Submillimeter water masers

Abstract

The excitation of maser emission in millimeter and submillimeter transitions of interstellar and circumstellar water is considered. Using an escape probability method, the equilibrium populations in 349 rotational states of both ortho- and para-water have been determined under varying conditions of gas temperature, density, water abundance and radiation field. Results indicate that under suitable conditions, collisional excitation of warm (400 K) interstellar gas may result in significant maser action in a total of 7 centimeter, millimeter, and submillimeter transitions of H₂O, while at somewhat higher gas temperatures (1000 K) a total of 22 centimeter, millimeter, and submillimeter H₂O transitions are found to mase. Under conditions of low atmospheric humidity, several of these transitions might be observed from mountaintop altitude, and all of them are potentially observable from airborne observatories. Under the assumption that water masers from different transitions originate in the same gas and that the emission is equally beamed, the ratio of maser luminosities from different transitions can be used to set limits on the temperature as well as the bundled quantity of hydrogen density, water abundance, and velocity gradient within the masing gas.