Lagrange Laboratory

Our knowledge of the internal structure of asteroids relies entirely on inferences from remote sensing observations of the surface and theoretical modeling. Many fundamental questions are still unresolved. Is the body a monolithic piece of rock or a rubble-pile, and how high is the porosity? What is the typical size distribution of the constituent blocks? Are these blocks homogeneous or heterogeneous? Direct measurements of an asteroid’s deep interior structure are needed to better understand asteroid accretion and their dynamic evolution. The characterization of the asteroids’ internal structure is crucial for science, planetary defense and exploration.In orbit sounding Radars are the most mature instruments capable of achieving the objective of characterizing the internal structure and heterogeneity, both for the benefit of science as well as for planetary defense or exploration. It is the goal of JuRa on cruise to probe Didymos binary system in 2027 and aim of two instruments under development for the ESA Ramses Mission and the Caltech Mission is to study the deep interior of Apophis in 2029. JuRaJuRa, the Juventas radar, onboard the ESA HERA mission, is a monostatic radar, BPSK coded and inherited from CONSERT/Rosetta. The center frequency of 60 MHz provides the capability to probe the internal structure to a depth of 100 meters or more, with a limited vertical resolution of about 20 m. Multipass processing makes it possible to create a 3D tomographic image of the interior. HERA was launched in October 2024 to deeply investigate the Didymos binary system and in particular its moonlet Dimorphos, five years after the DART/NASA impact. On the ESA mission HERA, the Juventas 6U CubeSat is carrying the Juventas Radar (JuRa).JuRa maps the backscatter coefficient (sigma zero - σ0) of the surface and of the subsurface, which quantifies the returned power per unit surface or volume. It is related to the degree of heterogeneity on the wavelength scale and to the dielectric contrast of heterogeneities, giving access to both, the sub-meter texture of the constituent material and larger scale structures.The main objective of JuRA is to characterize the asteroid interior, to identify internal geological structure such as layers, voids and sub-aggregates, to bring out the aggregate structure and to characterize its constituent blocks in terms of size distribution from sub-metric to global scale. The second objective is to estimate the average permittivity and to monitor its spatial variation in order to obtain information on its composition and porosity. The characterization of the constitutive material and its spatial organization both on the main and on the moon will help to better model the formation and stability conditions of the binary system and the response of Dimorphos to the DART impact. Radar to ApophisKnowledge of the internal structure of Apophis is crucial to better understand its accretion and dynamical evolution, to better study its stability conditions, and to model its response to the gravitational constraints imposed by its approach to induced by Earth close approach.For the RAMSES mission, the radar is operating in monostatic mode on one of the CubeSats and deeply inheriting from JuRa on Juventas/Hera, with minor modification. The radar is currently under development and under responsibility of Tyvak International, prime contractor for the CubeSat development. For the electronics (1U, 1kg), minor optimizations are proposed based on the experience from JuRa (mainly thermal optimization, adjustment of the network position and software modifications). The interfaces are revised depending on the carrying platform inheriting from Hera/Milani. For the antenna, a crossed dipole is proposed, as for JuRa on Juventas, providing measurements in full polarization. The cross dipole is consisting of 4 booms of 65g each. An optimization of the boom length and matching network will be required depending on the platform, on the antenna accommodation and on the solar array geometry.The Caltech Mission to Apophis is under study by Caltech and JPL. For his mission, the radar is a customized version of JuRa, operating in bistatic mode: the two satellites are maintained opposite from each other around Apophis, using semi-autonomous navigation based on optical cameras. Electronics on the two platforms measure the signal transmitted throughout Apophis, like done for CONSERT on Rosetta/ESA. Using the synchronization provided by the Inter Satellite Link (ISL), the radar provides an absolute measurement of the propagation delay between the platforms through the asteroid. This provides a direct measurement of the dielectric permittivity, which is related to composition and microporosity of the inner structure of Apophis. Partial coverage will provide slices of the body with an average characterization and its spatial variability to characterize large scale structures. Dense coverage will provide a larger diversity of observation angles, the bistatic mode will then allow a complete 3D tomography to recover the permittivity contrast throughout the volume.In addition to modifications depending on the new platform and the antenna accommodation and the adjustments based on JuRa lesson learned, the implementation of the bistatic mode requires a deeper revisit of the software and firmware to preserve synchronization of the two electronics, while the ISL is ensuring the control of the timing and the frequency.In this talk we will present the instruments, their status, performances and goals as well as the science objectives in the context of the different targets.

In this work, we present and characterize the galaxy cluster catalog detected by the WaZP cluster finder, which is not based on red-sequence identification, on the full six years of observations of the Dark Energy Survey (DES-Y6). The full catalog contains over 400k detected clusters with richnesses, Ngals, above 5 and that reach redshifts up to 1.3. We also provide a version of the catalog where the observation depth and richness computation are homogenized to be used for cosmology, containing 33k rich (Ngals >25) clusters. We compare our results with the previous WaZP catalog obtained from the DES first-year data release (DES-Y1). We find that essentially all clusters within the common footprint and depth limit are recovered. The deeper observations on DES-Y6 and the more complete available spectroscopic redshift sample lead to improvements in the redshifts of the clusters, resulting in an average scatter of 1.4% and offset of 0.2%. The optical clusters are also cross-matched with Sunyaev Zel'dovich Effect (SZE) cluster samples detected by the South Pole Telescope (SPT) and the Atacama Cosmology Telescope (ACT). We find that essentially all SZE clusters with reasonable overlapping footprint have a corresponding WaZP cluster. Conversely, 90% of the optical detections with richness greater than 150 have a counterpart in the deeper regions of the SZE surveys. Based on cross-match with the SZE catalogs, we also find that 15-20% of the SZE matched systems have more than one possible WaZP counterpart at the same redshift and within the SZE R500c, indicating possible interacting or unrelaxed systems. Finally, given the optical and SZE beams, WaZP and SZE centerings are found to be consistent. A more detailed study of the SZE-WaZP mass-richness relation will be presented in a separate paper.

International audience

Radiative shocks are shock waves where radiation plays a crucial role in shaping their dynamics and structure. This study investigates the influence of advanced radiation modeling, using the M1-multigroup moment method, on the properties of radiative shocks. We identify three key effects: the shock velocity, the hydrodynamic properties of the downstream medium, and the radiative precursor. Our simulations show that approaching the spectral effect of photons with the M1 multigroup reduces the shock velocity, resulting in a lower Mach number. Additionally, we find that precise radiation modeling enlarges the regions within the radiative precursor where gas and radiation are thermodynamically out of equilibrium. Finally, we compare the downstream quantities from our simulations with predictions from the commonly used diffusion regime approximation, identifying deviations of 1 to 8% in gas temperature and 3 to 15% in density. These findings highlight the importance of accurate modeling of the interaction between radiation and matter to advance our understanding of radiative shocks.

Vortex reconnections play a fundamental role in fluids.They increase the complexity of flow and develop small-scale motions.In this work, we report that in superfluids, they can also excite large scales. We numerically illustrate that during a superfluid vortex reconnection energy is injected into the thermal (normal) component of helium~II at small length scales, but is transferred nonlinearly to larger length scales, increasing the integral length scale of the normal fluid. We show, by studying about fifty different reconnections, that this inverse energy transfer is triggered by the helical imbalance generated in the normal fluid flow by the mutual friction force coupling the superfluid vortices and the normal component. We finally discuss the relevance of our findings to the problem of superfluid turbulence.

Linear Magnetic Holes (LMHs) are magnetic field depressions generated in the solar wind upstream of planetary and cometary shock. Some of those structures are reminiscent of mirror modes, thus possibly linked to the mirror mode instability driven by a temperature anisotropy in a large plasma beta environment. LMHs have also been found downstream of the shock, which suggests that they can survive its crossing (Karlsson et al. 2022). Using the new GPU-intensive kinetic hybrid model Menura (Behar et al. 2022), we present two-dimensional (2D 3V) simulations of individual solar-wind LMHs impacting a shock in quasi-perpendicular conditions. First, we feed an analytical model of stable LMHs of various size and depth with magnetic field and density variations in antiphase, oriented along the solar wind magnetic field, into the simulation. The LMHs are then left to propagate with and into the plasma flow, eventually impacting the shock, where they may cross into the induced magnetosheath. We look at the global and local effects of such crossings and how the structures' characteristics and their immediate vicinity change over time. We apply this setup to (i) a local quasi-perpendicular shock structure created by one reflecting boundary and (ii) a global simulation of a cometary environment, and compare with observational findings. This work is part of preliminary modelling efforts preparing for the upcoming ESA/JAXA Comet Interceptor mission.

According to CMB measurements, baryonic matter constitutes about $5\%$ of the mass-energy density of the universe. A significant population of these baryons, for a long time referred to as `missing', resides in a low density, warm-hot intergalactic medium (WHIM) outside galaxy clusters, tracing the ``cosmic web'', a network of large scale dark matter filaments. Various studies have detected this inter-cluster gas, both by stacking and by observing individual filaments in compact, massive systems. In this paper, we study short filaments (< 10 Mpc) connecting massive clusters ($M_{500} \approx 3\times 10^{14} M_{\odot}$) detected by the Atacama Cosmology Telescope (ACT) using the scattering of CMB light off the ionised gas, a phenomenon known as the thermal Sunyaev-Zeldovich (tSZ) effect. The first part of this work is a search for suitable candidates for high resolution follow-up tSZ observations. We identify four cluster pairs with an intercluster signal above the noise floor (S/N $>$ 2), including two with a tentative $>2\sigma$ statistical significance for an intercluster bridge from the ACT data alone. In the second part of this work, starting from the same cluster sample, we directly stack on ${\sim}100$ cluster pairs and observe an excess SZ signal between the stacked clusters of $y=(7.2^{+2.3}_{-2.5})\times 10^{-7}$ with a significance of $3.3\sigma$. It is the first tSZ measurement of hot gas between clusters in this range of masses at moderate redshift ($\langle z\rangle\approx 0.5$). We compare this to the signal from simulated cluster pairs with similar redshifts and separations in the THE300 and MAGNETICUM Pathfinder cosmological simulations and find broad consistency. Additionally, we show that our measurement is consistent with scaling relations between filament parameters and mass of the embedded halos identified in simulations.

AOC (Adaptive Optics system at Calern) is an adaptive optics bench being developed on the Epsilon (East) telescope of the two 1m telescopes of C2PU (Centre Pédagogique Planète Univers), Calern observatory, Observatoire de la Côte d'Azur (OCA), near Nice (France). It is installed at the F/35 Coudé focus, and aims at correcting the wavefront in the visible and in the near-infrared, using an ALPAO 11x11 actuator-across deformable mirror feeding a 10x10 Shack-Hartmann wavefront sensor. It is currently being upgraded with a First Light Imaging OCAM2 EMCCD detector, which will allow us to increase the number of available pixels per sub-aperture and the loop frequency, as well as to take advantage from a negligible read-out. The system is designed to work both in a standard stellar mode and in a more innovative planetary mode, capable of operating on planets such as Mars, Saturn and Jupiter, with a field up to 60arcsec. Such a wide field is much larger than the isoplanatic angle, therefore only the turbulent ground layer can be corrected, up to an altitude of a few thousand meters. Extensive simulations have been carried out of these two different modes. The system is now undergoing implementation and validation using a low order modal control (10 to 16 modes), but a zonal control will be implemented next to ensure performance improvement down to λ∼500nm. The loop frequency was limited to 500Hz by the speed of the Andor iXon camera, but its replacement by an OCAM2 camera will allow to reach between 1.5 and 2.2kHz (depending on binning). Theoretical performance of the system has been studied through simulations for the planetary mode and presented previously at SPIE. Here we present the implementation of the system and its preliminary performance on the sky and comparison with the simulations. In the future, we plan to better characterize the temporal transfer function as a function of the gain and the number of corrected modes, in order to implement an automatic gain control, both zonally in the visible and modally for near infrared applications. We are also implementing a control of the non-common path aberrations (NCPA). This is done by analysing the aberrations on a defocused stellar image, using the DONUT algorithm. We intend to characterize these NCPA in real-time directly from the observations. We also consider parameterizing the resulting point-spread function from the real-time statistics of the adaptive optics system to be used for further data analysis.

In this study, we explored the fundamental contrast limit of NIRCam coronagraphy observations, representing the achievable performance with post-processing techniques. This limit is influenced by photon noise and readout noise, with complex noise propagation through post-processing methods like principal component analysis. We employed two approaches: developing a formula based on simplified scenarios and using Markov Chain Monte Carlo (MCMC) methods, assuming Gaussian noise properties and uncorrelated pixel noise. Tested on datasets HIP 65426, AF Lep, and HD 114174, the MCMC method provided accurate but computationally intensive estimates. The analytical approach offered quick, reliable estimates closely matching MCMC results in simpler scenarios. Our findings showed the fundamental contrast curve is significantly deeper than the current achievable contrast limit obtained with post-processing techniques at shorter separations, being 10 times deeper at 0.5 ′′ and 4 times deeper at 1′′. At greater separations, increased exposure time improves sensitivity, with the transition between photon and readout noise dominance occurring between 2′′ and 3′′. We conclude the analytical approach is a reliable estimate of the fundamental contrast limit, offering a faster alternative to MCMC. These results emphasize the potential for greater sensitivity at shorter separations, highlighting the need for improved or new post-processing techniques to enhance JWST NIRCam sensitivity or contrast curve.