The observation of water, H₂O, by radioastronomy is often hampered because of the presence of water in Earth's amosphere. Only comparatively weak transitions can be detected as there will be non-zero transmittance at these frequencies through the atmosphere. Nevertheless, water was one of the first molecules to be detected in the interstellar medium. The J_KaKc = 6₁₆ – 5₂₃ ortho rotational transition of water, H₂O, was observed in Orion, SgrB2, and W49 displaying maser activity:
A. C. Cheung, D. M. Rank, C. H. Townes, D. D. Thornton, and W. J. Welch,
Detection of Water in Interstellar Regions by its Microwave Radiation
Nature 221, 626–628 (1969).
At the observed intensity levels, the detection could be viewed as secure. In the meantime, many additional transitions have been reported.

Singly deterated water, HDO, was reported first by
B. E. Turner, N. Fourikis, M. Morris, P. Palmer, and B. Zuckerman,
Microwave Detection of Interstellar HDO
Astrophys. J. 198, L125–L128 (1975).
The 1₁₁ – 1₁₀ transition was observed in emission toward Orion KL. Several other transitions have been reported in the meantime.

H₂¹⁸O was detected in emission through its 3₁₃ – 2₂₀ transition toward G34.3+0.2:
T. Jacq, C. Henkel, C. M. Walmsley, P. R. Jewell, and A. Baudry,
H₂¹⁸O in Hot Dense Molecular Cloud Cores
Astron. Astrophys. 199, L5–L8 (1988).
Emission features were also detected toward Orion KL, Sgr B2(N), and W51 e1/e2, but not unambiguous because possible overlap with lines of other molecules. In the meantime, other transitions have been detected, too.

H₂¹⁷O was detected in emission through its 1₁₀ – 1₁₀ transition toward Orion KL:
Å. Hjalmarson, P. Bergman, N. Biver, H.-G. Flor&eacite;n, U. Frisk, T. Hasegawa, K. Justtanont, B. Larsson, S. Lundin, M. Olberg, H. Olofsson, G. Persson, G. Rydbeck, A. Sandqvist,
Recent Astronomy Highlights from the Odin Satellite
Adv. Space Res. 36, 1031–1047 (2005).

The 1₁₀ – 1₁₀ transition of doubly deuterated water was detected in emission and absorption toward IRAS 16293-2422, a low-mass protostellar binary system:
H. M. Butner, S. B. Charnley, C. Ceccarelli, S. D. Rodgers, J. R. Pardo, B. Parise, J. Cernicharo, and G. R. Davis,
Discovery of Interstellar Heavy Water
Astrophys. J. 659, L137–L140 (2007).

E. A. Bergin, T. G. Phillips, C. Comito, et al.
presented in their account on
//Herschel// observations of EXtra-Ordinary Sources (HEXOS): The present and future of spectral surveys with //Herschel///HIFI
Astron. Astrophys. 521, Art. No. L20 (2010)
also an account of the detection of HD¹⁸O toward Orion KL. Four low-lying transitions were observed in emission. The derived D/H ration with respect to HDO is ~0.02.

Water in its v₂ = 1 excited vibrational state were detected in emission first toward two O-rich late-type stars VY CMa and W Hya:
K. M. Menten and G. J. Melnick,
Hot Water around Late-Type Stars – Detection of Two Millimeter-Wave Emission Lines from the //v//₂ Vibrationally Excited State
Astrophys. J. 341, L91–L94 (1989).

E. D. Tenenbaum, J. L. Dodd, S. N. Milam, N. J. Woolf, and L. M. Ziurys
describe in their work on
Comparative Spectra of Oxygen-rich Versus Carbon-rich Circumstellar Shells: VY Canis Majoris and IRC +10216 at 215−285 GHz
Astrophys. J. Lett. 720, L102–L107 (2010).
the detection of the 6_5,2 – 7_4,3 transition of H₂O in its v₂ = 2 state near 268.149 GHz.

T. Kamiński, H. S. P. Müller, M. R. Schmidt, I. Cherchneff, K. T. Wong, S. Brünken, K. M. Menten, J. M. Winters, C. A. Gottlieb, and N. A. Patel
mentioned in their account on
An observational study of dust nucleation in Mira (o Ceti) — II. Titanium oxides are negligible for nucleation at high temperatures
Astron. Astrophys. 599, Art. No. A59 (2017)
the identification of highly rotationally and vibrationally excited lines of H₂O, among them a formally ro-vibrational transition v₁ = 1 – v₃ = 1, 12_3,9 – 11_5,6 near 345.67 GHz.

A. Baudry, K. T. Wong, S. Etoka, A. M. S. Richards, H. S. P. Müller, F. Herpin, T. Danilovich, M. D. Gray, S. Wallström, D. Gobrecht, T. Khouri, L. Decin, C. A. Gottlieb, K. M. Menten, W. Homan, T. J. Millar, M. Montargès, B. Pimpanuwat, J. M. C. Plane, and P. Kervella
detected in their investigation
ATOMIUM: Probing the inner wind of evolved O-rich stars with new, highly excited H₂O and OH lines
Astron. Astrophys. 674, Art. No. A125 (2023)
10 highly rotationally or vibrationally excited lines of H₂O, of which 9 are new detections. The lines were obtained in selected ranges in the frequency region betweeb 222 and 269 GHz. Three of the lines involve the second triad, consisting of the states v₂ = v₃ = 1, v₁ = v₂ = 1, and v₂ = 3.

Contributor(s): H. S. P. Müller; 04, 2007; 03, 2024