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Astron. Astrophys. 315, L177-L180 (1996)

Infrared absorption of H2O toward massive young stars*

E.F. van Dishoeck and F.P. Helmich

Leiden Observatory, P.O.-Box 9513, 2300 RA Leiden, The Netherlands

Received 1 July 1996 / Accepted 21 August 1996

Abstract. We present ISO-SWS observations of absorption lines of gas-phase water within its bending vibrational mode at 6 img3.gifm 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, img4.gif , 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.

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

* Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA

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Last change: December 16, 1996
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