  Through Spitzer Space Telescope's observations, Su et al. (2005) show that
the Vega debris disc is dominated by grains which are small enough to be blown
out by radiation pressure. This implies the lifetime of Vega debris disc's
grains is relatively short, about 1000 years, and a continuous dust production
is necessary to maintain the observed debris disc. However, Krivov et al.
(2006)'s theoretical calculations show that the Vega debris disc is dominated
by 10 micro-meter grains, which would be in bound orbits and thus long-lived,
provided that the disc is in a steady state. In order to solve the above
contradiction, through dynamical simulations, we determine the grains' orbital
evolutions and density profiles and seek a model of size distribution which can
reproduce the observed surface brightness. Our results show that a
self-consistent dynamical model with a 1/R disc density profile can be
constructed when the grains have a power-law size distribution. Moreover, both
types of models, dominated by short-lived and long-lived grains, are consistent
with the observational data.
