Critical collapse in K-essence models
  We study gravitational collapse in K-essence model with shift symmetry. For
these models, we have the formation of two types of horizons, event and sonic.
For the particular case $K(X)=X+\beta X^2$ we found three different regimes. In
the weak field regime the scalar field disperses to infinity, in the very
strong regime both horizons form at the same time and finally for the
intermediate regime, the sonic horizon could form first or both horizons form
at the same time. The threshold of formation of black hole is found in the
regime where the sonic horizon forms first. We observe a universal behavior
with scaling parameter $\gamma\simeq 0.51$. Interestingly this universal
behavior is already encoded in the sonic horizon much before the black hole
forms and therefore the emergence of the event horizon.
