  We present a new analytical three-parameter formula to fit observed column
density profiles of prestellar cores. It represents a line-of-sight integral
through a spherically symmetric or disc-like isothermal cloud. The underlying
model resembles the Bonnor-Ebert model in that it features a flat central
region leading into a power-law decline \propto r^{-2} in density, and a
well-defined outer radius. However, we do not assume that the cloud is in
equilibrium, and can instead make qualitative statements about its dynamical
state (expansion, equilibrium, collapse) using the size of the flat region as a
proxy. Instead of having temperature as a fitting parameter, our model includes
it as input, and thus avoids possible inconsistencies. It is significantly
easier to fit to observational data than the Bonnor-Ebert sphere. We apply this
model to L1689B and B68. We show that L1689B cannot be in equilibrium but
instead appears to be collapsing, while our model verifies that B68 is not far
from being a hydrostatic object.
