H₂O Masers

Abstract

H₂O was the third interstellar molecule discovered in radio wavelengths. Cheung et al. (1969) detected the 22 GHz line corresponding to the 6₁₆ → 5₂₃ transition (figure 3.2) toward a number of star-forming regions using the 20 foot radio telescope of the University of California Hat Creek Observatory. The emission was rather strong, antenna temperatures as high as 60 K, even though the transition levels lie more than 600 K above ground and would require substantial excitation rates for population. Maser emission was suspected at the outset, and was promptly confirmed by the observations of Knowles et al. (1969). These were performed at the 85 foot reflector at the Maryland Point Observatory of the Naval Research Laboratory and added more defections, including the supergiant star VY CMa. The much larger aperture results in a smaller beam pattern, and since these are point sources the antenna temperatures were much higher (cf. eq. 2.3.29) — 1700 K in the case of W49. From the measured upper limit of 0.7′ on the source size it was deduced that the brightness temperature exceeds 50,000 K. Time variability suggested even smaller sizes, less than 0.005′, corresponding to brightness temperatures in excess of 10⁹ K (subsequent interferometry showed that some features in W49 have T _b ≳ 10¹⁴ K). Remarkably, a search for water radiation in the 22 GHz line had been proposed just prior to these observations by Snyder and Buhl (1969) who compiled a list of OH sources, suggested as potential candidates for H₂O detection. All four emission sources on their list were detected in the original discoveries!