Expansion parallax of the planetary nebula IC 418
  In this paper, we present radio continuum observations of the planetary
nebula IC 418 obtained at two epochs separated by more than 20 years. These
data allow us to show that the angular expansion rate of the ionization front
in IC 418 is 5.8 $\pm$ 1.5 mas yr$^{-1}$. If the expansion velocity of the
ionization front is equal to the expansion velocity of the gas along the line
of sight as measured by optical spectroscopy, then the distance to IC 418 must
be 1.1 $\pm$ 0.3 kpc. Recent theoretical predictions appropriate for the case
of IC 418, however, suggest that the ionization front may be expanding about
20% faster than the material. Under this assumption, the distance to IC 418
would increase to 1.3 $\pm$ 0.4 kpc.
