A molecular-line study of clumps with embedded high-mass protostar candidates

We present molecular line observations made with the IRAM 30-m telecopes of tile immediate surroundings of a sample of 11 candidate high-mass protosta rs. These observations are part of an effort to clarify the evolutionary st atus of a set of objects which we consider to be precursors of UC HII regio ns. In a preceding series of papers we have studied a sample of objects, which on the basis of their IR colours are likely to be associated with compact m olecular clouds. The original sample of 260 objects was divided approximate ly evenly into a High group, with IR colour indices [25-12] greater than or equal to 0.57 and [60-12] greater than or equal to 1.3, and a Low group wi th complementary colours. The FIR luminosity of the Low sources, their dist ribution in the In colour-colour diagram, and their lower detection rate in H2O maser emission compared to the High sources, led to the hypothesis tha t the majority of these objects represent an earlier stage in the evolution than the members of the Nigh group, which are mostly identifiable with UC HII regions. Subsequent observations led to the selection of 12 Low sources that have FIR luminosities indicating the presence of B2.5 to O8.5 V-0 sta rs, are associated with dense gas and dust, have (sub-)mm continuum spectra indicating temperatures of similar to 30 K, and have no detectable radio c ontinuum emission. One of these sources has been proposed by us to be a goo d candidate for the high-mass equivalent of a Class 0 object. In the presen t paper we present observations of the molecular environment of 11 of these 12 objects, with the aim to derive the physical parameters of the gas in w hich they are embedded, and to find further evidence in support of our hypo thesis that these sources are the precursors to UC HII regions. We find tha t the data are consistent with such an interpretation. All observed sources are associated with well-defined molecular clumps. Mas ses, sizes, and other parameters depend on the tracer used, but typically t he cores have average diameters of similar to0.5-1 pc (with a range of 0.2 to 2.2 pc), and masses of a few tens to a few thousand solar masses. Compar ed to a similar analysis of High sources, the present sample has molecular clumps that are more massive, larger, cooler, and less turbulent. They also tend to have a smaller ratio of virial-to-luminous mass, indicating they a re less dynamically stable than their counterparts in which the High source s are embedded. The large sizes suggest these clumps should still undergo s ubstantial contraction (their densities are similar to 10 times smaller tha n those of the High sources). The lower temperatures and small linewidths a re also expected in objects in an earlier evolutionary state. In various so urces indications are found for outflowing gas, though its detection is ham pered by the presence of multiple emission components in tile line spectra. There are also signs of self-absorption, especially in the spectra of (CO) -C-13 and HCO+. We find that the masses of the molecular clumps associated with our objects increase with L-fir, (M-clump proportional to L-fir(1.17)) , and that there is a (weak) relation between the clump mass and the mass o f the embedded protostellar object M-proto proportional to M-clump(0.30). T he large amount of observational data is necessarily presented in a compact , reduced form. Yet we supply enough information to allow further study. Th ese data alone cannot prove or disprove the hypothesis that among these obj ects a high-mass protostar is truly present. More observations, at differen t wavelenghts and spatial resolutions are needed to provide enough constrai nts on the number of possible interpretations.