from the surface of Phobos and returning it to Earth in 2031. OROCHI is a wide-angle visible-to-near-infrared (VNIR) 8-channel 8-camera multispectral imaging system for MMX, with a key objective of characterising the surface spectral diversity of the moons from orbit, during descent, and once landed on the surface of Phobos. Operating a new imaging system in a new environment requires preparation, but the development timelines and protections required of spaceflight hardware rarely allow for extensive ground-based operation trials to be performed with the final Flight Model of an instrument. In preparation for multiview and multispectral imaging with the MMX spacecraft from the surface of Phobos, we have developed a laboratory simulator of the MMX OROCHI multispectral imager (LOROS), and have used it to image pristine grains of asteroid Ryugu collected in aggregate as an analogue of the surface scattering properties of Phobos, at the JAXA Extraterrestrial Sample Curation Centre (ISO-6 Cleanroom). We describe LOROS and demonstrate equivalent performance to OROCHI, and present the results of multi-phase multispectral imaging of the Ryugu C9003 aggregate sample. We discuss the implications of the surface bidirectional-reflectance distribution function on near-field imaging with the unique 8-camera 8-channel configuration of OROCHI, in the context of resolving the subtle VNIR features expected of Phobos.
Lundi 17 juin 2024 à 16h00 (Salle de confĂ©rence du bĂątiment 17)
Koki Yumoto (Post-doctorant JAXA)
Primitive asteroids may retain the record of volatile-rich planetesimals formed during the early solar system evolution. Our understanding of their diversity relies heavily on ground-based telescope observations of visible spectra. However, the interpretation of the featureless visible spectra is often challenging. Comparing two near-Earth primitive asteroids Ryugu and Bennu encountered by Hayabusa2 and OSIRIS-REx is crucial in this regard because their visible spectra differ : Ryugu has a reddish color (Cb-type) while Bennu is blue (B-type). Despite the spectral difference, recent analyses of samples returned from Ryugu and Bennu indicate that they are both consistent with low-petrologic-type carbonaceous chondrites, suggesting that their compositions may not differ as much as previously assumed. So, what makes the spectra of primitive asteroids different ?
To address this question, we compared the spectral and photometric properties of Ryugu and Bennu in the 0.48â0.85 ÎŒm wavelength range using data from both remote sensing and sample analyses. The precise comparison of the two asteroids was made possible by cross-calibrating the two remote-sensing instruments onboard Hayabusa2 and OSIRIS-REx. We show that the spectral distributions of craters on Ryugu and Bennu follow a common trend line in the reflectanceâspectral slope diagram. In addition, the spectra of fresh craters on both asteroids are indistinguishable within the cross-calibration accuracy. The findings suggest that Ryugu and Bennu initially had similar visible spectra, but they evolved into spectrally distinct asteroids by processes such as (1) solar wind/micrometeorite bombardment, (2) solar heating, and (3) grain size/porosity evolution. We obtained multiple lines of evidence suggesting that the most plausible process may be (3). For instance, the spectral difference between coarse ( 1 mm) and fine (<300 ”m) grained Ryugu samples qualitatively aligns with the observed spectral evolution trend. Our model calculation shows that such opposite evolution of grain size/porosity may be simply explained by their difference in asteroid size. This hypothesis implies that asteroids with different spectral types can actually have similar compositions, and thus Ryugu/Bennu-like materials may be more widespread in the solar system than previously assumed.
Mercredi 12 juin 2024 à 11h00 (Salle de confĂ©rence du bĂątiment 17)
Ahmed Houeibib (LESIA)
We propagate relativistic test particles in the field of a steady three-dimensional MHD simulation of the solar wind. We use the MPI-AMRVAC code for the wind simulations and integrate the relativistic guiding center equations using a new third-order accurate time integration scheme to solve the particle trajectories. Diffusion in velocity space, given a particle-turbulence mean free path λ℠along the magnetic field, is also included. Preliminary results for 81 keV electrons injected at 0.139 AU heliocentric distance and mean free path λ℠=0.5 AU are reported. Pitch angle distribution are in a good qualitative agreement with measurements at 1 AU. For these electrons, an energy loss of roughly 10 % is observed, quasi-exclusively due to the curvature of the magnetic field.
Lundi 13 mai 2024 à 16h00 (Salle de confĂ©rence du bĂątiment 17)
Antranik SEFILIAN (Friedrich-Schiller University in Jena, Germany)
Main-sequence stars are commonly surrounded by debris discs analogous to the Solar Systemâs 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 bĂątiment 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 â 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 confĂ©rence du bĂątiment 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 confĂ©rence du bĂątiment 17)
Robin SULTANA (LESIA)
Titanâs 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 Cassiniâs 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 confĂ©rence du bĂątiment 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 confĂ©rence du bĂątiment 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 121102âs 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.
