Water Masers

Maser transitions have been observed in many important molecules and have been used to carry out surveys of the entire sky. The 22.235 GHz water maser transition is the strongest transition in the radio universe and represents an interesting candidate for an interstellar broadcast frequency. If extraterrestrial intelligence is trying to communicate with us, the choice of the broadcast frequency is an important one and would be known to all intelligent life. Of course it would have a different label, 22.235 GHz being a distinctly Earthly label, but it is a fundamental transition frequency and is observed everywhere. Other maser transitions include the 6.7 and 12.2 GHz methanol maser, the SiO maser v = 1, J = 7-6, 301.8 GHz, which occurs between levels in the first vibration state of the SiO molecule, and finally the OH maser first discovered in 1963 and buried deep in the 2n3/2 electronic state of the hydroxyl radical near 18 cm. This is actually four transitions at 1612, 1665, 1667 and 1720 MHz, all of which must be seen as a group but not necessarily of the same intensity. 

Maser The microwave equivalent of a laser that represents some of the strongest transition intensities in the radio region of the electromagnetic spectrum, e.g. the water maser at 22.235 GHz. 

DETECTION OF WATER MASER EMISSION FROM A CARBON STAR V778 CYGNI... 

Water maser emission has been detected towards V778 Cyg (Fig. 1) in the present observation. It is another evidence of the oxygen-rich nature of the circumstellar envelope around V778 Cyg, because water vapor has always been found around M stars. Thermochemical calculations also support the idea that the water molecules are present in the oxygen-rich environment but not in the carbon-rich one. The positional coincidence of the water maser with the optical star was found to be less than 0.5 arcsec (Deguchi et al. 1987), (Fig. 2). Unfortunately no other emission has been found, though Benson and Little-Marenin (1987) observed a water maser in EU And. Our negative detection of maser emission in EU And was probably due to the intensity variation of the object. 

Fig.2. A map of water maser emission toward V778 Cyg. The contour levels are -0.2, 0.2, 0.4, 0.6, and 0.8 normalized to the peak flux 5.9 Jy/beam.

Detection of Water Maser Emission from a Carbon Star V778 Cygni... 

Stern and Shull, 1991 [325] reported on water Masers, red giants, and Oort Clouds around other stars. Solar like stars will evolve to Red Giants as soon as their hydrogen supply is exhausted in their central cores. This so-called post main sequence phase implies an increase in luminosity by a factor of 1000. Such an increase will certainly affect the Oort cloud bodies. If we assume that such clouds are common around solar-like stars, then the question arises whether we would be able to detect such processes in giant star outflows. 

During the impact of comet Shoemaker Levy on Jupiter (1994), a 22 GHz water MASER emission line was observed. This line can be used as a diagnostic for the existence of water on exoplanets. An observational program (called ITASEL = Italian Search for Extraterrestrial Life) started 1999 using the 32 m dish of the Medic-ina radiotelescope and from 32 candidates four emissions can be suspected. If such observations can be definitely confirmed it will prove the importance of cometary impacts for the deposit of water on exoplanets (Cosmovici et al., 2006 [80]). 

While for a long time the radio line near 22 GHz was the only known water maser transition, in recent years various other water maser lines have been detected at millimeter and submillimeter wavelengths, some of them from the vibrationally excited bending mode. Masers can be emitted by several molecules The following species have been observed in stimulated emission from astronomical environments OH, CH, H22CO, H2O, NH3, CH3OH, SiS, HC3N, SiO, HCN, H. 

Maser detection in the interstellar medium was described by Elitzur, 1987 [118]. Water maser emission at 22 GHz may be a hint for cometary impacts that occur in extrasolar planetary systems. Such radiosignatures were proposed by Cos-movici et al., 2001 [79]. During the impact of comet Shoemaker-Levy 1994 on Jupiter observations made with the 32-m dish Medicina radiotelescope (Bolgna, Italy) revealed the delivery of water and for the first time also a water maser in the solar system. After the comet s nucleus exploded in the upper part of Jupiter s atmosphere, this maser was produced. 

Water masers are also found in galaxies and star forming galaxies. 

Water signatures in protostars have been observed already 30 years ago. Hydroxyl and Water Masers in Protostars were discussed e.g. by Litvak, 1969 [204]. The first detection of interstellar water molecules was made by Cheung et al., 1969 [67]. They analyzed the 1.35 cm microwave emission in the sources Sgr B 2, Orion and W 49. For numerical calculations, de Jong, 1973 [100] used the abundance of water relative to hydrogen as 10 that means for 10 molecules or atoms of hydrogen there is one H2O molecule. 

Codella and Palla, 1995 [71] reported on a search for water masers in protostellar disks. 160 IRAS sources were selected which define high-mass star forming regions. Water maser emission was found in 11 sources of this sample. This could mean that the population of OB-stars was overestimated by 50%. 

The water maser emission associated with the infrared centers IRS 1 and IRS 3 of the NGC 207 HR star-forming region was studied by Seth, Greenhill, and Holder, 2002 [305]. They used water masers as tracers for protostellar disks. NGC 2027 is a reflection nebula in the constellation of Orion. 

A possible relationship between the evolutionary stage of post-AGB stars and planetary nebulae (PNe) and the presence of water masers in their envelopes was investigated by Suarez, Gomez and Morata, 2007 [329]. They used a 70-m antenna in Robledo de Chavela (Spain) to search for the water maser transition at 22.235 GHz. They found two water maser emissions in planetary nebulae (IRAS 17443-2949 and IRAS 18061-2505), a water fountain in a post-AGB star (IRAS 16552-3050), and one in a source previously catalogued as a planetary nebula, but whose classification is uncertain (IRAS 17580-3111). 

A 22 GHz ortho-water maser in a dusty medium near late-type stars depends on several factors exchange of energy between dust and gas in the radiation field of a star, radiative cooling by water molecules and pumping of water masers (Babkovskaia, Poutanen, 2006 [14]). 

Walsh et al., 2008, [353] used Mopra Observations to map two one square degree regions along the galactic plane to find water masers. Young stellar objects (YSOs) were studied in the outer Galaxy by detection of water masers (Mochizuki, Hachisuka and Umemoto, 2009 [233]). 

Water masers in a galaxy provide proxies for identifying star forming regions and are therefore of particular interest. 

GHz water masers were found in four nearby galaxies. The locations of these water masers coincides with ultradense H-II regions. These regions correspond also to bright 24 pm emission areas (see Darling, Brogan and Johnson, 2009 [93] and 2008 [92]). 

Using the Expanded Very Large Array, Edmonds et al., 2008 [113] have conducted a search for 22.2 GHz H2O megamaser emission in the strongly lensed sub mm galaxy, SMM J16359 -I- 6612 at z = 2.517. They did not detect any water maser emission but were able to set an upper limit on the luminosity of any water maser present (LH20 5305 L ). 

Fig. 7.16 An artist conception of water maser emission in the accretion disk of the accretion disk and radio jet around the black hole in the heart of the Seyfert galaxy NGC 4258. This accretion disk material lies within a few tenths of a parsec from a supermassive black hole. The disk is warped, rotating differentially according to Kepler s laws. The inset at the bottom of the graphic is a radio spectrum (intensity as a function of frequency or velocity) of the water maser emission. The white glints on the disk surface show the locations of regions where maser emission has been detected. Image courtesy of NRAO/AUl and Artist John Kagaya (Hoshi No Techou)...

Supernova remnants sometimes show water emission (shock excited, e.g. in the supernova remnant 3 C 391, Reach and Rho, 1998 [277]). Woodall and Gray, 2007 [364] investigated 18 known supernova remnants for evidence of the 22.235-GHz water maser spectral line, using the 20-m telescope at the Onsala observatory in Sweden. Their results were negative, which was consistent with their theoretical models. Only in unusual environments can 22-GHz water masers be detected around supernova remnants. The Very Large Array was also used to search for water masers in the three supernova remnants W28, W44, and IC 443. Only one candidate maser was detected, in W44. Follow-up observations using the VLA at 1612 MHz demonstrate that the water maser in W44 is likely associated with a late-type star and not a shock in the supernova remnant (Claussen et al., 1999 [70]). 

Babkovskaia, N., Poutanen, J. A self-consistent model of a 22 GHz water maser in a dusty environment near late-type stars. Astron. Astrophys. 447,949-962 (2006)... 

Braatz, J.A., Gugliucci, N.E. The discovery of water maser emission from eight nearby galaxies. Astrophys. J. 678, 96-101 (2008)... 

Claussen, M.J., Goss, W.M., Frail, D.A., Seta, M. No water masers associated with supernova remnants. Astron. J. 117, 1387-1391 (1999)... 

Codella, C., Palla, F The nature of massive protosteUar candidates. I. A search for water masers towards color-selected IRAS sources. Astron. Astrophys. 302,528 (1995)... 

Cosmovid, C.B., Pogrebenko, S., Montebugnoli, S., Maccaferri, G. Radio searching for extrasolar cometary impacts at 22 Ghz (water MASER emission). In Catastrophic Events and Mass Extinctions Impacts and Beyond, p. 3002 (2001)... 

Darling, J., Brogan, C., Johnson, K. Ubiquitous water masers in nearby star-forming galaxies. Astrophys. J. Lett. 685, L39-L42 (2008)... 

Furuya, R.S., Kitamura, Y., Wootten, A., Claussen, M.J., Kawabe, R. Water maser survey toward low-mass young stellar objects in the northern sky with the Nobeyama 45 meter telescope and the very laige array. Astrophys. J. SuppL 144,71-134 (2003)... 

Mochizuki, N., Hachisuka, K., Umemoto, T Survey of Outer galaxy molecular lines associated with water masers. In Hagiwara, Y, Fomalont, E., Tsuboi, M., Yasuhiro, M. (eds.) Astronomical Society of the Pacific Conference Series, vol. 402, p. 384 (2009)... 

Stem, S.A., ShuU, J.M. Water masers, red giants, and Oort clouds around other stars. LPI Contrib. 765,208 (1991)... 

---