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Until recently, only three large main belt asteroids, Ceres, Vesta and Lutetia, had been imaged with a high level of detail, as they were visited by the space missions Dawn and Rosetta of NASA and the European Space Agency, respectively. The previously small number of detailed observations of asteroids meant that, until now, key characteristics such as their 3D shape or density had remained largely unknown. Between 2017 and 2019, we have been filling this gap by conducting a high-angular-resolution imaging survey of 42 large main-belt asteroids with VLT/SPHERE (ESO large programme; PI: P. Vernazza; ID: 199.C-0074), sampling the main compositional classes. These observations have allowed to cast some light on the following fundamental questions: – What is the diversity in shape among large asteroids and are the shapes close to equilibrium?– How do large impacts affect asteroid shape?– What is the bulk density of large asteroids and is there a relationship with their surface composition? Is there any evidence of differentiation among those bodies?– Is the density of those bodies that are predicted to be implanted bodies from the outer Solar System (P/D-types) compatible with that of small (D ≤ 300 km) trans-Neptunian objects?– What physical properties drive the formation of companions around large asteroids? Importantly, our survey along with previous observations provides evidence in support of the possibility that some C-complex bodies could be intrinsically related to IDP-like P- and D-type asteroids, representing different layers of a same body (C: core; P/D: outer shell). We therefore propose that P/ D-types and some C-types may have the same origin in the primordial trans-Neptunian disk. The main belt would thus host a population of former TNOs much more important than the one previously considered, consisting solely of P and D-type bodies. Here, we will present an overview of the results obtained from this survey ([1]) with a specific focus on the origin of a large fraction of the asteroid belt (typically C, P and D-type bodies). References [1] Vernazza, P., Ferrais, M., Jorda, L., et al. VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis. A&A 654, 2021.

International audience; Themis is an outer main-belt family comprising more than 4,000 dynamically well-established members (Nesvorny 2012), mainly B- and C-type asteroids (Florczak et al. 1999; Mothé-Diniz et al. 2005; Ziffer et al. 2011). This family is rather unique for a number of reasons:- It is believed to be the only main-belt family formed from the catastrophic disruption of a large (D>200 km) B-/C-type body (Brož et al. 2013). As such, it offers a unique view on the internal composition of a primitive asteroid.- Elst-Pizarro, one of the family member, was one of the first main belt comets to be discovered (Hsieh & Jewitt 2006).- 24 Themis is the first main belt asteroid for which water ice was detected at its surface (Campins et al. 2010; Rivkin & Emery 2010).- The low density values recorded for two family members (

We report the clear detection of C 2 and of abundant NH 2 in the first prominently active interstellar comet, 2I/Borisov. We observed 2I on three nights in November 2019 at optical wavelengths 4800-9300 Awith the Multi-Unit Spectroscopic Explorer (MUSE) integral-field spectrograph on the ESO/Very Large Telescope. These data, together with observations close in time from both 0.6-m TRAPPIST telescopes, provide constraints on the production rates of species of gas in 2I's coma. From the MUSE detection on all epochs of several bands of the optical emission of the C 2 Swan system, a rich emission spectrum of NH 2 with many highly visible bands, and the red (1-0) bandhead of CN, together with violet CN detections by TRAPPIST, we infer production rates of Q(C 2) = 1.1 × 10 24 mol s −1 , Q(NH 2) = 4.8 × 10 24 mol s −1 and Q(CN) = (1.8 ± 0.2) × 10 24 mol s −1. In late November at 2.03 au, 2I had a production ratio of C 2 /CN= 0.61, only barely carbon-chain depleted, in contrast to earlier reports measured further from the Sun of strong carbon-chain depletion. Thus, 2I has shown evolution in its C 2 production rate: a parent molecule reservoir has started sublimating. At Q(NH 2)/Q(CN) = 2.7, this second interstellar object is enriched in NH 2 , relative to the known Solar System sample.

International audience; Colours of the Outer Solar System Origins Survey (Col-OSSOS) is probing Kuiper belt object (KBO) surface properties via near simultaneous g,r and J photometry from Gemini North with additional u-band imaging from CFHT. We will present the latest results from the survey including the surface properties of two outlier objects from the Col-OSSOS color distribution that may represent a new surface type present within the small KBO population.

International audience; The small bodies in the outer Solar System, called trans-Neptunian objects (TNOs), have a variety of surface types and orbital classifications. The cold classicals, which formed in situ, and the dynamically excited objects, which were likely scattered into their current locations, have distinct surface types. These surface types can be robustly identified using photometry in the combination of g, r, and z bands. We investigate the 2:1 resonance, which is located just outside the cold classical belt, to see whether this resonance harbors trapped cold classical surfaces. These would have been captured during a period of smooth sweeping migration by Neptune. We have acquired photometry of 17 of our 34 targets in 2018, and our results exclude a slow smooth sweeping of the entire cold classical region by Neptune's 2:1 resonance.

International audience; We propose a new compositional model for Kuiper Belt Objects that can account for many of the observed properties of those bodies, including their colour, and spectral behaviour. This model successfully accounts for the links to other related bodies, including the satellite Phoebe. The model asserts that the majority of KBOs consist of reddened carbonaceous material. If true, this implies that neutral class KBOs share a primordial link with the carbonaceous asteroids, the same bodies that scattered in to become the C-types in the asteroid belt.

International audience; We present the utilisation of genetic algorithm to create asteroid models from lightcurves and VLT/SPHERE adaptive-optics images. The lightcurve-only non-convex SAGE method was extended to include disk-resolved data during the modelling, alongside lightcurves.

We present near-infrared spectroscopy of the sporadically active asteroid (6478) Gault collected on the 3 m NASA/Infrared Telescope Facility observatory in late 2019 March/early April. Long-exposure imaging with the 0.5 m NEEMO T05 telescope and previously published data simultaneously monitored the asteroid activity, providing context for our measurements. We confirm Gault is a silicate-rich (Q- or S-type) object likely linked to the (25) Phocaea collisional family. The asteroid exhibits substantial spectral variability over the 0.75-2.45 $\mu$m wavelength range, from unusual blue (s'=-13.5+/-1.1% $\mu$m-1 to typical red (s'=+9.1+/-1.2% $\mu$m-1) spectral slope, that does not seem to correlate with activity. Spectral comparisons with samples of ordinary chondrite meteorites suggest that the blue color relates to the partial loss of the asteroid dust regolith, exposing a fresh, dust-free material at its surface. The existence of asteroids rotating close to rotational break-up limit and having similar spectral properties as Gault further supports this interpretation. Future spectroscopic observations of Gault, when the tails dissipate, will help further testing of our proposed hypothesis., Comment: 8 pages, 5 figures, 2 tables, accepted for publication in ApJL

International audience; As part of our ESO large program (ID 199.C-0074), we observed asteroid (7) Iris with the VLT/SPHERE/ZIMPOL instrument throughout its rotation during two consecutive nights in October 2017 (five different epochs). Iris, which is one of the four D>200 km S-type main belt asteroids along with (3) Juno, (15) Eunomia and (29) Amphitrite, is an exceptional target for an adaptive optics (AO) campaign due to its large angular size as seen from the Earth (0.35'') during opposition. Considering the large size of Iris, one pixel represents 2.3 km at distance of Iris on our AO images. We identified several topographic features in the ZIMPOL AO images that we interpreted as impact craters. Crater identification was performed manually on the images, by looking for circular features with a clear brightness contrast. Craters were first extracted on each of the five epochs. We compared all the images within a given epoch to confirm the genuineness of the identified features, removing the possibility that they are deconvolution artifacts. In the end, we checked the pairing of craters between the different epochs, that lead to a total number of six individual craters with diameters larger than 20 km. We compared the number and size of the identified craters to the cratering records on Ceres and Vesta. We used the well established ADAM (All-Data Asteroid Modeling) inversion technique for the reconstruction of a highly-detailed 3D shape model (craters included), volume and the spin of Iris using its disk-integrated data (optical lightcurves) and disk-resolved images as inputs. We also estimated the bulk density to 2.4±0.3 g cm-3. No moons were identified in our images. This work was supported by the grant 18-09470S of the Czech Science Foundation.

International audience; Large (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, A8

International audience; The vast majority of the geological constraints (i.e., internal structure via the density, cratering history) for main belt asteroids have so far been obtained via dedicated interplanetary missions (e.g., Rosetta, DAWN). The high angular resolution of SPHERE/ZIMPOL (one pixel represents 3.6 x 3.6 mas on sky), the new-generation visible adaptive-optics camera at ESO/VLT, implies that such science objective can now be investigated from the ground for a large fraction of D>=100 km main-belt asteroids (most of these bodies possess an angular diameter around opposition larger than 100 mas). The sharp images acquired by this instrument can be used to constrain accurately the shape and thus volume of these bodies (hence density when combined with mass estimates) and to characterize the distribution and topography of D>=30 km craters across their surfaces. To make substantial progress in our understanding of the shape, internal compositional structure (i.e., density) and surface topography of large main belt asteroids, we are carrying out an imaging survey via an ESO Large program entirely performed in service mode with seeing constraints 100 km main-belt asteroids ( 35 out of 200 asteroids; our survey covers the major compositional classes) at high angular-resolution with VLT/SPHERE throughout their rotation (typically 6 epochs per target). Here, we will present a summary of the results obtained after one year of observations.

International audience; Asteroid (16) Psyche is the target of the NASA Psyche mission. It is considered as 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 association is however challenged by both its near- and mid-infrared spectral properties (e.g. Hardersen et al. Icarus 175, 2005; Takir et al. AJ, 153, 2017; Landsman et al. Icarus, 304, 2018). We observed (16) Psyche with ESO VLT SPHERE/ZIMPOL as part of our large program (ID 199.C-0074, PI Vernazza) between April 24 and June 6 2018. We use the high-angular resolution of these observations to reconstruct the 3D shape model of Psyche. When combined with the most recent estimates of its mass, the volume that we derive led to 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). Although our observations only covered the northern hemisphere of Psyche, they reveal the presence of two peculiar units in the front views, namely one low and one high brightness regions nicknamed Panthia and Meroe. Meroe unit is about 7% brighter than the surrounding region, whereas the Panthia unit is 8% fainter. Panthia is a depression with a width of 90 km and a depth of 10 km. We did not detect any moons around Psyche and estimate the minimum radius of a moon to be detected around Psyche to 730 ± 100 m at 150 km (so 0.2% x RHill) of the primary and 400 ± 100 m at 2000 km from the primary corresponding to 3% x RHill) where most of the satellites of >100-km asteroids have been seen so far (Yang et al. AJL 820:L35, 2016). Considering that the visible and near-infrared spectral properties of mesosiderites are similar to those of Psyche, there is merit to the initial hypothesis by Davis et al. (1999) that Psyche could be a plausible candidate parent body for mesosiderites (Viikinkoski et al. submitted to A&A, 2018). This material is based upon work partially supported by the National Science Foundation under Grant No 1743015.

International audience; IAUC 9282 available at Central Bureau for Astronomical Telegrams.

International audience; IAUC 9282 available at Central Bureau for Astronomical Telegrams.