Abstract
Context. The so-called Barbarian asteroids share peculiar, but common polarimetric properties, probably related to both their shape and composition. They are named after (234) Barbara, the first on which such properties were identified. As has been suggested, large scale topographic features could play a role in the polarimetric response, if the shapes of Barbarians are particularly irregular and present a variety of scattering/incidence angles. This idea is supported by the shape of (234) Barbara, that appears to be deeply excavated by wide concave areas revealed by photometry and stellar occultations. Aims. With these motivations, we started an observation campaign to characterise the shape and rotation properties of Small Main- Belt Asteroid Spectroscopic Survey (SMASS) type L and Ld asteroids. As many of them show long rotation periods, we activated a worldwide network of observers to obtain a dense temporal coverage. Methods. We used light-curve inversion technique in order to determine the sidereal rotation periods of 15 asteroids and the con- vergence to a stable shape and pole coordinates for 8 of them. By using available data from occultations, we are able to scale some shapes to an absolute size. We also study the rotation periods of our sample looking for confirmation of the suspected abundance of asteroids with long rotation periods. Results. Our results show that the shape models of our sample do not seem to have peculiar properties with respect to asteroids with similar size, while an excess of slow rotators is most probably confirmed.
Highlights
Shape modeling is of primary importance in the study of asteroid properties
Our goal is to evaluate the reliability of shape model details, and to check if a set of light-curves provides a good determination of the spin parameters
We have presented new observations here for 15 Barbarian or candidate Barbarian asteroids
Summary
In the last decades, ∼1000 asteroids have had a shape model determined by using inversion techniques; most of them are represented by convex shape models. This is due to the fact that the most used technique when inverting the observed light-curves of an asteroid – the so-called light-curve inversion – is proven to mathematically converge to a unique solution only if the convex hypothesis is enforced (Kaasalainen & Torppa 2001; Kaasalainen et al 2001). The classical light-curve inversion process cannot model concavities, Kaasalainen et al (2001) point out that the convex shape model obtained by the inversion is very close to the convex hull of the asteroid shape. Since the convex hull corresponds to the minimal envelope that contains the non-convex shape, the location of concavities corresponds to flat areas. Devogèle et al (2015) developed the flat surfaces
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