EDP Sciences
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Volume 418, Number 1, April IV 2004
Page(s) 117 - 129
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20034322

A&A 418, 117-129 (2004)
DOI: 10.1051/0004-6361:20034322

Water masers in dusty environments

N. Babkovskaia and J. Poutanen

Astronomy Division, PO Box 3000, 90014 University of Oulu, Finland
    e-mail: natalia.babkovskaia@oulu.fi
(Received 15 September 2003 / Accepted 22 December 2003 )

We study in detail a pumping mechanism for the  $\lambda=1.35$ cm maser transition   $6_{16} \rightarrow 5_{23}$ in ortho-H 2O based on the difference between gas and dust temperatures. The upper maser level is populated radiatively through $4_{14} \rightarrow 5_{05}$ and  $5_{05} \rightarrow 6_{16}$ transitions. The heat sink is realized by absorbing the 45  $\mu{\rm m}$ photons, corresponding to the  $5_{23} \rightarrow 4_{14}$ 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 $N_{\rm H_2O}\approx 10^{6}{-}10^{7}~{\rm ~cm}^{-3}$ 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  $6_{16} \rightarrow 5_{23}$ transition becomes inverted already at a temperature difference of  $\Delta T\sim 1$ K, while other possible masing transitions require a larger  $\Delta T\ga 30$ K. We identify the region of the parameter space where other ortho- and para-water masing transitions can appear.

Key words: ISM: dust, extinction -- masers -- radio lines: general -- methods: analytical -- methods: numerical

Offprint request: J. Poutanen, juri.poutanen@oulu.fi

© ESO 2004