Shear-free relativistic fluids and the absence of movable branch points

Abstract

The problem of determining the metric for a nonstatic shear-free spherically symmetric fluid (either charged or neutral) reduces to the problem of determining a one-parameter family of solutions to a second-order ordinary differential equation (ODE) containing two arbitrary functions f and g. Choices for f and g are determined such that this ODE admits a one-parameter family of solutions that have poles as their only movable singularities. This property is strictly weaker than the Painlevé property and it is used to identify classes of solvable models. It is shown that this procedure systematically generates many exact solutions including the Vaidya metric, which does not arise from the standard Painlevé analysis of the second-order ODE. Interior solutions are matched to exterior Reissner–Nordstrøm metrics. Some solutions given in terms of second Painlevé transcendents are described.