Submillimeter Observations of H_2(18) O in Molecular Clouds

Details Submillimeter Observations of H_2(18) O in Molecular Clouds Submillimeter Observations of H_2(18) O in Molecular Clouds

May 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994aas...184.1604z&link_type=abstract

American Astronomical Society, 184th AAS Meeting, #16.04; Bulletin of the American Astronomical Society, Vol. 26, p.883

Astronomy and Astrophysics

Astronomy

Scientific paper

Although water may be abundant in molecular clouds, measurements of the abundance are difficult because atmospheric absorption prevents access to transitions between low-lying levels. Highly excited transitions are observable from the ground, but they sample only hot and dense gas and are usually masing, and are not useful for deriving the abundance. We have detected the 547 GHz 110 - 101 ground-state transition of ortho-H_2(18) O, using our submillimeter SIS receiver flying aboard the NASA Kuiper Airborne Observatory. The 547 GHz line is in absorption towards Sgr B2 and is in emission in OMC-1. For Sgr B2, the source structure and radiative transfer modeling lead us to conclude that we are sampling the cool, quiescent molecular envelope rather than the hot core region. For ortho-H_2(18) O, only the 101 ground state is expected to have a significant population in this envelope. We derive an H_2(18) O column density of 1.1 times 10(14) \ cm(-2) , which gives H_2(18) O / H_2 ~ 7 times 10(-10) and H_2O / H_2 ~ 1.8 times 10(-7) , assuming (16) O / (18) O ~ 250. The velocity width of the 547 GHz emission line in OMC-1 is consistent with the 20\ km\ s(-1) outflow which is associated with 22 GHz maser emission, indicating that we may be sampling shock-excited gas. We have also made a tentative detection in OMC-1 of the 211-202 transition of para-H_2(18) O at 745 GHz. This work was supported in part by grants from NASA (NAG2-744 and NAGW-107), NASA/JPL, and an NSF Presidential Young Investigator grant to J. Zmuidzinas.

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