JAC Style guide: template
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.
Last Modification Date 1996/08/13 - Last Modification Author: gdw
Graeme Watt (gdw)