2014A&A...570A..51S

The earliest phases of high-mass star formation are poorly understood. Our goal is to determine the physical conditions and kinematic structure of massive starforming cloud clumps. We analyse H₂O 557GHz line profiles observed with HIFI toward four positions in two infrared-dark cloud clumps. By comparison with ground-based C¹⁷O, N₂H⁺, CH₃OH, and NH₃ line observations, we constrain the volume density and kinetic temperature of the gas and estimate the column density and abundance of H₂O and N₂H⁺. The observed water lines are complex with emission and absorption components. The absorption is redshifted and consistent with a cold envelope, while the emission is interpreted as resulting from proto-stellar outflows. The gas density in the clumps is ∼10⁷cm^–3. The o-H₂O outflow column density is 0.3-3.0x10¹⁴cm^–2. The o-H₂O absorption column density is between 1.5x10¹⁴ and 2.6x10¹⁵cm^–2 with cold o-H₂O abundances between 1.5x10^–9 and 3.1x10^–8. All clumps have high gas densities (∼10⁷cm^–3) and display infalling gas. Three of the four clumps have outflows. The clumps form an evolutionary sequence as probed by H₂O N₂H⁺, NH₃, and CH₃OH. We find that G28-MM is the most evolved, followed by G11-MM and then G28-NH₃. The least evolved clump is G11-NH₃ which shows no signposts of starformation; G11-NH₃ is a high-mass pre-stellar core.