Size distributions of dust in circumstellar debris discs

Details Size distributions of dust in circumstellar debris discs Size distributions of dust in circumstellar debris discs

Oct 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000a%26a...362.1127k&link_type=abstract

Astronomy and Astrophysics, v.362, p.1127-1137 (2000)

Astronomy and Astrophysics

Astrophysics

46

Stars: Circumstellar Matter, Stars: Individual: Begin {Equation}Ta Pic, Stars: Planetary Systems

Scientific paper

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 begin {equation}ta 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 (alpha -meteoroids) and smaller ones blown away by radiation pressure (begin {equation}ta-meteoroids). Although begin {equation}ta-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 alpha -meteoroids not only by mutual collisions, but also by impacts of begin {equation}ta-meteoroids becomes significant. This flattens the distribution of alpha -meteoroids in the size regime adjacent to the blow-out limit and shifts the cross section-dominating sizes from a few micrometres to ga 10 mum . The overall distribution shows essentially three different slopes: steeper ones for both begin {equation}ta-meteoroids and large alpha -meteoroids and a gentler one for alpha -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 ~ 10 mum in size) well agree with the observational data available. Although particular calculations were made for the begin {equation}ta 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.

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