Sizes, Shapes, and Satellites of Asteroids from Occultations

The population of large 100+ km asteroids is thought to be primordial. As such, they are the most direct witnesses of the early history of our Solar System available. Those among them with satellites allow study of the mass, and hence density and internal structure. We study here the dynamical, physical, and spectral properties of the triple asteroid (107) Camilla from lightcurves, stellar occultations, optical spectroscopy, and high-contrast and high-angular-resolution images and spectro-images.Using 80 positions measured over 15 years, we determine the orbit of its larger satellite, S/2001 (107) 1, to be circular, equatorial, and prograde, with root-mean-square residuals of 7.8 mas, corresponding to a sub-pixel accuracy. From 11 positions spread over three epochs only, in 2015 and 2016, we determine a preliminary orbit for the second satellite S/2016 (107) 1. We find the orbit to be somewhat eccentric and slightly inclined to the primary's equatorial plane, reminiscent of the properties of inner satellites of other asteroid triple systems. Comparison of the near-infrared spectrum of the larger satellite reveals no significant difference with Camilla. Hence, both dynamical and surface properties argue for a formation of the satellites by excavation from impact and re-accumulation of ejecta in orbit.We determine the spin and 3-D shape of Camilla. The model fits well each data set: lightcurves, adaptive-optics images, and stellar occultations. We determine Camilla to be larger than reported from modeling of mid-infrared photometry, with a sphericalvolume-equivalent diameter of 254 ± 36 km (3 σ uncertainty), in agreement with recent results from shape modeling (Hanus et al., 2017, A&A 601). Combining the mass of (1.12 ± 0.01) × 10 19 kg (3 σ uncertainty) determined from the dynamics of the satellites and the volume from the 3-D shape model, we determine a density of 1,280 ± 130 kg·m −3 (3 σ uncertainty). From this density, and considering Camilla's spectral similarities with (24) Themis and (65) Cybele (for which water ice coating on surface grains was reported), we infer a silicate-to-ice mass ratio of 1-6, with a 10-30% macroporosity.

Section: Stellar Occultationsmentioning

confidence: 99%

Physical, spectral, and dynamical properties of asteroid (107) Camilla and its satellites

Pajuelo

Carry

Vachier

et al. 2018

“…In this case, the occulted star was TYC 4909-00873-1. The observing network was coordinated by the IOTA group (International Occultation Timing Association, http://www.occultations.org/) according to predictions made regularly by Dunham et al (2013). Figure 13 displays the resulting occultation chords for the event derived by D.…”

Section: The Main-belt Asteroid 216 Kleopatramentioning

confidence: 99%

Dumb-bell-shaped equilibrium figures for fiducial contact-binary asteroids and EKBOs

Descamps¹

2015

In this work, we investigate the equilibrium figures of a dumb-bell-shaped sequence with which we are still not well acquainted. Studies have shown that these elongated and nonconvex figures may realistically replace the classic "Roche binary approximation" for modeling putative peanut-shaped or contact binary asteroids. The best-fit dumb-bell shapes, combined with the known rotational period of the objects, provide estimates of the bulk density of these objects. This new class of mathematical figures has been successfully tested on the observed light curves of three noteworthy small bodies: main-belt asteroid 216 Kleopatra, Trojan asteroid 624 Hektor and Edgeworth-Kuiper-belt object 2001 QG 298 . Using the direct observations of Kleopatra and Hektor obtained with high spatial resolutiontechniques and fitting the size of the dumb-bell-shaped solutions, we derived new physical characteristics in terms of equivalent radius, 62.5 ± 5 km and 92 ± 5 km, respectively, and bulk density, 4.4 ± 0.4 gcm -3 and 2.43 ± 0.35 gcm -3 , respectively. In particular, the growing inadequacy of the radar shape model for interpreting any type of observations of Kleopatra (light curves, AO images, stellar occultations) in a satisfactory manner suggests that Kleopatra is more likely to be a dumb-bell-shaped object than a "dog-bone."

“…Four stellar occultations by Sylvia have been reported in the past 30 years, but only a total of four chords (three for an event, one for another, none for the two others) have been collected (Dunham et al 2012). These occultation data are therefore useless to scale Sylvia's shape model (Kaasalainen et al 2002).…”

Section: Stellar Occultation Datamentioning

confidence: 99%

Physical and dynamical properties of the main belt triple Asteroid (87) Sylvia

Berthier

Vachier

Marchis

et al. 2014

We present the analysis of high angular resolution observations of the triple asteroid (87) Sylvia collected with three 8-10 m class telescopes (Keck, VLT, Gemini North) and the Hubble Space Telescope. The moons' mutual orbits were derived individually using a purely Keplerian model. We computed the position of Romulus, the outer moon of the system, at the epoch of a recent stellar occultation which was successfully observed at less than 15 km from our predicted position, within the uncertainty of our model. The occultation data revealed that the moon, with a surface-area equivalent diameter D S = 23.1 ± 0.7 km, is strongly elongated (axes ratio of 2.7 ± 0.3), significantly more than single asteroids of similar size in the main-belt. We concluded that its shape is probably affected by the tides from the primary. A new shape model of the primary was calculated combining adaptive-optics observations with this occultation and 40 archived light-curves recorded since 1978. The difference between the J 2 = 0.024 +0.016 −0.009 derived from the 3-D shape model assuming an homogeneous distribution of mass for the volume equivalent diameter D V = ✩ Based on observations collected at the European Southern Observatory,