Search for massive protostar candidates in the southern hemisphere. II. Dust continuum emission

In an ongoing effort to identify and study high-mass protostellar candidates we have observed in various tracers a sample of 235 sources selected from the IRAS Point Source Catalog, mostly with 8 < -30', with the SEST antenna at millimeter wavelengths. The sample contains 142 Low sources and 93 High, which are believed to be in different evolutionary stages. Both sub-samples have been studied in detail by comparing their physical properties and morphologies. Massive dust clumps have been detected in all but 8 regions, with usually more than one clump per region. The dust emission shows a variety of complex morphologies, sometimes with multiple clumps forming filaments or clusters. The mean clump has a linear size of 60.5 pc, a mass of 6320 M QQQ ? for a dust temperature Td = 30 K, an H2 density of 9.5 x 105 cm-3, and a surface density of 0.4 g cm-2. The median values are 0.4 pc, 102 M QQQ ?, 4 x 104 cm-3, and 0.14 g cm-2, respectively. The mean value of the luminosity-to-mass ratio, L/M QQQ ? 99 L QQQ ?/M QQQ ?, suggests that the sources are in a young, pre-ultracompact HII phase. We have compared the millimeter continuum maps with images of the mid-IR MSX emission, and have discovered 95 massive millimeter clumps non-MSX emitters, either diffuse or point-like, that are potential prestellar or precluster cores. The physical properties of these clumps are similar to those of the others, apart from the mass that is 63 times lower than for clumps with MSX counterpart. Such a difference could be due to the potential prestellar clumps having a lower dust temperature. The mass spectrum of the clumps with masses above M 6 100 M QQQ ? is best fitted with a power-law dN/M 8 M-a with a = 2.1, consistent with the Salpeter (1955) stellar IMF, with a = 2.35. On the other hand, the mass function of clumps with masses 10 M QQQ ? QQQ ? M QQQ ? 120 M QQQ ? is better fitted with a power law of slope a = 1.5, more consistent with the mass function of molecular clouds derived from gas observations.