Microwave detection of interstellar HDO

Abstract

Results are reported for observations of the 1(11)-1(10) transition of HDO in emissions at a wavelength of 3.72 mm toward the Kleinman-Low nebula (KL) in Orion A. A preliminary abundance ratio of deuterated to nondeuterated water is calculated, and possible excitation mechanisms are considered for the detected HDO transition. Analysis indicates that the present transition has a population inversion for a wide range of kinetic temperatures, total particle densities, and projected densities of HDO, and that the HDO line probably represents thermal or quasi-thermal emission from interstellar water molecules. It is concluded that most of the oxygen in KL is probably contained in water molecules provided the ratio of deuterated water to nondeuterated water is about equal to the ratio of D to H.