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 .
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 prestellar cores , which are believed to represent the stage of star formation that precedes the formation of a protostar .
29 of the 52 cores were detected at 850Â \mu m at varying levels of signal-to-noise ratio greater than 3 \sigma 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 } \sim 15 under typical assumptions .
The detected cores were split into ‘ bright ’ cores and ‘ intermediate ’ cores , depending on their peak flux density at 850Â \mu m. Those with peak 850- \mu m flux densities greater than 170Â mJy/beam were designated ‘ bright ’ cores .
Those with peak 850- \mu m flux densities less than this value were designated ‘ intermediate ’ cores .
This dividing line corresponds to a mean detection limit of 10 \sigma at peak , and an approximate A _ { V } \sim 50 under typical assumptions .
13 of the 29 detected cores 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 prestellar 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 prestellar core is roughly \sim 3 \times 10 ^ { 5 } years , while that of a bright core is \sim 1.5 \times 10 ^ { 5 } years .
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 prestellar core lifetime , do not predict the relative numbers of cores seen at each stage .