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.