The physical conditions in IRDC clumps from Herschel/HIFI observations of H₂O^{⋆,⋆⋆,⋆⋆⋆}

¹ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
e-mail: russ@sron.nl
² Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
³ Max-Planck-Institut für Radioastronomie, PO Box 20 24, 53010 Bonn, Germany
⁴ Univ. Bordeaux, LAB, UMR 5804, 33270 Floirac, France
⁵ CNRS, LAB, UMR 5804, 33270 Floirac, France
⁶ ASTRON Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD, Dwingeloo, The Netherlands

Received: 31 March 2014
Accepted: 30 July 2014

Abstract

Context. The earliest phases of high-mass star formation are poorly understood.

Aims. Our goal is to determine the physical conditions and kinematic structure of massive starforming cloud clumps.

Methods. We analyse H₂O 557 GHz 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⁺.

Results. 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 ~10⁷ cm^-3. The o-H₂O outflow column density is 0.3−3.0 × 10¹⁴ cm^-2. The o-H₂O absorption column density is between 1.5 × 10¹⁴ and 2.6 × 10¹⁵ cm^-2 with cold o-H₂O abundances between 1.5 × 10^-9 and 3.1 × 10^-8.

Conclusions. All clumps have high gas densities (~10⁷ cm^-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.