Polarization of asteroids. Synthetic curves and characteristic parameters

A data base of polarization measurements has been constituted for about 100 asteroids, mainly observed in the blue and green domains, for which no significant wavelength effect is detected. Individual empirical phase curves have been obtained for about 35 asteroids, using polynomial and trigonometric fits that lead to similar results within the phase angle range where data are available. Phase curves are comparable for asteroids which belong to the same taxonomic type. Significant analogies are found between the three “synthetic” curves drawn for asteroids of igneous type (S, M and E). The analogies are even greater between the three curves obtained for asteroids of C, G and P types that exhibit a deeper minimum of polarization and a steeper slope at inversion than the previous ones. The CCD polarization method, previously developed for comets, has been adapted to polarimetric observations of asteroids (up to magnitude 15) from the Pic du Midi Observatory. The comparison between the synthetic curves and the new data typically allows us to estimate that 1992 AC is an igneous object. Polarimetry is found to be an additional tool to determine the taxonomic superclass, or possibly class, of asteroids that happen to be observed at relatively large phase angles.