A. Bacmann 1, Ph. André 1, A. Abergel 2, J.-Ph. Bernard 2, J.-L. Puget 2, S. Bontemps 3, & D. Ward-Thompson 4
1 DSM/DAPNIA/SAp, CEA-Saclay, 91191 Gif sur Yvette Cedex FRANCE
2 IAS Orsay, France
3 Stockholm Observatory, Sweden
4 University of Wales, Cardiff, U.K.
An ISOCAM survey has been carried out in order to detect and map a
sample of 23 starless dense cores (presumably precollapse) in
absorption against the diffuse mid-infrared background. The goal is to
gain insight into the structure of prestellar dense cores. In previous
submillimeter continuum studies of such starless cores, it was found
that the derived column density profiles did not follow a single
power-law in
throughout their full extent but flattened out
near their centre (Ward-Thompson et al. 1994, André et al. 1996). These
observations however could not constrain the density profiles farther
than 10000 AU.
The present absorption study uses ISOCAM's sensitivity and angular
resolution to determine the structure of the cores at angular radii
that extend beyond the limits of sensitivity of the submillimeter
continuum maps. Among the 23 cores observed in our survey, half of them
show absorption features. The starless cores studied here all show an
H2 column density profile that flattens in the centre, which confirms
the above-mentioned submillimeter emission results. More interestingly,
beyond a radius of 4000-5000 AU, the typical column density profile
steepens with the distance from the core centre and gets steeper than
until eventually the cores merge with the low-density ambient
molecular cloud. Thus the cores present a sharp edge at R
15000-30000
AU and appear to be decoupled from the parent cloud, providing a finite
reservoir of mass for subsequent star formation. This could suggest
that the cores, initially magnetically supported, may have become
supercritical (eg. Ciolek & Mouschovias 1994) or that they are confined
by external pressure (see Bertoldi & McKee 1992).