WIDESPREAD WATER-VAPOR EMISSION IN ORION

We have discovered spatially extended emission in the 3(13)-2(20) line of water vapor in Orion. This is the first time that extended water emission has been found in the interstellar medium. Many strong narrow features with flux density approximately (1-15) x 10(3) Jy have been detected around Ori A-IRc2. These features appear superposed over a weaker (approximately 10(3) Jy) high velocity plateau with a half-power size of almost-equal-to 45'', and a total extent of 80'' x 80''. Narrow lines with intensities approximately 25-200 Jy, are detected at the velocity of the molecular cloud for all the positions observed along the molecular ridge. These lines are not arising from point sources and the different emission regions are well resolved by our 15'' beam. Narrow and blueshifted emission is also found along a filament containing the S6 source 100'' south of IRc2. From statistical equilibrium and radiative transfer calculations for the physical conditions of the Orion molecular cloud, we conclude that the nature of the observed water vapor emission is dominated by maser emission. The water abundance is estimated to be larger than 10(-5), implying that water vapor is a substantial component of the gas phase in warm-molecular clouds and one of its most important gas coolants.