Negative Pressure and Naked Singularities in Spherical Gravitational   Collapse
  Assuming the weak energy condition, we study the nature of the non-central shell-focussing singularity which can form in the gravitational collapse of a spherical compact object in classical general relativity. We show that if the radial pressure is positive, the singularity is covered by a horizon. For negative radial pressures, the singularity will be covered if the ratio of pressure to the density is greater than -1/3 and naked if this ratio is $\leq -1/3$. 
F. I. Cooperstock, S. Jhingan, P. S. Joshi and T. P. Singh