H2O Maser Emission in Circumstellar Envelopes around AGB Stars: Physical Conditions in Gas-Dust Clouds

Abstract

The pumping of 22.2-GHz H$_2$O masers in the circumstellar envelopes of asymptotic giant branch stars has been simulated numerically. The physical parameters adopted in the calculations correspond to those of the circumstellar envelope around IK Tau. The one-dimensional plane-parallel structure of the gas-dust cloud is considered. The statistical equilibrium equations for the H$_2$O level populations and the thermal balance equations for the gas-dust cloud are solved self-consistently. The calculations take into account 410 rotational levels belonging to the five lowest vibrational levels of H$_2$O. The stellar radiation field is shown to play an important role in the thermal balance of the gas-dust cloud due to the absorption of emission in rotational-vibrational H$_2$O lines. The dependence of the gain in the 22.2-GHz maser line on the gas density and H$_2$O number density in the gas-dust cloud is investigated. Gas densities close to the mean density of the stellar wind, 10$^7-10^8$ cm$^{-3}$, and a high relative H$_2$O abundance, more than 10$^{-4}$, have been found to be the most likely physical conditions in maser sources.