  We explore the formation of cosmic string Y-junctions when strings of two
different types collide, which has recently become important since string
theory can yield cosmic strings of distinct types. Using a model containing two
types of local U(1) string and stable composites, we simulate the collision of
two straight strings and investigate whether the dynamics matches that
previously obtained using the Nambu-Goto action, which is not strictly valid
close to the junction. We find that the Nambu-Goto action performs only
moderately well at predicting when the collision results in the formation of a
pair of Y-junctions (with a composite string connecting them). However, we find
that when they do form, the late time dynamics matches those of the Nambu-Goto
approximation very closely. We also see little radiative emission from the
Y-junction system, which suggests that radiative decay due to bridge formation
does not appear to be a means via which a cosmological network of such string
would rapidly lose energy.
