Abstract
This paper studies the structures of debris discs, focusing on the conditions that can form an axisymmetric-looking outer disc from systems with inner clumps. The main conclusion is that as long as the dominated dust grains are smaller than the blowout size, it is easy to form an axisymmetric-looking outer debris disc, which is part of a quasi-steady state of the whole system. This quasi-steady state is established through the balance between grain generations and a continuous outflow of grains. Assuming there is an event that starts planetesimal collisions and the corresponding grain generations, this balance can be approached in a few thousand years. This result suggests that a quasi-steady-state picture could solve the possible mass budget problem of Vega's outer debris disc. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.