  Two-dimensional melting transitions for model colloids in presence of a
one-dimensional external periodic potential are investigated using Monte Carlo
simulation and Finite Size Scaling techniques. Here we explore a hard disk
system with commensurability ratio $p=\sqrt{3}a_s/(2d)=2$, where $a_s$ is the
mean distance between the disks and $d$ the period of the external potential.
Three phases, the modulate liquid, the locked smectic and the locked floating
solid are observed, in agreement with other experimental and analytical
studies. Various statistical quantities like order parameters, their cumulants
and response functions, are used to obtain a phase diagram for the transitions
between these three phases.
