2010A&A...510A..86C

The earliest stages of high-mass star formation have yet to be characterised well, because high-angular resolution observations are required to infer the properties of the molecular gas hosting the newly formed stars. We search for high-mass molecular cores in a large sample of 15 high-mass star-forming regions that are observed at high-angular resolution, extending a pilot survey based on a smaller number of objects. The sample was chosen from surveys of H₂O and OH masers to favour the earliest phases of high-mass star formation. Each source was first observed with the 32-m single-dish Medicina antenna in the (1, 1) and (2, 2) inversion transitions at 1.3cm of ammonia, which is an excellent tracer of dense gas. High-resolution maps in the NH₃(2, 2) and (3, 3) lines and the 1.3 cm continuum were obtained successively with the VLA interferometer. We detect continuum emission in almost all the observed star-forming regions, which corresponds to extended and UCHII regions created by young stellar objects with typical luminosities of ∼10⁴L_☉. However, only in three cases do we find a projected overlap between HII regions and H₂O and OH maser spots. On the other hand, the VLA images detect eight ammonia cores closely associated with the maser sources. The ammonia cores have sizes of ∼10⁴AU, and high masses (up to 10⁴M_☉), and are very dense (from ∼10⁶ to a few x10⁹cm^–3). The typical relative NH₃ abundance is ≤10^–7, in agreement with previous measurements in high-mass star-forming regions. The statistical analysis of the distribution between H₂O and OH masers, NH₃ cores, and HII regions confirms that the earliest stages of high-mass star formation are characterised by high-density molecular cores with temperatures of on average ≥30K, either without a detectable ionised region or associated with a hypercompact HII region.