β Pictoris light variations: II. Scattering by a dust cloud

We explain the observed photometric variations of the star β Pic of November 1981 (Lecavelier et al. 1995) in terms of scattering and occultation by a dust cloud that is orbiting the star. The calculations were made for different phase functions for scattering and diffraction. We derived the parameters of the cloud that are compatible with the observed light curve: in particular the distance from the star (between 0.45 and 4 AU) and the effective scattering surface (a few times 10²⁴ cm²). However a spherical dust cloud is inconsistent with the observed photometric variations because its large size cannot explain the short duration of the dip in the light curve. A model consisting of a flat cloud, that is elongated in the orbital plane and has a pointed shape with the largest optical depth closest to the star, can explain the light curve. We suggest that such a cloud could be the result of one large comet that passes the line of sight to the star near periastron, or of a fragmented comet of the type as the Shoemeker-Levi fragments. The model suggests that the comet passes the star at a distance of about 0.4 AU and a dust mass of about 2 × 10²¹ g. The crucial test of these models is the possible presence (in the case of an orbiting dust cloud) or absence (in the case of a comet) of periodic recurrences of the short time photometric variations.