Asteroid (16) Psyche’s primordial shape: A possible Jacobi ellipsoid

Context. Asteroid (16) Psyche is the largest M-type asteroid in the main belt and the target of the NASA Psyche mission. It is also the only asteroid of this size (D >  200 km) known to be metal rich. Although various hypotheses have been proposed to explain the rather unique physical properties of this asteroid, a perfect understanding of its formation and bulk composition is still missing. Aims. We aim to refine the shape and bulk density of (16) Psyche and to perform a thorough analysis of its shape to better constrain possible formation scenarios and the structure of its interior. Methods. We obtained disk-resolved VLT/SPHERE/ZIMPOL images acquired within our ESO large program (ID 199.C-0074), which complement similar data obtained in 2018. Both data sets offer a complete coverage of Psyche’s surface. These images were used to reconstruct the three-dimensional (3D) shape of Psyche with two independent shape modeling algorithms (MPCD and ADAM). A shape analysis was subsequently performed, including a comparison with equilibrium figures and the identification of mass deficit regions. Results. Our 3D shape along with existing mass estimates imply a density of 4.20  ±  0.60 g cm−3, which is so far the highest for a solar system object following the four telluric planets. Furthermore, the shape of Psyche presents small deviations from an ellipsoid, that is, prominently three large depressions along its equator. The flatness and density of Psyche are compatible with a formation at hydrostatic equilibrium as a Jacobi ellipsoid with a shorter rotation period of ∼3h. Later impacts may have slowed down Psyche’s rotation, which is currently ∼4.2 h, while also creating the imaged depressions. Conclusions. Our results open the possibility that Psyche acquired its primordial shape either after a giant impact while its interior was already frozen or while its interior was still molten owing to the decay of the short-lived radionuclide 26Al.Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme 199.C-0074 (principal investigator: P. Vernazza). P. Vernazza, A. Drouard, M. Ferrais and B. Carry were supported by CNRS/INSU/PNP. J.H. and J.D. were supported by grant 18-09470S of the Czech Science Foundation and by the Charles University Research Programme no. UNCE/SCI/023. E.J. is F.R.S.-FNRS Senior Research Associate. The work of TSR was carried out through grant APOSTD/2019/046 by Generalitat Valenciana (Spain). This work was supported by the MINECO (Spanish Ministry of Economy) through grant RTI2018-095076-B-C21 (MINECO/FEDER, UE)

A basin-free spherical shape as an outcome of a giant impact on asteroid Hygiea

(10) Hygiea is the fourth largest main belt asteroid and the only known asteroid whose surface composition appears similar to that of the dwarf planet (1) Ceres1,2, suggesting a similar origin for these two objects. Hygiea suffered a giant impact more than 2 Gyr ago3 that is at the origin of one of the largest asteroid families. However, Hygeia has never been observed with sufficiently high resolution to resolve the details of its surface or to constrain its size and shape. Here, we report high-angular-resolution imaging observations of Hygiea with the VLT/SPHERE instrument (~20 mas at 600 nm) that reveal a basin-free nearly spherical shape with a volume-equivalent radius of 217 ± 7 km, implying a density of 1,944 ± 250 kg m−3 to 1σ. In addition, we have determined a new rotation period for Hygiea of ~13.8 h, which is half the currently accepted value. Numerical simulations of the family-forming event show that Hygiea’s spherical shape and family can be explained by a collision with a large projectile (diameter ~75–150 km). By comparing Hygiea’s sphericity with that of other Solar System objects, it appears that Hygiea is nearly as spherical as Ceres, opening up the possibility for this object to be reclassified as a dwarf planet.P.V., A.D. and B.C. were supported by CNRS/INSU/PNP. M.Brož was supported by grant 18-04514J of the Czech Science Foundation. J.H. and J.D. were supported by grant 18-09470S of the Czech Science Foundation and by the Charles University Research Programme no. UNCE/SCI/023. This project has received funding from the European Union’s Horizon 2020 research and innovation programmes under grant agreement nos 730890 and 687378. This material reflects only the authors’ views, and the European Commission is not liable for any use that may be made of the information contained herein. TRAPPIST-North is a project funded by the University of Liège, in collaboration with Cadi Ayyad University of Marrakech (Morocco). TRAPPIST-South is a project funded by the Belgian Fonds (National) de la Recherche Scientifique (F.R.S.-FNRS) under grant FRFC 2.5.594.09.F. E.J. and M.G. are F.R.S.-FNRS Senior Research Associates

Including the pyramid optical gains into analytical models

International audienceFourier-filtering wavefront sensors (WFS), such as the pyramid of Zernike WFS, are shown to be highly sensitive.They are becoming the baseline for future adaptive optics (AO) systems for astronomy. The next generationExtremely Large Telescopes (ELTs) will be equipped with such sensitive WFS. However the main drawback ofthese sensors is a quick loss of linearity when subject to strong turbulence residuals.Two major methods can be identified to simulate the AO point-spread-function (PSF): the end-to-endsimulation and the analytical model. The first one propagates random samples of phase screens through a fullysimulated AO loop, it can thus reproduce fine spatial and temporal effects, inlcuding the WFS non linearities.The second method is based on analytical formulas that provide a quick simulation with a good understanding ofthe AO system (separation of the AO error terms) but require a linear response of the system.We develop here a method to include the non linearities of the WFS into analytical formulas. It consequentlyimproves the accuracy of the simulation and enables to describe with good accuracy Fourier-filtering WFS. We testour method against end-to-end simulations, and derive possible applications for AO system design or performanceestimation

Toward the full control of NCPA with the pyramid wavefront sensor: mastering the optical gains

International audienceThe pyramid wavefront sensor is an asset for an AO system thanks to its sensitivity. However, because itsa nonlinear sensor it comes with operational challenges. A convolutional method and a gain sensing cameraallow to track the optical gains, which encode the sensitivity variations due to the nonlinearities. Tracking andcompensating the optical gains is necessary to perform extreme adaptive optics and to operate the pyramidoff-zero to compensate for the NCPA.This study focuses on the reliability of this method. A numerical twinof the bench PAPYRUS, developed for this study, shows a improvement of the performance by a factor 2.7 onthe Strehl Ratio when compensating for the optical gains. The convolutional method is implemented for thePAPYRUS bench, allowing the first on-sky tracking of optical gains. The next main steps are to compensate forthe optical gains in real-time, then to offset the pyramid in order to optimise fiber-injection, to compensate forNCPA and to provide AO generated dark hole for high-contrast imaging

Toward the full control of NCPA with the pyramid wavefront sensor: mastering the optical gains

International audienceThe pyramid wavefront sensor is an asset for an AO system thanks to its sensitivity. However, because itsa nonlinear sensor it comes with operational challenges. A convolutional method and a gain sensing cameraallow to track the optical gains, which encode the sensitivity variations due to the nonlinearities. Tracking andcompensating the optical gains is necessary to perform extreme adaptive optics and to operate the pyramidoff-zero to compensate for the NCPA.This study focuses on the reliability of this method. A numerical twinof the bench PAPYRUS, developed for this study, shows a improvement of the performance by a factor 2.7 onthe Strehl Ratio when compensating for the optical gains. The convolutional method is implemented for thePAPYRUS bench, allowing the first on-sky tracking of optical gains. The next main steps are to compensate forthe optical gains in real-time, then to offset the pyramid in order to optimise fiber-injection, to compensate forNCPA and to provide AO generated dark hole for high-contrast imaging

PAPYRUS: Second stage adaptive optics with a vector Zernike wavefront sensor

International audiencePAPYRUS: Second stage adaptive optics with a vector Zernike wavefront senso

Pallas's formation and internal structure: New insights from VLT/SPHERE

International audienceLarge (D>100km) asteroids are the most direct remnants of the building blocks of planets. (2) Pallas is the third largest asteroid and the parent body of a small collisional family. Its spectral properties indicate a B-type surface, meaning Pallas is most likely linked to carbonaceous chondrite meteorites. Disc-resolved images have revealed a nearly hydrostatic shape overprinted by long-wavelength concavities (Schmidt et al. 2009, Carry et al. 2010). This was interpreted as evidence for an early phase of internal heating subsequent to Pallas's formation, followed by several large impact craters (Schmidt & Castillo-Rogez 2012). Recent estimates of Pallas's density, 2.40±0.25 g/cm3 (Schmidt et al. 2009), 3.40±0.90 g/cm3 (Carry et al. 2010) and 2.72±0.17 g/cm3 (Hanus et al. 2017), are rather inconsistent and prevent from differentiating among the various models proposed for its internal structure (Schmidt & Castillo-Rogez 2012). This currently limits our understanding of the formation and thermal evolution of Pallas. We report new high-angular resolution observations of Pallas collected in the frame of the SPHERE large survey of the asteroid belt (see Talk by P. Vernazza) with the adaptive-optics-fed SPHERE ZIMPOL camera on the VLT. 40 images acquired at 8 epochs provide a full longitudinal coverage of Pallas's southern hemisphere, with Pallas being resolved with ˜120 pixels along its longest axis. The optimal angular resolution of each image was restored with Mistral (Fusco et al. 2002), a myopic deconvolution algorithm optimised for images with sharp boundaries, which allows the identification of many craters and geological features on Pallas. A precise 3D-shape reconstruction was achieved with the ADAM software (Viikinkoski et al. 2015), providing a high precision estimate of Pallas's 3D shape, volume and hence density. Those are used to explore Pallas's early thermal evolution, its subsequent collisional evolution, and its current internal structure and composition. [1] Carry et al. 2010, Icarus, 205, 460 [2] Fusco et al. 2002, SPIE, 4839, 1065 [3] Hanus et al. 2017, A&A, 601, A114 [4] Schmidt et al. 2009, Science, 326, 275 [5] Schmidt & Castillo-Rogez, Icarus, 218, 478 [6] Viikinkoski et al. 2015, A&A, 576, A

Pallas's formation and internal structure: New insights from VLT/SPHERE

International audienceLarge (D>100km) asteroids are the most direct remnants of the building blocks of planets. (2) Pallas is the third largest asteroid and the parent body of a small collisional family. Its spectral properties indicate a B-type surface, meaning Pallas is most likely linked to carbonaceous chondrite meteorites. Disc-resolved images have revealed a nearly hydrostatic shape overprinted by long-wavelength concavities (Schmidt et al. 2009, Carry et al. 2010). This was interpreted as evidence for an early phase of internal heating subsequent to Pallas's formation, followed by several large impact craters (Schmidt & Castillo-Rogez 2012). Recent estimates of Pallas's density, 2.40±0.25 g/cm3 (Schmidt et al. 2009), 3.40±0.90 g/cm3 (Carry et al. 2010) and 2.72±0.17 g/cm3 (Hanus et al. 2017), are rather inconsistent and prevent from differentiating among the various models proposed for its internal structure (Schmidt & Castillo-Rogez 2012). This currently limits our understanding of the formation and thermal evolution of Pallas. We report new high-angular resolution observations of Pallas collected in the frame of the SPHERE large survey of the asteroid belt (see Talk by P. Vernazza) with the adaptive-optics-fed SPHERE ZIMPOL camera on the VLT. 40 images acquired at 8 epochs provide a full longitudinal coverage of Pallas's southern hemisphere, with Pallas being resolved with ˜120 pixels along its longest axis. The optimal angular resolution of each image was restored with Mistral (Fusco et al. 2002), a myopic deconvolution algorithm optimised for images with sharp boundaries, which allows the identification of many craters and geological features on Pallas. A precise 3D-shape reconstruction was achieved with the ADAM software (Viikinkoski et al. 2015), providing a high precision estimate of Pallas's 3D shape, volume and hence density. Those are used to explore Pallas's early thermal evolution, its subsequent collisional evolution, and its current internal structure and composition. [1] Carry et al. 2010, Icarus, 205, 460 [2] Fusco et al. 2002, SPIE, 4839, 1065 [3] Hanus et al. 2017, A&A, 601, A114 [4] Schmidt et al. 2009, Science, 326, 275 [5] Schmidt & Castillo-Rogez, Icarus, 218, 478 [6] Viikinkoski et al. 2015, A&A, 576, A

Pallas's formation and internal structure: New insights from VLT/SPHERE

International audienceLarge (D>100km) asteroids are the most direct remnants of the building blocks of planets. (2) Pallas is the third largest asteroid and the parent body of a small collisional family. Its spectral properties indicate a B-type surface, meaning Pallas is most likely linked to carbonaceous chondrite meteorites. Disc-resolved images have revealed a nearly hydrostatic shape overprinted by long-wavelength concavities (Schmidt et al. 2009, Carry et al. 2010). This was interpreted as evidence for an early phase of internal heating subsequent to Pallas's formation, followed by several large impact craters (Schmidt & Castillo-Rogez 2012). Recent estimates of Pallas's density, 2.40±0.25 g/cm3 (Schmidt et al. 2009), 3.40±0.90 g/cm3 (Carry et al. 2010) and 2.72±0.17 g/cm3 (Hanus et al. 2017), are rather inconsistent and prevent from differentiating among the various models proposed for its internal structure (Schmidt & Castillo-Rogez 2012). This currently limits our understanding of the formation and thermal evolution of Pallas. We report new high-angular resolution observations of Pallas collected in the frame of the SPHERE large survey of the asteroid belt (see Talk by P. Vernazza) with the adaptive-optics-fed SPHERE ZIMPOL camera on the VLT. 40 images acquired at 8 epochs provide a full longitudinal coverage of Pallas's southern hemisphere, with Pallas being resolved with ˜120 pixels along its longest axis. The optimal angular resolution of each image was restored with Mistral (Fusco et al. 2002), a myopic deconvolution algorithm optimised for images with sharp boundaries, which allows the identification of many craters and geological features on Pallas. A precise 3D-shape reconstruction was achieved with the ADAM software (Viikinkoski et al. 2015), providing a high precision estimate of Pallas's 3D shape, volume and hence density. Those are used to explore Pallas's early thermal evolution, its subsequent collisional evolution, and its current internal structure and composition. [1] Carry et al. 2010, Icarus, 205, 460 [2] Fusco et al. 2002, SPIE, 4839, 1065 [3] Hanus et al. 2017, A&A, 601, A114 [4] Schmidt et al. 2009, Science, 326, 275 [5] Schmidt & Castillo-Rogez, Icarus, 218, 478 [6] Viikinkoski et al. 2015, A&A, 576, A

Physics-based model of the adaptive-optics-corrected point spread function: Applications to the SPHERE/ZIMPOL and MUSE instruments

International audienceContext. Adaptive optics (AO) systems greatly increase the resolution of large telescopes, but produce complex point spread function (PSF) shapes, varying in time and across the field of view. The PSF must be accurately known since it provides crucial information about optical systems for design, characterization, diagnostics, and image post-processing.Aims. We develop here a model of the AO long-exposure PSF, adapted to various seeing conditions and any AO system. This model is made to match accurately both the core of the PSF and its turbulent halo.Methods. The PSF model we develop is based on a parsimonious parameterization of the phase power spectral density, with only five parameters to describe circularly symmetric PSFs and seven parameters for asymmetrical ones. Moreover, one of the parameters is the Fried parameter r0 of the turbulence’s strength. This physical parameter is an asset in the PSF model since it can be correlated with external measurements of the r0, such as phase slopes from the AO real time computer (RTC) or site seeing monitoring.Results. We fit our model against end-to-end simulated PSFs using the OOMAO tool, and against on-sky PSFs from the SPHERE/ZIMPOL imager and the MUSE integral field spectrometer working in AO narrow-field mode. Our model matches the shape of the AO PSF both in the core and the halo, with a relative error smaller than 1% for simulated and experimental data. We also show that we retrieve the r0 parameter with sub-centimeter precision on simulated data. For ZIMPOL data, we show a correlation of 97% between our r0 estimation and the RTC estimation. Finally, MUSE allows us to test the spectral dependency of the fitted r0 parameter. It follows the theoretical λ6/5 evolution with a standard deviation of 0.3 cm. Evolution of other PSF parameters, such as residual phase variance or aliasing, is also discussed