Polarization of Asteroid (387) Aquitania: The newest member of a class of large inversion angle asteroids

We present new imaging polarimetric observations of two Main Belt asteroids, (234) Barbara and (387) Aquitania, taken in the first half of 2008 using the Dual-Beam Imaging Polarimeter on the University of Hawaii 2.2 m telescope, located on Mauna Kea, Hawaii. Barbara had been previously shown to exhibit a very unusual polarization-phase curve by Cellino, A., Belskaya, I.N., Bendjoya, Ph., di Martino, M., Gil Hutton, R., Muinonen, K., Tedesco, E.F., 2006. Icarus 180, 565-567]. Our observations confirm this result and add Aquitania to the growing class of large inversion angle objects. Interestingly, these asteroids show spinel features in their IR spectra suggesting a mineralogical origin to the phase angle-dependent polarimetric features. As spinel is associated with calcium-aluminum-rich inclusions and carbonaceous chondrites, these large inversion angle asteroids may represent some of the oldest surfaces in the Solar System. Circular as well as linear polarization measurements were obtained but circular polarization was not detected.