INFRARED-ABSORPTION OF H2O TOWARD MASSIVE YOUNG STARS

We present ISO-SWS observations of absorption lines of gas-phase water within its bending vibrational mode at 6 mu m toward four massive you ng stars, which cover a range in physical parameters. Hot water with a n excitation temperature >200 K is detected toward GL 2136 and GL 4176 , in addition to GL 2591 discussed by Helmich et al. (this volume). Th e abundance of water with respect to H-2 is high in these regions, sim ilar to (2 - 3) 10(-5), and comparable to the solid H2O 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 den sity of warm gas along the line of sight. Infrared observations of a l arger variety of sources may provide insight into the relative importa nce of evaporation of grain mantles vs. high temperature gas-phase che mistry in producing the observed high abundance of H2O.