A SUBMILLIMETER 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', (CO)-C-13 maps show that 17 of the cores have structure in the form of one or more clumps, with significant dep artures from spherical symmetry. The clumps were surveyed in the submi llimetre continuum, but only 12 were detected. In all cases no more th an one clump in each of the Myers cores was detected in the continuum, no matter how many (CO)-C-13 clumps it contained. Five of the clumps were mapped in the continuum, to demonstrate that they are true emissi on peaks. The continuum peaks are not always exactly coincident with t he (CO)-C-13 peaks, indicating that the (CO)-C-13 may be optically thi ck. For the first time a size difference is found between the starless cores and the cores with IRAS sources: the continuum clumps in the ce ntres 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 approximately 10(5)-10(6) CM-3. Photometry of the clumps shows that they have insuf ficient bolometric luminosities to be consistent with the earliest pha se of accreting protostars predicted by the Standard Protostellar Mode l. The lifetimes of the clumps derived from statistical considerations are shown to be too long for the cores to be undergoing free-fall col lapse, 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 starle ss cores with submillimetre continuum detections are therefore hypothe sized to be pre-protostellar in nature, and sites of future star forma tion. However, none of the mapped clumps shows the steep, rho(r) is-pr oportional-to r-2, power-law radial density profile predicted by the S tandard Protostellar Model. All have profiles that flatten out near th eir centres. This means either that the cores have not yet reached thi s stage in their evolution, or that cores do not achieve such steep de nsity profiles prior to star formation, due to support by some other m echanism, such as a magnetic field. Previous observations may have fai led 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 co res during ambipolar diffusion.