An ISOCAM absorption survey of the structure of pre-stellar cloud cores

We present the results of a mid-infrared (lambda similar or equal to 7 mu m ) imaging survey of a sample of 24 starless dense cores carried out at an a ngular resolution of 6" with the ISOCAM camera aboard the Infrared Space Ob servatory (ISO). The targeted cores are believed to be pre-stellar in natur e and to represent the initial conditions of low-mass, isolated star format ion. In previous submillimeter dust continuum studies of such pre-stellar c ores, it was found that the derived column density profiles did not follow a single power-law such as N-H2 proportional to (r) over bar(-1) throughout their full extent but flattened out near their centre. These submillimeter observations however could not constrain the density profiles at radii gre ater than similar to 10000 AU. The present absorption study uses ISOCAM's s ensitivity to map these pre-stellar cores in absorption against the diffuse mid-infrared background. The goal was to determine their structure at radi i that extend beyond the limits of sensitivity of the submillimeter continu um maps and at twice as good an angular resolution. Among the 24 cores obse rved in our survey, a majority of them show deep absorption features. The s tarless cores studied here all show a column density profile that flattens in the centre, which confirms the submillimeter emission results. Moreover, beyond a radius of similar to 5000-10000 AU, the typical column density pr ofile steepens with distance from core centre and gets steeper than N-H2 pr oportional to (r) over bar(-1), until it eventually merges with the low-den sity ambient molecular cloud. At least three of the cores present sharp edg es at R similar to 15000 - 30000 AU and appear to be decoupled from their p arent clouds, providing finite reservoirs of mass for subsequent star forma tion.