  We present the results of a mid-infrared (7 micron) imaging survey of a
sample of 24 starless dense cores carried out at an angular resolution of 6
arcsec with the ISOCAM camera aboard the Infrared Space Observatory (ISO). The
targeted cores are believed to be pre-stellar in nature and to represent the
initial conditions of low-mass, isolated star formation. In previous
submillimeter dust continuum studies of such pre-stellar cores, it was found
that the derived column density profiles did not follow a single power-law such
as N[H2] \propto r^(-1) throughout their full extent but flattened out near
their center. These submillimeter observations however could not constrain the
density profiles at radii greater than ~ 10000 AU. The present absorption study
uses ISOCAM's sensitivity to map these pre-stellar cores in absorption against
the diffuse mid-infrared background. The goal was to determine their structure
at radii that extend beyond the limits of sensitivity of the submillimeter
continuum maps and at twiceas good an angular resolution. Among the 24 cores
observed in our survey, a majority of them show deep absorption features. The
starless cores studied here all show a column density profile that flattens in
the center, which confirms the submillimeter emission results. Moreover, beyond
a radius of ~ 5000-10000 AU, the typical column density profile steepens with
distance from core center and gets steeper than N[H2] \propto r^(-1), until it
eventually merges with the low-density ambient molecular cloud. At least three
of the cores present sharp edges at R ~ 15000-30000 AU and appear to be
decoupled from their parent clouds, providing finite reservoirs of mass for
subsequent star formation.
