A. Cellino, S. Bagnulo, P. Tanga, B. Novaković and M. Delbò

A successful search for hidden Barbarians in the Watsonia asteroid family

Figure 1. Phase - polarization data (in R light) for the targets of the present investigation, compared with the polarization curves (in V light) of the (234) Barbara, and of (12) Victoria, a large L-class asteroid which does not exhibit the Barbarian behaviour. Black symbols: the seven targets exhibiting the Barbarian plarimetric behaviour; red symbols: our two targets that display a "normal" polarimetric behaviour; small blue symbols and blue curve: available data for (234) Barbara (Masiero & Cellino 2009), and the corresponding best-fit curve using the linear-exponential relation P_r = A[e^α/B òÈÒ 1] + C · α, where ¾‘ is the phase angle in degrees; dashed, green curve: the best-fit curve for the L-class asteroid (12) Victoria (for the sake of clearness, individual observations of this asteroid are not shown).

Abstract

Barbarians, so named after the prototype of this class (234) Barbara, are a rare class of asteroids exhibiting anomalous polarimetric properties. Their very distinctive feature is that they show negative polarization at relatively large phase-angles, where all "normal" asteroids show positive polarization. The origin of the Barbarian phenomenon is unclear, but it seems to be correlated with the presence of anomalous abundances of spinel, a mineral usually associated with the so-called Calcium Aluminum-rich inclusions (CAIs) on meteorites. Since CAIs are samples of the oldest solid matter identified in our solar system, Barbarians are very interesting targets for investigations. Inspired by the fact that some of the few known Barbarians are members of, or very close to the dynamical family of Watsonia, we have checked whether this family is a major repository of Barbarians, in order to obtain some hints about their possible collisional origin. We have measured the linear polarization of a sample of nine asteroids which are members of the Watsonia family within the phase-angle range 17° òÈÒ 21°. We found that seven of them exhibit the peculiar Barbarian polarization signature, and we conclude that the Watsonia family is a repository of Barbarian asteroids. The new Barbarians identified in our analysis will be important to confirm the possible link between the Barbarian phenomenon and the presence of spinel on the surface.

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Last Revised: 2013 December 4th