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

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