High density molecular clumps around protostellar candidates

Abstract

There are several indications that strong H2O maser emission arises at the very beginning of the evolu- tion of a massive star and disappears when an Hii region becomes detectable in the radio continuum. If this is the case, one expects to nd dense hot molecular gas surround- ing embedded far IR sources coincident with H2 Om asers. 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 re- gions 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 conrm the hypoth- esis that the H2O masers not associated with com- pact radio continuum emission are indeed located at the centre of high density clumps within a molec- ular cloud; b) to use several molecular transitions (namely: 13 CO(2 1), CS(3 2), C 34 S(2 1), C 34 S(3 2), C 34 S(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 00 30 00 . In this paper we present the large amount of data ob- tained at Pico Veleta in a compressed way, but still su- ciently ample to give usable informations for further stud- ies. General results from a rst analysis of the data are also presented. Our rst goal is amply veried since in all cases and in molecules tracing high density gas we nd a barely re- solved peak at the position of the maser, conrming 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 dierent molecules, as well as of other