  We study numerically the Schwinger-Dyson equations for the coupled system of
gluon and ghost propagators in the Landau gauge and in the case of pure gauge
QCD. We show that a dynamical mass for the gluon propagator arises as a
solution while the ghost propagator develops an enhanced behavior in the
infrared regime of QCD. Simple analytical expressions are proposed for the
propagators, and the mass dependency on the $\Lambda_{QCD}$ scale and its
perturbative scaling are studied. We discuss the implications of our results
for the infrared behavior of the coupling constant, which, according to fits
for the propagators infrared behavior, seems to indicate that $\alpha_s (q^2)
\to 0$ as $q^2 \to 0$.
