We present shape models and volume estimates of twenty asteroids based on relative photometry and adaptive optics images. We discuss error estimation and the effects of myopic deconvolution on shape ...solutions. For further analysis of the information capacities of data sources, we also present and discuss ambiguity and uniqueness results for the reconstruction of nonconvex shapes from photometry.
ABSTRACT Anhydrous pyroxene-rich interplanetary dust particles (IDPs) have been proposed as surface analogs for about two-thirds of all C-complex asteroids. However, this suggestion appears to be ...inconsistent with the presence of hydrated silicates on the surfaces of some of these asteroids, including Ceres. Here, we report the presence of enstatite (pyroxene) on the surface of two C-type asteroids (Ceres and Eugenia) based on their spectral properties in the mid-infrared range. The presence of this component is particularly unexpected in the case of Ceres, because most thermal evolution models predict a surface consisting of hydrated compounds only. The most plausible scenario is that Ceres' surface has been partially contaminated by exogenous enstatite-rich material, possibly coming from the Beagle asteroid family. This scenario questions a similar origin for Ceres and the remaining C-types, and it possibly supports recent results obtained by the Dawn mission (NASA) that Ceres may have formed in the very outer solar system. Concerning the smaller D ∼ 200 km C-types such as Eugenia, both their derived surface composition (enstatite and amorphous silicates) and low density (<1.5 g cm−3) suggest that these bodies accreted from the same building blocks, namely chondritic porous, pyroxene-rich IDPs and volatiles (mostly water ice), and that a significant volume fraction of these bodies has remained unaffected by hydrothermal activity likely implying a late accretion. In addition, their current heliocentric distance may best explain the presence or absence of water ice at their surfaces. Finally, we raise the possibility that CI chondrites, Tagish-Lake-like material, or hydrated IDPs may be representative samples of the cores of these bodies.
•We scale asteroid shape models by Keck II adaptive optics observations.•We compute the volume-equivalent diameters for 48 asteroids.•We compare these sizes with those based on IRAS, WISE and AKARI ...data.•We derive the bulk densities for 40 asteroids with known mass estimates.
We select 50 main-belt asteroids with a diameter between 20 and 400km for which we have (i) shape models derived by the lightcurve inversion method and (ii) resolved observations of good quality collected with the Keck II adaptive optics (AO) system in the near-infrared. We derive the size of these asteroids by minimizing the difference between the contours from deconvolved AO images and the projected silhouettes calculated from the shape model at the time of the AO observations. We compute the volume-equivalent diameters for 48 of these asteroids. For 15 of them, we remove the ambiguity of the pole orientation typical for shape models derived by the lightcurve inversion method. We have found that our equivalent diameters are smaller by 3%, 7%, and 2% compared with the effective diameters derived from mid-IR photometric observations provided by IRAS, WISE and AKARI. For 40 asteroids with previously determined mass estimates, we compute their bulk densities and discuss the mass–density dependence with respect to taxonomic types.
The Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument is a multipurpose high-contrast imaging platform designed for the discovery and detailed characterization of exoplanetary systems ...and serves as a testbed for high-contrast imaging technologies for ELTs. It is a multiband instrument which makes use of light from 600 to 2500 nm, allowing for coronagraphic direct exoplanet imaging of the inner 3λ/D from the stellar host. Wavefront sensing and control are key to the operation of SCExAO. A partial correction of low-order modes is provided by Subaru's facility adaptive optics system with the final correction, including high-order modes, implemented downstream by a combination of a visible pyramid wavefront sensor and a 2000-element deformable mirror. The well-corrected NIR (y-K bands) wavefronts can then be injected into any of the available coronagraphs, including but not limited to the phase-induced amplitude apodization and the vector vortex coronagraphs, both of which offer an inner working angle as low as 1λ/D. Noncommon path, low-order aberrations are sensed with a coronagraphic low-order wavefront sensor in the infrared (IR). Low noise, high frame rate NIR detectors allow for active speckle nulling and coherent differential imaging, while the HAWAII 2RG detector in the HiCIAO imager and/or the CHARIS integral field spectrograph (from mid-2016) can take deeper exposures and/or perform angular, spectral, and polarimetric differential imaging. Science in the visible is provided by two interferometric modules: VAMPIRES and FIRST, which enable subdiffraction limited imaging in the visible region with polarimetric and spectroscopic capabilities respectively. We describe the instrument in detail and present preliminary results both on-sky and in the laboratory.
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
26
Al.
Context. Disk-integrated photometric data of asteroids do not contain accurate information on shape details or size scale. Additional data such as disk-resolved images or stellar occultation ...measurements further constrain asteroid shapes and allow size estimates. Aims. We aim to use all the available disk-resolved images of approximately forty asteroids obtained by the Near-InfraRed Camera (Nirc2) mounted on the W.M. Keck II telescope together with the disk-integrated photometry and stellar occultation measurements to determine their volumes. We can then use the volume, in combination with the known mass, to derive the bulk density. Methods. We downloaded and processed all the asteroid disk-resolved images obtained by the Nirc2 that are available in the Keck Observatory Archive (KOA). We combined optical disk-integrated data and stellar occultation profiles with the disk-resolved images and use the All-Data Asteroid Modeling (ADAM) algorithm for the shape and size modeling. Our approach provides constraints on the expected uncertainty in the volume and size as well. Results. We present shape models and volume for 41 asteroids. For 35 of these asteroids, the knowledge of their mass estimates from the literature allowed us to derive their bulk densities. We see a clear trend of lower bulk densities for primitive objects (C-complex) and higher bulk densities for S-complex asteroids. The range of densities in the X-complex is large, suggesting various compositions. We also identified a few objects with rather peculiar bulk densities, which is likely a hint of their poor mass estimates. Asteroid masses determined from the Gaia astrometric observations should further refine most of the density estimates.
Aims. Over the past decade, discoveries of multiple and binary asteroid systems have played a significant role in our general understanding of small solar system bodies. Direct observations of ...satellites of asteroids are rare and difficult since they require the use of already over-subscribed facilities such as adaptive optics (AO) on large 8−10 m class telescopes and the Hubble Space Telescope (HST). The scarcity of data and the long temporal baseline of observations (up to 10 years) significantly complicate the determination of the mutual orbits of these systems. Methods. We implemented a new approach for determining the mutual orbits of directly-imaged multiple asteroids using a genetic-based algorithm. This approach was applied to several known binary asteroid systems (22 Kalliope, 3749 Balam, and 50 000 Quaoar) observed with AO systems and HST. This statistical method is fast enough to permit the search for an orbital solution across a large parameter space and without a priori information about the mutual orbit. Results. From 10 years of observation, we derived an orbital solution for Linus, companion of (22) Kalliope, with an accuracy close to the astrometric limit provided by the AO observations, assuming a purely Keplerian orbit. A search for non-Keplerian orbit confirmed that a J2 ~ 0 is the best-fitting solution. We show that the precession of the nodes could be detected without ambiguity, implying that Kalliope’s primary may have an inhomogeneous internal structure. HST astrometric observations of Weywot, companion of the trans-Neptunian object (50 000) Quaoar, were used to derive its mass and its bulk density, which appears to be higher than the density of other TNOs. Finally, we derived a bundle of orbital solutions for (3749) Balam, with equally good fits, from the limited set of astrometric positions. They provide a realistic density between 1.3 and 3.7 g/cm3 for this S-type asteroid.
We prove the existence of stationary turbulent flows with arbitrary positive vortex circulation on non-simply connected domains. Our construction yields solutions for all real values of the inverse ...temperature with the exception of a quantized set, for which blow-up phenomena may occur. Our results complete the analysis initiated in Ricciardi and Zecca (2016).
•We determine the parameters needed for the dynamics of 45 Eugenia and 87 Sylvia.•We study the satellites’ orbits using numerical modeling and observations.•The difference between theory and model ...points to differenciated interior.•We propose a two-layered model to explain the dynamical polar oblateness.
Satellites of asteroids are a widely used way to determine the masses and other dynamical parameters of their systems. We make such a determination here, using data from the two triple-asteroid systems, (45) Eugenia and (87) Sylvia, that come from the Hubble Space Telescope and adaptive optics on three 8–10m class telescopes (Keck, VLT, Gemini North). First, we determined what set of parameters can be precisely derived from a limited number of observations. Next, we deduced constraints on the inner structure of the main body of the systems. To do this, we fit our dynamical model ODIN to simulated observations to determine at what confidence level we can constrain the dynamical parameters. We deduced that the mass of the satellites is too small to be fitted with confidence, while the polar oblateness J2 of the primary can be constrained. Then, ODIN was fitted to observations of the systems to obtain the values of the polar oblateness J2. The difference between the theoretical value of J2, and our estimate deduced from the motion of the satellites, suggests that Eugenia is differentiated. We explore the properties needed by a dense and spherical core, surrounded by a less dense layer, to explain this difference. The lack of agreement between the different dynamical studies of Sylvia prevents us from making any definitive conclusion, but the large range of possible solutions points to an interesting inner structure.
(16) Psyche: A mesosiderite-like asteroid? Viikinkoski, M.; Vernazza, P.; Hanuš, J. ...
Astronomy and astrophysics (Berlin),
11/2018, Volume:
619
Journal Article, Web Resource
Peer reviewed
Open access
Context. Asteroid (16) Psyche is the target of the NASA Psyche mission. It is considered one of the few main-belt bodies that could be an exposed proto-planetary metallic core and that would thus be ...related to iron meteorites. Such an association is however challenged by both its near- and mid-infrared spectral properties and the reported estimates of its density. Aims. Here, we aim to refine the density of (16) Psyche to set further constraints on its bulk composition and determine its potential meteoritic analog. Methods. We observed (16) Psyche with ESO VLT/SPHERE/ZIMPOL as part of our large program (ID 199.C-0074). We used the high angular resolution of these observations to refine Psyche’s three-dimensional (3D) shape model and subsequently its density when combined with the most recent mass estimates. In addition, we searched for potential companions around the asteroid. Results. We derived a bulk density of 3.99 ± 0.26 g cm−3 for Psyche. While such density is incompatible at the 3-sigma level with any iron meteorites (∼7.8 g cm−3), it appears fully consistent with that of stony-iron meteorites such as mesosiderites (density ∼4.25 g cm−3). In addition, we found no satellite in our images and set an upper limit on the diameter of any non-detected satellite of 1460 ± 200 m at 150 km from Psyche (0.2% × RHill, the Hill radius) and 800 ± 200 m at 2000 km (3% × RHill). Conclusions. Considering that the visible and near-infrared spectral properties of mesosiderites are similar to those of Psyche, there is merit to a long-published initial hypothesis that Psyche could be a plausible candidate parent body for mesosiderites.
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