Constraining the shape and density of binary asteroid (121) Hermione

Context(121) Hermione is a large binary asteroid [1] located at the outer edge of the asteroid belt in the Cybele region, where asteroids are thought to be linked to the outer Solar System. Hermione has a Ch/Cgh-type that has been linked to CM chondrites. Adaptive optics observations between 2003 and 2008 suggest a rare bilobate shape for the primary [2,3]. However, Hermione's shape and bulk density (ranging between 1.4 and 2 g.cm-3) remain poorly constrained to this day.AimWe acquired spatially resolved images and optical lightcurves of Hermione during its close apparition of September 2021. It was the best chance in 13 years to acquire such high angular resolution images (angular diameter = 0.14"). We aimed to constrain Hermione's 3D shape, hence its volume, and the orbit of its satellite, hence the mass of the system. Combining the volume and the mass allows to constrain the bulk density with high accuracy.MethodsWe obtained 8 series of 5 images with the SPHERE/ZIMPOL instrument on the Very Large Telescope (ESO Program ID 107.22UT.001; PI: P. Vernazza). These images were combined with optical lightcurves and stellar occultations by the ADAM and MPCD methods [4,5] to reconstruct the asteroid's 3D shape. For the determination of the satellite's orbit, we complemented the SPHERE images with a compilation of archival data from other large ground-based AO instruments (KeckII/NIRC2, ESO/VLT/NACO and Gemini-North/NIRI). Then, we used the meta-heuristic algorithm Genoid [6] to accurately determine the orbital elements.ResultsThe determined volume and mass of Hermione yield a new higher bulk density of ~1.7 g.cm-3, more compatible with its Ch/Cgh classification. We will also present our analyse of the shape and compare it with other elongated Ch/Cgh asteroids

The stellar occultations by the largest satellite of the dwarf planet Haumea, Hi'iaka

Two stellar occultations by the largest satellite of the dwarf planet Haumea, Hi'iaka, were predicted to happen on April, 6th and 16th, 2021. Additional high accuracy astrometric analysis was carried out in order to refine the prediction for April 6th, using several telescopes in the 1.2-m to 2-m range, with the final shadow path crossing North Africa. We successfully detected the first event from TRAPPIST-North telescope at Oukaïmeden Observatory (Morocco). Although it was recorded from only one site, this first detection allowed us to improve the prediction for the second that crossed North America from East to West. We had a good success recording six positive detections and several negative detections that constrain the shape and size of the body. The light curves obtained from the different observatories provide the time at which the star disappears and reappears, which are translated into chords (the projected lines on the sky-plane as observed from each location). Additionally, we carried out a campaign to study Hi'iaka's rotational light-curve, studying the residuals of Haumea's rotational light-curve to a four-order Fourier fit. We obtained the rotational phases at the times of the occultations, which is critical for the analysis of the occultations, given that Hi'iaka is clearly non-spherical. Our preliminary results show that Hi'iaka indeed has a triaxial shape with a larger effective diameter than what has been published so far. The preliminary results and their implications will be discussed in this talk

Spitzer observations of mutual events in the binary system (617) Patroclus-Menoetius

We report Spitzer observations of the binary Trojan system (617) Patroclus-Menoetius during two mutual events, when respectively one component shadowed and occulted the other. Observing the thermal response to mutual shadowing with spectral ( 8--33 µm) and temporal resolution allowed us to determine the system's thermal inertia in a uniquely direct way. Furthermore, our analysis provided an accurate determination of the system's size which is methodologically independent of the estimate by Berthier et al. (this session). Our results allow a more reliable estimate of the system's bulk density (the total mass was determined from the system's mutual orbit; Marchis et al., 2006; Berthier et al., this session). This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. This material is partly based upon work supported by the national Aeronautics and Space Administration issue through the Science Mission Directorate Research and Analysis Programs number NNG05GF09G

2003 AZ84: Size, shape, albedo and first detection of topographic features

We analyze two multi-chord stellar occultations by the Trans-Neptunian Object (TNO) 2003 AZ84 observed on February 3, 2012 and November 15, 2014.They provide different elliptical limb fits that are consistent to within their respective error bars, but could also suggest a possible precession of the object (assumed here to be a Maclaurin spheroid). The derived equatorial radius and oblateness are R[SUB]e[/SUB] = 393 ± 7 km and ɛ = 0.057 in 2014 and R[SUB]e[/SUB] = 414 ± 13 km and ɛ = 0.165 in 2012, respectively. Those results are consistent with single-chord events observed in January 2011 and December 2013. The figures above provide geometric visual albedos of p[SUB]V(2014)[/SUB] = 0.112 ± 0.008 and p[SUB]V(2012)[/SUB] = 0.114 ± 0.020. Using the Maclaurin assumption, combined with possible rotational periods of 6.67 h and 10.56 h, we estimate density upper limits of 1.89 ± 0.16g/cm[SUP]3[/SUP] and 0.77 ± 0.07g/cm[SUP]3[/SUP] for the two dates, respectively.The 2014 event provides (for the first time during a TNO occultation) a grazing chord with a gradual disappearance of the star behind 2003[SUB]AZ[/SUB]84's limb that lasts for more than 10 seconds. We rule out the possibility of a localized dust concentration as it would imply very high optical depth for that cloud. We favor a local topographic feature (chasm) with minimum width and depth of 22 ± 2.5 km and 7 ± 2.0 km, respectively. Features with similar depths are in fact observed on Pluto's main satellite, Charon, which has a radius of about 605 km, comparable to that of 2003[SUB]AZ[/SUB]84

Physical properties of Hi'iaka from stellar occultation data

Two very bright stellar occultations by Hi'iaka, the largest satellite of the dwarf planet Haumea, were predicted to take place during in April 2021. Since the uncertainty on Hi'iaka's shadow path was large due to uncertainty on Hi'iaka's position with respect to Haumea, we performed an observational campaign using medium-sized telescopes to obtain high accuracy astrometric data of Hi'iaka's orbit around Haumea. The astrometric data allowed us to successfully observe the first stellar occultation on April 6[SUP]th[/SUP], with final path crossing North Africa. We only obtained one positive chord in this event from the TRAPPIST-North telescope at Oukaïmeden Observatory (Morocco), but thanks to this detection, we were able to obtain a more accurate path for the second event on April 16[SUP]th[/SUP]. The second shadow path was predicted to cross the continental US from East to West. We carried out a huge observational campaign involving more than 50 professional and amateur observatories across the US and southern Canada. The final path of this second stellar occultation moved slightly to the North of the predicted path and, as a result, we were able to obtain 5 positive chords and negative chords only from the south of the shadow. We also collected photometric data in order to obtain Hi'iaka's rotational light-curve and calculate its three-dimensional shape. The rotational light-curve was obtained by observing the unresolved system of Haumea-Hi'iaka and removing Haumea's rotational light-curve from the data. Using Hi'iaka's rotational light-curve we obtained the rotational phase at which each stellar occultation took place, which allowed us to obtain a three-dimensional model of the satellite. Preliminary results from the stellar occultation show that Hi'iaka, with a triaxial shape as suggested in previous publications, is larger than what has been thought before and with a similar albedo to that of Haumea. In this talk we will present our analysis and preliminary results of some of Hi'iaka's physical properties