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

We present the results of a mid-infrared (λ ' 7 μm) imaging survey of a sample of 24 starless dense cores carried out at an angular resolution of 6′′ 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 NH2 ∝ r̄−1 throughout their full extent but flattened out near their centre. 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 twice as 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 centre, which confirms the submillimeter emission results. Moreover, beyond a radius of ∼ 5000−10000AU, the typical column density profile steepens with distance from core centre and gets steeper than NH2 ∝ r̄−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.