Through Spitzer Space Telescopes 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 discs 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.
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