POLARIZATION OF ASTEROIDS - SYNTHETIC CURVES AND CHARACTERISTIC PARAMETERS

A data base of polarization measurements has been constituted for abou t 100 asteroids, mainly observed in the blue and green domains, for wh ich no significant wavelength effect is detected. Individual empirical phase curves have been obtained for about 35 asteroids, using polynom ial and trigonometric fits that lead to similar results within the pha se angle range where data are available. Phase curves are comparable f or asteroids which belong to the same taxonomic type. Significant anal ogies are found between the three ''synthetic'' curves drawn for aster oids of igneous type (S, M and E). The analogies are even greater betw een the three curves obtained for asteroids of C, G and P types that e xhibit a deeper minimum of polarization and a steeper slope at inversi on than the previous ones. The CCD polarization method, previously dev eloped for comets, has been adapted to polarimetric observations of as teroids (up to magnitude 15) from the Pic du Midi Observatory. The com parison between the synthetic curves and the new data typically allows us to estimate that 1992 AC is an igneous object. Polarimetry is foun d to be an additional tool to determine the taxonomic superclass, or p ossibly class, of asteroids that happen to be observed at relatively l arge phase angles.