2013A&A...556A..16G

The details of the process of massive star formation are still elusive. A complete characterization of the first stages of the process from an observational point of view is needed to constrain theories on the subject. In the past 20 years we have made a thorough investigation of colour-selected IRAS sources over the whole sky. The sources in the northern hemisphere were studied in detail and used to derive an evolutionary sequence based on their spectral energy distribution. To investigate the first stages of the process of high-mass star formation, we selected a sample of massive clumps previously observed with the Swedish-ESO Submillimetre Telescope at 1.2mm and with the ATNF Australia Telescope Compact Array at 1.3cm. We want to characterize the physical conditions in such sources, and test whether their properties depend on the evolutionary stage of the clump. With ATCA we observed the selected sources in the NH₃(1, 1) and (2, 2) transitions and in the H₂O(6₁₆-5₂₃) maser line. Ammonia lines are a very good temperature probe that allow us to accurately determine the mass and the column, volume, and surface densities of the clumps. We also collected all data available to construct the spectral energy distribution of the individual clumps and to determine if star formation is already occurring through observations of its most common signposts, thus putting constraints on the evolutionary stage of the source. We fitted the spectral energy distribution between 1.2mm and 70µm with a modified black body to derive the dust temperature and independently determine the mass. We find that the clumps are cold (T∼10-30K), massive (M∼10²-10³M_☉), and dense (n(H₂)>10⁵cm^–3) and that they have high column densities (N(H₂)∼10²³cm^–2). All clumps appear to be potentially able to form high-mass stars. The most massive clumps appear to be gravitationally unstable, if the only sources of support against collapse are turbulence and thermal pressure, which possibly indicates that the magnetic field is important in stabilizing them. After investigating how the average properties depend on the evolutionary phase of the source, we find that the temperature and central density progressively increase with time. Sources likely hosting a ZAMS star show a steeper radial dependence of the volume density and tend to be more compact than starless clumps.