The physical conditions in IRDC clumps from Herschel/HIFI observations of H2O

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. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia with important participation of NASA.Tables 4, 6, 8, 10, 11, and Appendix A are available in electronic form at http://www.aanda.orgFinal Herschel and APEX data used in the paper (FITS files) are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/570/A51