  We study the topological defects in the classical Heisenberg antiferromagnet
in two dimensions on a triangular lattice (HAFT). While the topological
analysis of the order parameter space indicates that the defects are of $Z_2$
type, consideration of the energy leads us to a description of the low--energy
stationary points of the action in terms of $\pm$ vortices, as in the planar XY
model. Starting with the continuum description of the HAFT, we show
analytically that its partition function can be reduced to that of a
2--dimensional Coulomb gas with logarithmic interaction. Thus, at low
temperatures, the correlation length is determined by the spinwaves, while at
higher temperatures we expect a crossover to a Kosterlitz--Thouless type
behaviour. The results of recent Monte Carlo calculations of the correlation
length are consistent with such a crossover.
