D-Brane Dynamics Near NS5-Branes

We use the Dirac-Born-Infeld action to study the real time dynamics of a $Dp$-brane propagating in the vicinity of $NS5$-branes. This problem is closely related to tachyon condensation on an unstable D-brane, with the role of the tachyon played by the radial mode on the D-brane. As the D-brane approaches the fivebranes, its equation of state approaches that of a pressureless fluid. The pressure goes to zero at late times like $\exp(-\alpha t)$; $\alpha$ is a function of the number of fivebranes and of the angular momentum of the D-brane. For unstable D-branes a similar equation of state is taken to signal the decay of the D-brane into closed string radiation. We propose that in our case the D-brane decays into modes propagating in the fivebrane throat, and show that this is consistent with spacetime expectations. We also argue that for radial motions of the D-brane deep inside the throat, the rolling process is described by an exactly solvable worldsheet conformal field theory.