Physical processes

We distinguish between the two possibilities indicated in the title by analysing the physical process operating in the β Pic system. Based on recent models of the disc (Artymowicz et al. 1989) and the information on gaseous constituents of the disc (Vidal-Madjar et al. 1986, Lagrange-Henri et al. 1988) we consider the following processes, which we expect to determine the size distribution of grains and influence the disc appearance:

1. 1 Inter-particle collisions. In the densest parts of the disc (∼ 20 to 50 AU from the star) grains collide typically once in several hundred orbits (∼ 103 yr). At 100 AU, the time-scale is 105 yr and at 1000 AU of order 108 yr. The outcome of a typical collision, which from our knowledge of the disc geometry occurs at impact speeds ∼ 0.1 times the local Keplerian velocity, is the erosional cratering of larger particles and the destructive shattering of smaller ones. No agglomeration through grain sticking is possible.

2. 2 Poynting-Robertson (P-R) effect. In most previous work, the P-R drag was suggested to play a dominant role. This is not correct. The P-R time-scale for even the smallest (∼ 2 µm-sized) particles is too long, ∼ 4 × 106 yr at 100 AU and increasing with the square of the radius. Whenever collisions act on shorter time-scales, the P-R drag effectively acts on the total mass of the disc, not just the smallest grains, hence the time-scales given are merely lower limits.

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