The non-convex shape of (234) Barbara, the first Barbarian

TL;DR

This study determines the complex, concave 3D shape of asteroid (234) Barbara using lightcurve and occultation data, revealing no satellite presence and exploring links to its unique polarimetric features.

Contribution

It introduces a detailed shape model of (234) Barbara based on extensive observational data, clarifying its morphology and polarimetric behavior without requiring a satellite.

Findings

Shape exhibits large concave areas.

No satellite detected from VLTI data.

Shape linked to polarimetric properties.

Abstract

Asteroid (234) Barbara is the prototype of a category of asteroids that has been shown to be extremely rich in refractory inclusions, the oldest material ever found in the Solar System. It exhibits several peculiar features, most notably its polarimetric behavior. In recent years other objects sharing the same property (collectively known as "Barbarians") have been discovered. Interferometric observations in the mid-infrared with the ESO VLTI suggested that (234) Barbara might have a bi-lobated shape or even a large companion satellite. We use a large set of 57 optical lightcurves acquired between 1979 and 2014, together with the timings of two stellar occultations in 2009, to determine the rotation period, spin-vector coordinates, and 3-D shape of (234) Barbara, using two different shape reconstruction algorithms. By using the lightcurves combined to the results obtained from stellar occultations, we are able to show that the shape of (234) Barbara exhibits large concave areas. Possible links of the shape to the polarimetric properties and the object evolution are discussed. We also show that VLTI data can be modeled without the presence of a satellite.