Tereschenko I. Brightness opposition effect of lowalbedo asteroids.

The work is devoted to investigation of phase dependences of brightness of lowalbedo asteroids. The observations were carried out for 135 nights and composite lightcurves, phase dependences of brightness (including small phase angles < 1 deg), absolute magnitudes, color indexes and rotation periods were obtained for 14 asteroids. As a result of the work it was revealed that the magnitude-phase dependence of asteroid 190 Ismene does not show nonlinear increase of brightness in range of opposition effect (OE) below to 0.3 deg. The magnitude-phase relations in two standard V and R bands were obtained for asteroid 2867 Steins that is a target of European space mission Rosetta. The brightness behavior with phase angle of this asteroid indicates that this object belongs to rare E-type. An analysis of magnitude-phase relations of 33 lowalbedo asteroids allowed us to discriminate the three groups of asteroids using their brightness behavior in the range of OE. About 20% asteroids in the considered sample show very small (less than 0.04 mag) values of the OE amplitude. It seems that the shadow hiding mechanism is the only one responsible for the OE of these objects. The correlation of OE amplitude and asteroid albedo was found. Amplitude of the OE tends to increase with albedo which is opposite to trend expected for shadow hiding mechanism. It gives evidence that another mechanism is responsible for nonlinear increase of magnitude for dark asteroids. It is shown that the OE amplitudes of lowalbedo asteroids within the accuracy of measurement do not depend on wavelengths in the spectral range 0.36-0.8 mkm.