3782 Celle: Discovery of a binary system within the Vesta family of asteroids

Photometric observations of the minor planet (3782) Celle, which has been associated both dynamically and spectroscopically with the Vesta asteroid family, were obtained using the 1.8-m Vatican Advanced Technology Telescope during September 2001 and December 2002–January 2003. Analysis of these data reveals a normal rotational lightcurve ( P = 3.84 h , amplitude =0.10–0.15 mag). During the 2002–2003 run, anomalous attenuation events were observed lasting for about 2.6–3.5 h that varied in amplitude from 0.15–0.3 mag. The attenuations were of two distinct types that can clearly be identified as primary and secondary occultation/eclipses similar to those that have been previously observed in known minor planet binary systems (Pravec et al., 2000). We therefore interpret our data as clear evidence that (3782) Celle is actually an asynchronous binary system with an orbital period of 36.57 ± 0.03 h (Ryan et al., 2003). A preliminary model, based on spherical components, yields a primary-to-secondary diameter ratio of 0.43 ± 0.01 and a combined bulk density of 2.2 ± 0.4 gm / cm 3 for the two components. Because these objects are likely to be composed of basaltic fragments, this density is indicative of a moderate to a highly fractured internal structure for at least one, if not both, of the binary components. Since the Vesta family is believed to have been created via a cratering event, this finding has important implications for understanding possible ejecta re-accumulation and satellite formation in subcatastrophic collisions.