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Astron. Astrophys. 362, 1127-1137 (2000)


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Size distributions of dust in circumstellar debris discs

A.V. Krivov 1,2, I. Mann 1 and N.A. Krivova 1,2

1 Max-Planck-Institut für Aeronomie, 37191 Katlenburg-Lindau, Germany (krivov,mann,natalie@linmpi.mpg.de)
2 Astronomical Institute, St. Petersburg University, 198504 St. Petersburg, Russia

Received 29 May 2000 / Accepted 31 August 2000

Abstract

The size distribution of particles in a dust disc is determined by, and holds the key to, sources, sinks and dynamics of grains. Here we derive the size distribution in circumstellar debris discs, exemplified by the disc of [FORMULA] Pictoris, by modelling the dynamical evolution of the circumstellar dust, dominated by collisions. The whole disc is considered as consisting of two dust populations: larger grains moving in bound orbits ([FORMULA]-meteoroids) and smaller ones blown away by radiation pressure ([FORMULA]-meteoroids). Although [FORMULA]-meteoroids leave the disc in hyperbolic trajectories, they are continuously replenished by collisions, so that at any time the disc contains a substantial population of small particles. As a consequence, the fragmentation of [FORMULA]-meteoroids not only by mutual collisions, but also by impacts of [FORMULA]-meteoroids becomes significant. This flattens the distribution of [FORMULA]-meteoroids in the size regime adjacent to the blow-out limit and shifts the cross section-dominating sizes from a few micrometres to [FORMULA]. The overall distribution shows essentially three different slopes: steeper ones for both [FORMULA]-meteoroids and large [FORMULA]-meteoroids and a gentler one for [FORMULA]-meteoroids with sizes just above the blow-out limit. This resembles the size distribution of interplanetary dust particles in the Solar system which, however, is shaped by different mechanisms. The basic features of the modelled size distribution (the presence of a substantial population of small hyperbolic particles in the disc, the dominance of grains [FORMULA] in size) well agree with the observational data available. Although particular calculations were made for the [FORMULA] Pic disc, our basic qualitative conclusions directly apply to the debris discs around other Vega-type stars with low gas contents and similar or somewhat lower optical depths.

Key words: stars: circumstellar matter – stars: individual: fi Pic – stars: planetary systems

Present address: Nonlinear Dynamics Group, Institute of Physics, University of Potsdam, Postfach 601553, 14415 Potsdam, Germany (krivov@agnld.uni-potsdam.de)

Send offprint requests to: A.V. Krivov

© European Southern Observatory (ESO) 2000

Online publication: October 30, 2000

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