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Astron. Astrophys. 328, 321-330


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$\beta$ Pictoris light variations

II. Scattering by a dust cloud

H.J.G.L.M. Lamers1,2, A. Lecavelier des Etangs3,4, and A. Vidal-Madjar3

1Astronomical Institute, University of Utrecht, Princetonplein 5, 3584 CC, Utrecht, The Netherlands
2SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands
3Institut d'Astrophysique de Paris, CNRS, 98 bis boulevard Arago, F-75014 Paris, France
4NCRA, Tata Institute of Fundamental Research, Post Bag 3, Ganeshkhind, Pune University Campus, Pune 411 007, India

Received 9 December 1996 / Accepted 30 April 1997

Abstract

We explain the observed photometric variations of the star $\beta$ 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 $2 \times 10^{21}$ 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.

Key words: stars: $\beta$ Pic - circumstellar matter - dust - scattering - comets: general


© European Southern Observatory (ESO) 1997

Online publication: October 30, 1997
Last change: March 24, 1998
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