1999A&AS..136..333C

There are several indications that strong H₂O maser emission arises at the very beginning of the evolution of a massive star and disappears when an Hii region becomes detectable in the radio continuum. If this is the case, one expects to find dense hot molecular gas surrounding embedded far IR sources coincident with H₂O masers. In order to test this hypothesis, we have used the Pico Veleta 30-m radiotelescope to search for molecular line emission towards a sample of 12 H₂O masers located in regions of massive star formation, but not directly associated with bright compact radio Hii regions, with the intention to identify the sites of newly born massive (proto)stars in their earliest stages and to study their properties. Our main goals were: a) to confirm the hypothesis that the H₂O masers not associated with compact radio continuum emission are indeed located at the centre of high density clumps within a molecular cloud; b) to use several molecular transitions (namely: ¹³CO(2-1), CS(3-2), C³⁴S(2-1), C³⁴S(3-2), C³⁴S(5-4), HCN(1-0), CH₃CN(8-7), CH₃CN(12-11), HCO⁺(1-0), CH₃OH(3-2), CH₃OH(5-4)) in order to derive information on the size, kinematics, temperature, density, and ionisation degree of the molecular gas in the places where star formation has just begun, as well as to search for the presence of outflows on scale sizes of 10"-30". In this paper we present the large amount of data obtained at Pico Veleta in a compressed way, but still sufficiently ample to give usable informations for further studies. General results from a first analysis of the data are also presented. Our first goal is amply verified since in all cases and in molecules tracing high density gas we find a barely resolved peak at the position of the maser, confirming the validity of our selection criteria. Our sample thus provides a valid reference list of regions of massive star formation in their earliest phases. As far as the second goal is concerned, the large variety of intensity ratios of different molecules, as well as of other derived parameters, point out that the molecular clumps where star formation is taking place are far from identical and that chemical evolution and influence of the newborn star may amply affect the line intensity ratios. In some cases small scale (seconds of arc) outflows were detected, not necessarily related to the minute of arc scale outflows present in the same regions. More detailed studies of each region are presented in separate papers.