Water masers in dusty environments

Abstract

We study in detail a pumping mechanism for the λ = 1.35 cm maser transition 616 → 523 in ortho-H2O based on the difference between gas and dust temperatures. The upper maser level is populated radiatively through 414 → 505 and 5 05 → 616 transitions. The heat sink is realized by absorbing the 45 μm photons, corresponding to the 523 → 414 transition, by cold dust. We compute the inversion of maser level populations in the optically thick medium as a function of the hydrogen concentration, the water-to-dust mass ratio, and the difference between the gas and the dust temperatures. The main results of the numerical simulations are interpreted in terms of a simplified four-level model. We show that the maser strength depends mostly on the product of hydrogen concentration and the dust-to-water mass ratio but not on the size distribution of the dust particles or their type. We also suggest approximate formulae that describe accurately the inversion and can be used for fast calculations of the maser luminosity. Depending on the gas temperature, the maximum maser luminosity is reached when the water concentration NH2O ≈ 106-107 cm-3 times the dust-to-hydrogen mass ratio, and the inversion completely disappears at densities just an order of magnitude larger. For a dust temperature of 130 K, the 616 → 523 transition becomes inverted already at a temperature difference of δT ∼ 1 K, while other possible masing transitions require a larger δT ≥ 30 K. We identify the region of the parameter space where other ortho- and para-water masing transitions can appear.