Abundant gas-phase H2O in absorption toward massive protostars

We present infrared spectra of gas-phase H2O around 6 μm toward 12 deeply embedded massive protostars obtained with the Short Wavelength Spectrometer on board the Infrared Space Observatory (ISO). The $\nu_2$ ro-vibrational band has been detected toward 7 of the sources and the excitation temperatures indicate an origin in the warm gas at $T_{{\rm ex}}\ga 250$ K. Typical derived gas-phase H2O abundances are ~$5\times10^{-6}{-}6\times10^{-5}$, with the abundances increasing with the temperature of the warm gas. The inferred gas/solid ratios show a similar trend with temperature and suggest that grain-mantle evaporation is important. The increasing gas/solid ratio correlates with other indicators of increased temperatures. If the higher temperatures are due to a larger ratio of source luminosity to envelope mass, this makes gas-phase H2O a good evolutionary tracer. Comparison with chemical models shows that three different chemical processes, ice evaporation, high-T chemistry, and shocks, can reproduce the high inferred gas-phase H2O abundances. In a forthcoming paper each of these processes are investigated in more detail in comparison with data from the Long Wavelength Spectrometer on board ISO and the Submillimeter Wave Astronomy Satellite (SWAS). Comparison with existing SWAS data indicates that a jump in the H2O abundance is present and that the observed $\nu_2$ ro-vibrational band traces primarily the warm inner envelope.