Water masers in dusty environments

Abstract

Abstract. We study in detail a pumping mechanism for the λ = 1.35 cm maser transition 6 16 → 5 23 in ortho-H 2 O based on the difference between gas and dust temperatures. The upper maser level is populated radiatively through 4 14 → 5 05 and 5 05 → 6 16 transitions. The heat sink is realized by absorbing the 45 µm photons, corresponding to the 5 23 → 4 14 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 N H 2 O ≈ 10 6 −10 7 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 6 16 → 5 23 transition becomes inverted already at a temperature difference of ∆T ∼ 1 K, while other possible masing transitions require a larger ∆T > ∼ 30 K. We identify the region of the parameter space where other ortho-and para-water masing transitions can appear.

Mechanisms of Dipolar Ortho/Para-H2O Conversion in Ice

“…Thus the ortho.para-ratios of water molecules in the gas phase are only an indirect measure of the temperature of the surface temperature of the ice crystals from where they descend and the fate of the molecule in the course of the evaporation has to be taken into account. This result could be the explanation for the unexpected experimentally observed ortho.para-ratios in the clouds of comets [34][35][36][37][38][39][40].…”

“…For water vapor in comets non-equilibrium ortho.para-ratios have been observed the origin of which are still unexplained. [34][35][36][37][38][39][40] In these experiments the population ratio of the paraand ortho-states of water is employed as a measure for the spin temperature of the water vapor in the cloud of the comet. For example experiments showed for the comet C 2001 Q4 a typical gaseous water temperature of 30K [40].…”

H2O maser emission in circumstellar envelopes around AGB stars: Physical conditions in gas-dust clouds

“…In this case, for each photon of the emission being amplified there must be one or more infrared "sink" photons, which must either be absorbed by cold dust or escape from the resonance region to complete the pumping cycle. The collisional H 2 O maser pumping is most efficient under conditions when the dust temperature is much lower than the gas temperature (Bolgova et al 1977;Chandra et al 1984;Yates et al 1997;Babkovskaia and Poutanen 2004). The higher the dust temperature in the gas-dust cloud, the lower the gain.…”

“…The first term contains the unknowns at a given level population iteration step; the second term is calculated based on the level populations and source functions S † derived in the preceding iteration. Equation ( 4) is substituted into the system of equations for the level populations (3) and "new" level populations are calculated. We chose the local operator from Rybicki and Hummer (1991) as the Λ *operator.…”

A warped m= 2 water maser disc in V778 Cyg?

“…In order to estimate the maser power, we solve simultaneously the radiative transfer and statistical balance equations for the first 45 rotational levels of the ground and (0 1 0) excited vibrational levels of ortho‐H 2 O. We employ the escape‐probability method in order to solve the radiative transfer equation (for details, see Babkovskaia & Poutanen 2004, 2006). For a disc half‐thickness of H ∼ 10 14 cm at the outer disc edge (obtained by considering the conditions for hydrostatic equilibrium, see below), we obtain a maser absorption coefficient of κ=−4.5 × 10 −14 cm −1 .…”