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 delta < similar to 30 degrees, 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 similar to 0.5 pc, a mass of similar to 320 M-circle dot for a dust temperature T-d = 30 K, an H-2 density of 9.5 x 10(5) cm(-3), and a surface density of 0.4 g cm(-2). The median values are 0.4 pc, 102 M-circle dot, 4 x 10(4) cm(-3), and 0.14 g cm(-2), respectively. The mean value of the luminosity-to-mass ratio, L/M similar or equal to 99 L-circle dot/M-circle dot, 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 similar to 3 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 similar to 100 M-circle dot is best fitted with a power-law dN/dM proportional to M-alpha with alpha = 2.1, consistent with the Salpeter ( 1955) stellar IMF, with alpha = 2.35. On the other hand, the mass function of clumps with masses 10 M-circle dot less than or similar to M less than or similar to 120 M-circle dot is better fitted with a power law of slope alpha = 1.5, more consistent with the mass function of molecular clouds derived from gas observations.