Search for massive protostellar candidates in the southern hemisphere -: I.: Association with dense gas

We have observed two rotational transitions of both CS and (CO)-O-17, and the 1.2 mm continuum emission towards a sample of 130 high-mass protostellar candidates with delta < - 30 degrees. This work represents the first step of the extension to the southern hemisphere of a project started more than a decade ago aimed at the identification of massive protostellar candidates. Following the same approach adopted for sources with delta >= - 30 degrees, we have selected from the IRAS Point Source Catalogue 429 sources which potentially are compact molecular clouds on the basis of their IR colours. The sample has then been divided into two groups according to the colour indices [25 - 12] and [60 - 12]: the 298 sources with [25 - 12] >= 0.57 and [60 - 12] >= 1.30 have been called High sources, the remaining 131 have been called Low sources. In this paper, we check the association with dense gas and dust in 130 Low sources. We have obtained a detection rate of similar to 85% in CS, demonstrating a tight association of the sources with dense molecular clumps. Among the sources detected in CS, similar to 76% have also been detected in (CO)-O-17 and similar to 93% in the 1.2 mm continuum. Millimeter-continuum maps show the presence of clumps with diameters in the range 0.2- 2 pc and masses from a few M-. to 10(5) M-.; H-2 volume densities computed from CS line ratios lie between similar to 10(4.5) and 10(5.5) cm(-3). The bolometric luminosities of the sources, derived from IRAS data, are in the range 10(3)- 10(6) L-., consistent with embedded high-mass objects. Based on our results and those found in the literature for other samples of high-mass young stellar objects, we conclude that our sources are massive objects in a very early evolutionary stage, probably prior to the formation of an H.. region. We propose a scenario in which High and Low sources are both made of a massive clump hosting a high-mass protostellar candidate and a nearby stellar cluster. The difference might be due to the fact that the 12 mu m IRAS flux, the best discriminant between the two groups, is dominated by the emission from the cluster in Lows and from the massive protostellar object in Highs.