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Detection of the 464 GHz ground state line of HDO in W3(OH)/(H2O)

H2O is one of the most important interstellar molecules, but observations of it in interstellar clouds are greatly hampered by the Earth's atmosphere. However, deuterated water, HDO, possesses a number of (non--masing) low- excitation lines which can be observed from Earth. Although HDO was discovered already in 1975 through observations of the 1(1,0) - 1(1,1) transition at 80.6 GHz, most recent work has focussed on higher-lying transitions in the 230 GHz range. Because HDO is a light hydride, its ground-state 1(0,1) - 0(0,0) transition occurs in the submillimeter region, at 464 GHz. The line has been detected in Orion-KL by Schulz et al. (1991, A&A 246, L55), but searches in other sources have so far proved elusive.

Helmich, van Dishoeck and Jansen (1996, A&A in press) used the JCMT to search for the 464 GHz line toward several young stellar objects in the W3 giant molecular cloud. The line is clearly detected toward the position of the water masers, W3(H2O) (see Figure), and toward the compact H II region W3(OH), which lies only 7" to the west. These data form only the second detection of this line in the interstellar medium. The line was not seen toward W3 IRS5 and IRS4, but the conditions for these observations were not as good as those toward W3(H2O).

Together with observations of higher-lying lines at 241 and 225 GHz, the 464 GHz data allow constraints on the HDO excitation. A detailed analysis shows that the higher levels are most likely populated by intense far-infrared radiation due to warm dust (T(dust) > 100 K, E(B-V) > 500 mag). Combined with H218O data recently obtained by Gensheimer et al. (1996, A&A in press), the deuteration of water is found to be lower than that of other species, [HDO]/[H2O] = (2 - 6) 10(-4), but comparable to that found for other "hot cores". One possible explanation is that the W3 cloud never went through a very cold phase in its history.

F.P. Helmich, E.F. van Dishoeck and D.J. Jansen

Leiden Observatory, The Netherlands

Figure 1: Double side band spectrum of the "hot core" W3(H2O) obtained with receiver C2 and the DAS in the 250 MHz band width mode. Total integration time (on+off) is 90 minutes. The CH3OH (11,2 - 11,1) A+- line originates in the upper (image) sideband. Because of different calibrations of the two side-bands, its intensity should be multiplied by a factor of 1.4.

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Last Modification Date 1996/08/13 - Last Modification Author: gdw
Graeme Watt (gdw)
Contact: Holly Thomas. Updated: Tue Aug 17 17:32:19 HST 2004

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