The initial conditions of isolated star formation - VI. SCUBA mappingof pre-stellar cores

Observations have been carried out with the Submillimetre Common-User Bolometer Array (SCUBA) at the James Clerk Maxwell Telescope (JCMT) of regions of comparatively isolated star formation in molecular cloud cores. Some 52 starless cores were observed, which are molecular cloud cores that do not contain any sign of protostellar activity such as infrared sources or bipolar outflows. These are all therefore candidate pre-stellar cores, which are believed to represent the stage of star formation that precedes the formation of a protostar. Of the 52 cores, 29 were detected at 850 μm at varying levels of signal-to-noise ratio greater than 3σ at peak, while 23 of the cores were observed but not detected. The mean detection lower limit of the data corresponds roughly to an A_V~ 15 under typical assumptions. The detected cores were split into `bright' cores and `intermediate' cores, depending on their peak flux density at 850 μm. Those with peak 850-μm flux densities greater than 170 mJy beam^-1 were designated `bright' cores. Those with peak 850-μm flux densities less than this value were designated `intermediate' cores. This dividing line corresponds to a mean detection limit of 10σ at peak, and an approximate A_V~ 50 under typical assumptions. Of the 29 detected cores, 13 are found to be bright and 16 are intermediate. The data are combined with our previously published ISO data, and the physical parameters of the cores, such as density and temperature, are calculated. The bright cores are detected with sufficiently high signal-to-noise ratio to allow their structure to be mapped. Radial flux density profiles of these show flattened inner regions and sharp boundaries, consistent with previous observations of pre-stellar cores. Detailed fitting of the bright core radial profiles shows that they are not critical Bonnor-Ebert spheres, in agreement with previous findings. However, we find that intermediate cores, such as B68 (which has previously been claimed to be a Bonnor-Ebert sphere), may in fact be consistent with the Bonnor-Ebert criterion, suggesting perhaps that cores pass through such a phase during their evolution. We also find that the masses of the bright cores have a mean value of approximately the same order as their virial masses. We make rough estimates of core lifetimes based on the statistics of detections and find that the lifetime of a pre-stellar core is roughly ~3 × 10⁵ yr, while that of a bright core is ~1.5 × 10⁵ yr. Comparisons with some models that regulate collapse using either magnetic fields or turbulence show that no model can match all of the data. Models that are tuned to fit the total pre-stellar core lifetime do not predict the relative numbers of cores seen at each stage.