A submillimetre continuum survey of pre-protostellar cores

Results are presented of a submillimetre continuum survey of 21 Myers cores that have no known infrared (near-IR or IRAS) associations – the so-called ‘starless cores’. $$^{13}{\rm CO}$$ maps show that 17 of the cores have structure in the form of one or more clumps, with significant departures from spherical symmetry. The clumps were surveyed in the submillimetre continuum, but only 12 were detected. In all cases no more than one clump in each of the Myers cores was detected in the continuum, no matter how many $$^{13}{\rm CO}$$ clumps it contained. Five of the clumps were mapped in the continuum, to demonstrate that they are true emission peaks. The continuum peaks are not always exactly coincident with the $$^{13}{\rm CO}$$ peaks, indicating that the $$^{13}{\rm CO}$$ may be optically thick. For the first time a size difference is found between the starless cores and the cores with IRAS sources: the continuum clumps in the centres of starless cores are all less centrally peaked and more diffuse than the equivalent continuum clumps previously found in Myers cores with IRAS sources. Nevertheless, the starless cores are more centrally condensed than a constant-density sphere. Mass and density estimates show that the continuum peaks are true density peaks, of $$\sim 10^5-10^6\,\rm cm^{-3}$$. Photometry of the clumps shows that they have insufficient bolometric luminosities to be consistent with the earliest phase of accreting protostars predicted by the Standard Protostellar Model. The lifetimes of the clumps derived from statistical considerations are shown to be too long for the cores to be undergoing free-fall collapse, but are consistent with ambipolar diffusion time-scales. All of the clumps are found to have masses close to their virial masses, as expected during the quasi-static ambipolar diffusion phase. The starless cores with submillimetre continuum detections are therefore hypothesized to be pre-protostellar in nature, and sites of future star formation. However, none of the mapped clumps shows the steep, $$\rho(r) \propto r^{-2}$$, power-law radial density profile predicted by the Standard Protostellar Model. All have profiles that flatten out near their centres. This means either that the cores have not yet reached this stage in their evolution, or that cores do not achieve such steep density profiles prior to star formation, due to support by some other mechanism, such as a magnetic field. Previous observations may have failed to observe this flattening, due to their lower angular resolution. The radial profiles of the continuum clumps are, however, consistent with those predicted by a more recent theory of magnetic support of cores during ambipolar diffusion.