Infrared absorption of H_2_O toward massive young stars.

Abstract

We present ISO-SWS observations of absorption lines of gas-phase water within its bending vibrational mode at 6μm toward four massive young stars, which cover a range in physical parameters. Hot water with an excitation temperature >200K is detected toward GL 2136 and GL 4176, in addition to GL 2591 discussed by Helmich et al. (1996A&A...315L.173H). The abundance of water with respect to H_2_ is high in these regions, ~(2-3)x10^-5^, and comparable to the solid H_2_O abundance. In contrast, no gas-phase water absorption lines are seen toward NGC 7538 IRS9. The amount of gas-phase water is correlated with the column density of warm gas along the line of sight. Infrared observations of a larger variety of sources may provide insight into the relative importance of evaporation of grain mantles vs. high temperature gas-phase chemistry in producing the observed high abundance of H_2_O.