Shape, size and multiplicity of main-belt asteroidsI. Keck Adaptive Optics survey

Abstract

This paper presents results from a high spatial resolution survey of 33 main-belt asteroids with diameters >40 km using the Keck II Adaptive Optics (AO) facility. Five of these (45 Eugenia, 87 Sylvia, 107 Camilla, 121 Hermione, 130 Elektra) were confirmed to have satellite. Assuming the same albedo as the primary, these moonlets are relatively small (∼5% of the primary size) suggesting that they are fragments captured after a disruptive collision of a parent body or captured ejecta due to an impact. For each asteroid, we have estimated the minimum size of a moonlet that can positively detected within the Hill sphere of the system by estimating and modeling a 2-σ detection profile: in average on the data set, a moonlet located at 2/100 × R Hill (1/4 × R Hill ) with a diameter larger than 6 km (4 km) would have been unambiguously seen. The apparent size and shape of each asteroid was estimated after deconvolution using a new algorithm called AIDA. The mean diameter for the majority of asteroids is in good agreement with IRAS radiometric measurements, though for asteroids with a D < 200 km, it is underestimated on average by 6-8%. Most asteroids had a size ratio that was very close to those determined by lightcurve measurements. One observation of 104 Klymene suggests it has a bifurcated shape. The bi-lobed shape of 121 Hermione described in Marchis et al. [Marchis, F., Hestroffer, D., Descamps, P., Berthier, J., Laver, C., de Pater, I., 2005c. Icarus 178, 450-464] was confirmed after deconvolution. The ratio of contact binaries in our survey, which is limited to asteroids larger than 40 km, is surprisingly high (∼6%), suggesting that a non-single configuration is common in the main-belt. Several asteroids have been analyzed with lightcurve inversions. We compared lightcurve inversion models for plane-of-sky predictions with the observed images (9 Metis, 52 Europa, 87 Sylvia, 130 Elektra, 192 Nausikaa, and 423 Diotima, 511 Davida). The AO images allowed us to determine a unique photometric mirror pole solution, which is normally ambiguous for asteroids moving close to the plane of the ecliptic (e.g., 192 Nausikaa and 52 Europa). The photometric inversion models agree well with the AO images, thus confirming the validity of both the lightcurve inversion method and the AO image reduction technique.

Photometry and models of selected main belt asteroids

“…27 we compare both our models with the adaptive optics image from Marchis et al (2006). Unfortunately, because of the rather regular look of (94) Aurora in this viewing geometry, none of the solutions can be excluded on this basis.…”

“…These were the only observations of this object from the southern Earth hemisphere. Schevchenko et al (2006) and Marchis et al (2006) refined the albedo and diameter values to 169 km and 0.0446; and to 168.88 km respectively.…”

“…We also compared this model to the image of (679) Pax from AO observations made by Marchis et al (2006), Fig. 28.…”

Photometry and models of selected main belt asteroids

“…A comparison of the inversion models with the adaptive optics technique was succesfully made in, e.g., Marchis et al (2006). In most cases, this allowed the rejection of the mirror pole solution, and the shape outline was confirmed.…”

Physical models of asteroids from sparse photometric data

“…A set of lightcurves from several apparitions with different observing geometries contains so much information about the spin state and the shape of an asteroid, that those physical parameters can be unambiguously derived using lightcurve inversion. The lightcurve inversion method developed by and has led to about a hundred asteroid models (Kaasalainen et al 2002bTorppa et al 2003) and its reliability has been proven by ground truths from, e.g., Kaasalainen et al ( , 2005 and Marchis et al (2006). However, it is not necessary to have lightcurves that densely cover the rotational phase for deriving a plausible physical model of an asteroid.…”