Detection of Water in Interstellar Regions by Its Microwave Radiation

A. C. Cheung et al.

Editor’s Note

As radio receivers became capable of observations at ever shorter wavelengths, new molecules were detected in space. A. C. Cheung and colleagues here report the discovery of radio emission from water in three regions in the sky. The brightness temperature of the emission from the Orion Nebula and the W 49 region was surprisingly high, which puzzles the authors because the energetically excited state of the molecules they observed has a natural lifetime of just 10 seconds. The observations therefore implied that the water molecules were being constantly excited. It turns out that collisions with molecular hydrogen cause this excitation—but molecular hydrogen had not yet been discovered in space. 中文

MICROWAVE emission from the 6 ₁₆ →5 ₂₃ rotational transition of H ₂ O has been observed from the directions of Sgr B 2, the Orion Nebula and the W 49 source. This radiation, at 1.35 cm wavelength, was detected with the twenty foot radio telescope at the Hat Creek Observatory using techniques described earlier for the detection of the NH ₃ spectrum ¹ . In the case of Sgr B 2, the H ₂ O emission is from the same direction in which considerable NH ₃ is observed (unpublished work of A. C. C. et al. ), although there is reason to believe the two molecular species may not be closely associated. Strong H ₂ O radiation producing an antenna temperature of 14°K is observed from the Orion Nebula (where no NH ₃ was detected), and an antenna temperature at least as high as 55° was found for H ₂ O radiation from W 49. 中文

Fig. 1 shows the spectral intensity near the H ₂ O resonance in the approximate direction of Sgr B 2, the antenna being pointed at the position α ₁₉₅₀ =17 h 44 m 23 s. δ ₁₉₅₀ = –28°25′. The observed antenna temperature is plotted as a function of Doppler velocity. The mean Doppler velocity of about 70 km s ^–1 for the spectral line observed is not very different from the 68 km s ^–1 mean velocity found for one of the OH emission and broad OH absorption features observed in this region ² , the 62 km s ^–1 Doppler velocify of a small nearly HII region ³ , and the velocity of about 58 km s ^–1 found for NH ₃ (unpublished work of A. C. C. et al. ) observed in this direction. The results shown in Fig. 1 were obtained with filters producing a spectral resolution of about 1.3 MHz. 中文

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Fig. 2 shows the antenna temperature as a function of Doppler velocities observed in the Orion Nebula at α ₁₉₅₀ = 5h 32 m 57 s ± 4 s and δ ₁₉₅₀ = – 5° 25.5′±1.0′. In Orion, the radiation intensity was sufficiently high to make practical the use of filters producing a spectral resolution of about 350 kHz. In Fig. 2 the solid line represents the continuum temperature as it was measured with filters of width 2 MHz; the plotted points represent observations made with the filters having bandwidths of 350 kHz. These points are, in some places, closer together than the filter widths of 350 kHz because measurements were made during different runs in which the central frequencies of the filters were displaced in order to examine the structure of the spectral line, which is composite, representing at least two distinct Doppler velocities. The mean Doppler velocity of about +7 km s ^–1 found for the water line is somewhat different from the HII Doppler shifts in the range of –2 to –6 km s ^–1 found in the same direction ^3,4 . OH radiation from this direction shows Doppler velocities near 20 km s ^–1 and 7 km s ^–1 (ref. 2); the latter coincides more closely with the H ₂ O velocity. 中文

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Fig. 3 shows similar data for the still more intense source W 49, at α ₁₉₅₀ = 19 h 7 m 55 s±4 s and δ ₁₉₅₀ = 9° 0.4′±1′. Over most of the spectral line, the narrow filters of width 350 kHz were used and several narrow and intense features were revealed. The measured antenna temperature was as high as 55° K, but allowance for atmospheric attenuation of this source, which was rather low in the sky during the observations, raises its effective antenna temperature to somewhat more than 70°. Presumably, narrower filters would give still higher antenna temperatures at some frequencies. The solid lines on either side of the diagram show the radiation intensity observed with filters about 2 MHz wide. Radiation present on the extreme right of Fig. 3 seems to be above the continuum level, showing that H ₂ O radiation also occurs within the band of this last wide filter. The Doppler shift of OH in W 49 ranges from –2 km s ^–1 to +23 km s ^–1 (ref. 2) and that of the HII region is +6 km s ^–1 (ref. 3). These are in the same general range, but by no means similar to the dominant values of Doppler shifts shown in Fig. 3. 中文

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In all three figures, the continuum temperatures should be scaled down by a factor of two, because the heterodyne detection system received continuum radiation in both side bands. 中文

The Orion and W 49 sources were not large enough in angular size noticeably to broaden the beam width of 8.8′ in drift scans across the sources. Thus we conclude that these radiating sources are not larger in angular diameter than 3′. 中文

The radiation found is attributed to H ₂ O because its frequency coincides very closely to that found for H ₂ O in the laboratory, and no other known atomic or molecular species can explain the observations. The Doppler shifts plotted in the figures represent small departures from the laboratory frequency of 22,235.22 MHz for H ₂ O, for which 10 km s ^–1 corresponds to a frequency shift of 0.75 MHz. No other simple molecular of atomic species which can be expected in interstellar space is known to produce a line within several tens of MHz of this frequency except NH ₃ , the (3,1) inversion transition of which lies at 22,234.53 MHz. This is a rather weak NH ₃ transition, but it is still important to examine carefully the possibility that the observed radiation might be caused by NH ₃ . A search was therefore made for other stronger transitions of NH ₃ . The (3,3) transition is somewhat more than an order of magnitude stronger than this (3,1) transition at excitation temperatures above 50° K, while the (1,1) transition is similarly more intense at temperatures below 50° K. Even at excitation temperatures considerably above 50°K, the (1,1) transition is always more than twice as intense as the (3,1) transition. A search for these two NH ₃ transitions in the Orion Nebula was made. They were not found, with the upper detection limit of antenna temperature being set at less than 0.07°K for both the (1,1) and the (3,3) inversion transition. This limit is almost two orders of magnitude less than the intensity of radiation found at about 22,235 MHz, and hence we conclude that the radiation we have observed cannot be caused by the (3,1) transition of NH ₃ , and can be explained only by H ₂ O. 中文

In the case of Sgr B 2, the situation is more complex. Rather strong NH ₃ radiation was, in fact, observed from the same (or nearly the same) direction as the H ₂ O radiation. The (1,1), (2,2), (3,3) and (4,4) inversion transitions of NH ₃ were all found with antenna temperatures between 0.5° K and 1.5° K (unpublished work of A. C. C. et al. ). The possibility that the line observed at 22,235 MHz is caused by NH ₃ may be ruled out by the following argument, however. The NH ₃ rotational states (1,1), (2,2), (3,3) and (4,4) can be excited by collision, but are metastable, with exceedingly long radiation lifetimes. The (3,1) state can radiate in the infrared, however, by making a transition to the (2,1) state. The mean radiation lifetime of the upper state is about 50 s; thus unless the state is excited in a comparable time, it should be very much less populated than any of the states which were observed. The (4,3) state also radiates with a lifetime comparable with that of the (3,1) state. Thus a search was made for (4,3) level inversion radiation to find out whether or not some excitation mechanism was producing a significant population in the (4,3) level and, by inference, also producing an appreciable number of NH ₃ molecules in the (3,1) state. The (4,3) inversion radiation was not observed, the limit of detection being an antenna temperature of 0.07° K. The (4,3) transition searched for is about an order of magnitude more intense than the (3,1) transition. If the observed antenna temperature of 1° K in Sgr B 2 at 22,235 MHz were caused by the (3,1) transition, the (4,3) transition would have an antenna temperature of 10° K. No such temperature is observed, so we conclude that the observed radiation at 22,235 MHz must be caused by H ₂ O rather than this weak NH ₃ transition. 中文

In the case of W 49, a search was made for the NH ₃ (1,1) inversion transition. It was not found, and an upper limit of 0.5° could be established for the antenna temperature at its frequency. 中文

There is, of course, H ₂ O in the Earth’s atmosphere. One might wonder if it could produce the observed radiation. It can be eliminated as a possible source for several reasons. One is that the antenna beam is switched from one part of the sky to a neighbouring part, so that only radiation which varies very rapidly with angle is detected. A second reason is that the microwave resonant line was detected in the three particular fixed directions in space, and not nearby. Still a third is that an atmospheric water line would have occurred at the laboratory frequency, which corresponds to a Doppler shift of +15 km s ^–1 in Fig. 1 and –30 km s ^–1 in Fig. 2. 中文

It has previously been suggested that the 6 ₁₆ →5 ₂₃ transition of H ₂ O could be of interest to radio astronomy ⁵ , and recently a substantial proposal for its detection has been made by Snyder and Buhl at the AAS meeting, Dallas, December 1968. But it is surprising that the transition is as strong as observed, because it involves levels of rotational energy of 456 cm ^–1 which can radiate to lower states in about 10 s. They require moderately high temperatures and frequent excitations, either by collisions or by radiation. Rotational states of NH ₃ which can similarly radiate have not been found, indicating that the NH ₃ and H ₂ O have been detected in rather different regions. Presumably the H ₂ O is present in rather special regions of higher than normal excitation. The matrix element for this H ₂ O line is appreciably less than that for the NH ₃ inversion levels. Thus if there is thermal equilibrium between the 6 ₁₆ and 5 ₂₃ states, the population in these two levels, rather high above the ground state, must have a column density greater than that found for NH ₃ (ref. 1), or about 10 ¹⁷ cm ^–2 in the Sgr B 2 cloud. 中文

In Orion, the actual microwave brightness temperature of the source would have to be at least as high as a few hundred degrees, and in W 49 at least as great as about one thousand degrees. The actual temperatures may be much higher if these sources are smaller than the upper limit of 3’ of arc given, or if they are optically thin. The high intensity, very narrow, lines suggest that perhaps thermal equilibrium does not occur and that there may even be maser action. Further study of the distribution and condition of H ₂ O in interstellar space is clearly needed. If thermal equilibrium does in fact apply, this intense microwave radiation from H ₂ O and the existence of strong HDO transitions in the radio region should allow an interesting measurement of the hydrogen–deuterium abundance ratio. 中文

We thank Professor Harold Weaver for giving us his positions of OH clouds in the Sagittarius region and for discussions, and Paul Rhodes for his help with the observations. This work is supported in part by NASA, the US Office of Naval Research and the US National Science Foundation. 中文

( 221 , 626-628; 1969)

A. C. Cheung, D. M. Rank and C. H. Townes: Department of Physics, University of California, Berkeley.

D. D. Thornton and W. J. Welch: Radio Astronomy Laboratory and Department of Electrical Engineering, University of California, Berkeley.

Received January 13, 1969.

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References: FRbRkd7jcxnRV5KGE7s0YcMsRQ7IPpETdLjh1Q1u1bdQtYeuD3QVxDgGPFW31qZI

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5. Townes, C. H., in Fourth IAU Symp., Manchester, 1955 (edit. by van de Hulst, H. C.) (Cambridge Univ. Press, 1957).