Physical studies of Apollo-Amor asteroids: UBVRI photometry of 1036 Ganymed and 1627 Ivar

Abstract

Photoelectric lightcurves of the Amor asteroids 1036 Ganymed and 1627 Ivar were obtained during June–December 1985. For Ganymed a drastic change in the shape of the lightcurve was found together with a significant increase of the synodic rotation period. This indicates a large change in the viewing conditions during the apparition and a complex interrelationship between these changes and the probably very irregular shape of the asteroid. The mean synodic peroid is 10.3 hr and the sense of rotation is retrograde. For Ivar, a decrease of the synodic period (4.8 hr) is found, yielding a prograde sense of rotation. The phase curves for both asteroids show strong deviations from the H- G magnitude system phase function at large phase angles. Because of this fact and the lack of observations at phase angles <17° the determined absolute magnitude H(0°) and the slope parameter G are very uncertain. For Ganymed H = 9.50 and G = 0.33, and for Ivar H = 13.24 and G = 0.65. Pole solutions were tried for both asteroids, yielding unreliable results for Ganymed, while for Ivar two independent solutions are in good mutual agreement. We emphasize the opportunity to obtain additional observations to clarify the complex nature of both asteroids during the 1989 (Ganymed) and the 1990 (Ivar) apparitions. The UBVRI color indices found are typical for S asteroids.