LESIA - Observatoire de Paris

Nouvelles possibilités d'analyses atmosphériques avec Monte Carlo

2024-12-03T15:31:23Z

Gone with the stellar wind : characterizing atmospheric escape in exoplanets

2024-11-26T13:27:55Z

Some exoplanets orbit so close to their star that their atmosphere can be partially “blown away” by intense stellar radiation. This phenomenon of atmospheric escape plays a key role in planetary evolution, shaping the structure of the upper atmosphere. Photo-evaporation may even lead to the formation of super-Earth planets through the complete evaporation of mini-Neptunes' atmospheres. In recent years, the study of the metastable helium triplet in the infrared (1083.3 nm) has made it possible to probe atmospheric escape processes from the ground using high-resolution spectrographs, enabling unprecedented measurements. We analyzed the transits of 15 exoplanets observed with SPIRou (CFHT, R=70,000), searching for helium signatures to characterize these atmospheric losses. In this seminar, I will present the physical processes at play in these extreme environments and the analytical tools and models we developed to interpret our observations.

The talk will be available via Zoom at : Zoom Link.

The Hubble Tension : Recent Results from HST and JWST by the SH0ES Team

2024-11-20T15:55:35Z

The 5-sigma tension between the local measurement and the early universe prediction of the Hubble constant (H0) may be the most exciting development in modern cosmology, pointing towards the possibility of new physics beyond lambda-CDM. In my talk, I will present the latest H0 measurement by the SH0ES team, based on the Cepheid and Type Ia supernova distance ladder, and will discuss recent improvements from Gaia, HST and (...)

- Saison 2024-2025

Changing disc compositions via internal photoevaporation

2024-06-25T08:33:53Z

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.

Toward Multiview-Multispectral Sensing from the Martian Moons eXploration Spacecraft : Imaging Ryugu Samples with the Laboratory OROCHI Simulator

2024-06-21T12:44:49Z

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.

Unraveling the opposite spectral evolutions on Ryugu and Bennu

2024-06-11T09:08:36Z

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 0.48–0.85 μm 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 reflectance–spectral 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 µm) 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.

Propagation of Solar Energetic Particles in 3D MHD Simulations of the Solar Wind

2024-05-27T09:26:43Z

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.

Planet-debris disc interactions : The role of disc gravity and observational implications

2024-05-07T08:33:09Z

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.

Hydrogenated atmospheres of lava planets : Atmospheric structure and emission spectra

2024-04-25T09:00:58Z

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.

Diagnostics of interplanetary electron beams using X-ray and Radio data from Solar Orbiter

2024-04-23T07:57:09Z

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.