Photocentre displacement of minor planets: analysis of HIPPARCOS astrometry

Abstract

We present a study of the phase effect (or photocentre offset) on the astrometry of minor planets. Analysis of the high precision Hipparcos observations of the largest asteroids shows the validity of the model. Minor planets Ceres, Pallas, Vesta and Iris show a brightness distribution close to the lunar scattering properties, while Juno and Hebe are found to be more uniformly bright objects. For general purposes, the observed astrometric positions of asteroids could be corrected for the systematic photocentre offset by assuming in a first approximation a spherical body with a brightness distribution following the law of Buratti & Veverka (1983). Based on data from the Hipparcos astrometry satellite\hfill\break Present address: Osservatorio Astronomico di Torino, Str. Osservatorio 20, I-10025 Pino Torinese, Italy (e-mail: hestro@to.astro.it)