2 Crimean Astrophysical Observatory, Nauchny, 334413 Crimea, Ukraine

Results of UBVRI polarimetric observations of 16 asteroids obtained in 1995-1998 are presented. The main purposes were to investigate polarization-phase dependence at low phase angles and to estimate wavelength dependence of polarization for different asteroid types.

Polarization - phase dependences down to phase angles less than 1 deg were obtained for asteroids 20 Massalia (S-type) and 64 Angelina (E-type). Any indication on possible existence of so called polarization opposition effect [1,2] was not found within the accuracy of observations (about 0.06% in the B band). Measured negative polarization branches of the asteroids show an asymmetrical shape. The phase angle at which the polarization minimum occurs completely coincide with the angle of opposition effect beginning in magnitude-phase dependences of asteroids 20 Massalia and 64 Angelina. Thorough analysis of available data on other asteroids for which both negative polarization and opposition effect were measured confirmed the conclusion. Relative contribution of coherent backscattering mechanism to both phenomena is discussed.

Wavelength dependences of negative polarization in the region of 0.37-0.82 microns were measured for 15 asteroids of 7 different types. Within a particular type asteroids display similar wavelength behavior of negative polarization. Results of both new and available measurements of UBVRI polarization of asteroids are analyzed.

REFERENCES: [1] Mishchenko (1993). Astrophys. J. 411, p.351. [2] Kiselev N.N., Shakhovskoy N.M., Efimov Yu.S. (1996). Icarus 120, p.408.