Black hole solutions in 2+1 dimensions

We give circularly symmetric solutions for null fluid collapse in (2+1)-dimensional Einstein gravity with a cosmological constant. The fluid pressure P and energy density ρ are related by P=kρ (k≤1). The long time limit of the solutions is a black hole whose horizon structure depends on the value of k. The k=1 solution is the Banados-Teitelboim-Zanelli black hole metric in the long time static limit, while the k<1 solutions give other, ‘‘hairy,’’ black hole metrics in this limit. © 1995 The American Physical Society.