Infrared absorption of H2O toward massive young stars

E.F. van Dishoeck - F.P. Helmich

Abstract:

We present ISO-SWS observations of absorption lines of gas-phase water within its bending vibrational mode at 6 microns toward four massive young stars, which cover a range in physical parameters. Hot water with an excitation temperature >200 K is detected toward GL 2136 and GL 4176, in addition to GL 2591 discussed by Helmich et al. (this volume). The abundance of water with respect to H₂ is high in these regions, (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 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 H₂O.

Keywords:
ISM: molecules - ISM: clouds - ISM: individual: AFGL 2591, AFGL 2136, AFGL 4176, NGC 7538 IRS9

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