The initial conditions of isolated star formation - III. Millimetre continuum mapping of pre-stellar cores

We present the results of 1.3-mm continuum mapping observations of eight pre-stellar cores, taken with the IRAM 30-m telescope equipped with the 19-channel MPIfR bolometer array. The new 1.3-mm data, which were obtained in the `on-the-fly' mapping mode, have higher angular resolution and sensitivity than previous surveys, reaching an rms noise level of ~ 3-9 mJy per 13-arcsec beam. The present study supports the conclusions of our previous James Clerk Maxwell Telescope (JCMT) survey and suggests that, in contrast with some theoretical predictions, most pre-stellar cores have flat inner density gradients that only approach rho (r) ~ r^-2 beyond a few thousand au. Several of the cores have a filamentary-like morphology and are apparently fragmented in a small number (~ 2-4) of subclumps. This implies that the initial conditions for protostellar collapse depart significantly from a singular isothermal sphere. We also note quantitative disagreement in time-scales with published ambipolar diffusion models for the quasi-static evolution of molecular cloud cores under the influence of a uniform magnetic field. We speculate that turbulent processes might help to explain our observations.