Two-period lightcurve of 1994 AW1: Indication of a binary asteroid?

Abstract

An analysis of the complex lightcurve of the Earth-approaching asteroid 1994 AW1 is presented. We have found that the lightcurve canbe explained as the sum of two components having different periods, P1&#610.4666 +- 0.0005 day and P2&#610.104972 +- 0.000006 day, confirming and refining the finding by S. Mottola et al. (Icarus 117, 62-70, 1995). It is the first case of detection of a double-period lightcurve for a fast rotator. We present a hypothesis of an occulting/eclipsing binary asteroid that can explain the observed lightcurve character. The estimated ratio of the diameters of the secondary to the primary os 0.5:1, the orbital perios P&#612P1, and the eccentricity is small. The rotation of the primary is not synchronized with the orbital motion and has a period of P2. The origin and evolution of the binary system may be related to close encounters with the Earth, as suggested by the theory of W.F. Bottke and H. J. Melosh (Nature 281, 51-53, 1996). An alternative hypothesis for 1994 AW1 as a single body in the excited rotation state requires further study. Since the relaxation time of the excited rotation is much shorter than the lifetime of asteroid in itscurrent orbit, it would require a relatively recent setting of the excited rotation.