All Figures

$\begin{figure} \par\includegraphics[width=8cm,clip]{0322fig1.ps} \end{figure}$ Figure 1: A portion of the diagram of ortho-H₂O showing the rotational levels involved in the pumping of the $6_{16} \rightarrow 5_{23}$ maser.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{0322fig2.ps} \end{figure}$ Figure 2: Dependence of the 6₁₆-5₂₃maser absorption coefficient $\alpha_{\rm m}/(10^{-14}{\rm~cm}^{-1})$ on the half-thickness of the slab for six sets of parameters $(N_{\rm H_2},f_{\rm H_2O}/f_{\rm d})$ : (1) 10⁹,10^-2; (2) 10¹⁰,10^-3; (3) 10¹¹,10^-4; (4) 10⁹,10^-4; (5) 10¹⁰,10^-5; (6) 10¹¹,10^-6. Other parameters are fixed at $T_{\rm d}=130$ K, T=250 K, $f_{\rm d}=0.01$ . At a slab thickness less that 10¹³ cm, the effect of the boundary becomes significant. For higher H, the maser becomes saturated when $\tau _{\rm m}=30$ .
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{0322fig3.ps} \end{figure}$ Figure 3: Levels of constant maser absorption coefficient $\alpha_{\rm m}/(10^{-14}~{\rm~cm}^{-1})$ (solid curves) and maser optical depth $\tau _{\rm m}$ (dotted curves) in the plane slab half-thickness - gas temperature for $N_{\rm H_2}=10^{9}~{\rm~cm}^{-3}$ and $f_{\rm H_2O}/f_{\rm d}=10^{-2}$ . Other parameters are fixed at $T_{\rm d}=130$ K, $f_{\rm d}=0.01$ .
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In the text

$\begin{figure} \par\includegraphics[width=14cm,clip]{0322fig4.ps} \end{figure}$ Figure 4: Contour plots of the levels of constant inversion $\Delta n_{\rm m}$ (dotted curves) and constant optical depth (solid curves) for the 6₁₆-5₂₃ transition in the plane hydrogen concentration - water-to-dust mass ratio. A slab of half-thickness H=10¹⁴ cm and dust in the form of amorphous ice of the size $a=0.1~\mu{\rm m}$ was assumed, with a dust-to-gas mass ratio $f_{\rm d}=0.01$ . The panels correspond to the following temperatures a) $T_{\rm d}=30$ K, T=250 K; b) $T_{\rm d}=130$ K, T=132 K; c) $T_{\rm d}=130$ K, T=250 K; d) $T_{\rm d}=130$ K, T=400 K. The dashed contour in panel b) corresponds to the negligible photon escape case $p=p^{\rm c}=\delta$ .
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{0322fig5.ps} \end{figure}$ Figure 5: Contour plots of the levels of constant 6₁₆-5₂₃ maser optical depth $\tau _{\rm m}$ (solid curves) in the plane gas-dust temperature difference - water-to-dust mass ratio for the following set of parameters $N_{\rm H_2}=10^{9}~{\rm~cm}^{-3}$ , $T_{\rm d}=130$ K, $f_{\rm d}=0.01$ , and H=10¹⁴ cm. The dashed curves show the corresponding contours if the de Jong mechanism is not operating (i.e. the photon escape from the slab is neglected and the K₂ and L₂ terms in Eq. (12) are omitted).
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{0322fig6.ps} \end{figure}$ Figure 6: Temperature dependence of the maser optical depth for the same six sets of parameters $(N_{\rm H_2},f_{\rm H_2O}/f_{\rm d})$ as in Fig. 2. We fixed H=10¹⁴ cm, $T_{\rm d}=130$ K, $f_{\rm d}=0.01$ . The solid curve corresponds to analytical formula (45) with free normalization.
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In the text

$\begin{figure} \par\includegraphics[width=14cm,clip]{0322fig7.ps} \end{figure}$ Figure 7: Contours of constant maser optical depth for the strongest masing transitions of ortho-water for $T_{\rm d}=130$ K, T=400 K. The other parameters are the same as in Fig. 4.
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In the text

$\begin{figure} \par\includegraphics[width=14cm,clip]{0322fig8.ps} \end{figure}$ Figure 8: Same as in Fig. 7, but for para-water.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{0322fig9.ps} \end{figure}$ Figure 9: Isolines of the zero inversion of the strongest masing transitions for the same parameters as in Fig. 5. The labels on the contours correspond to the numbering in Table 1. Panel a) is for $N_{\rm H_2}=10^{10}~{\rm~cm}^{-3}$ ; b) corresponds to $N_{\rm H_2}=10^{9}~{\rm~cm}^{-3}$ . The inversion of the maser level populations exists on the right side of the corresponding isoline. The dashed curves on the left show the zero inversion for the $6_{16} \rightarrow 5_{23}$ maser when the de Jong mechanism is turned off.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{0322fig10.ps} \end{figure}$ Figure 10: Same as Fig. 9, but for para-water. The labels on the contours correspond to the numbering in Table 2.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{0322fig1.ps} \end{figure}$	Figure 1: A portion of the diagram of ortho-H₂O showing the rotational levels involved in the pumping of the $6_{16} \rightarrow 5_{23}$ maser.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{0322fig3.ps} \end{figure}$	Figure 3: Levels of constant maser absorption coefficient $\alpha_{\rm m}/(10^{-14}~{\rm~cm}^{-1})$ (solid curves) and maser optical depth $\tau _{\rm m}$ (dotted curves) in the plane slab half-thickness - gas temperature for $N_{\rm H_2}=10^{9}~{\rm~cm}^{-3}$ and $f_{\rm H_2O}/f_{\rm d}=10^{-2}$ . Other parameters are fixed at $T_{\rm d}=130$ K, $f_{\rm d}=0.01$ .
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$\begin{figure} \par\includegraphics[width=8cm,clip]{0322fig6.ps} \end{figure}$	Figure 6: Temperature dependence of the maser optical depth for the same six sets of parameters $(N_{\rm H_2},f_{\rm H_2O}/f_{\rm d})$ as in Fig. 2. We fixed H=10¹⁴ cm, $T_{\rm d}=130$ K, $f_{\rm d}=0.01$ . The solid curve corresponds to analytical formula (45) with free normalization.
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$\begin{figure} \par\includegraphics[width=14cm,clip]{0322fig7.ps} \end{figure}$	Figure 7: Contours of constant maser optical depth for the strongest masing transitions of ortho-water for $T_{\rm d}=130$ K, T=400 K. The other parameters are the same as in Fig. 4.
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$\begin{figure} \par\includegraphics[width=14cm,clip]{0322fig8.ps} \end{figure}$	Figure 8: Same as in Fig. 7, but for para-water.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{0322fig10.ps} \end{figure}$	Figure 10: Same as Fig. 9, but for para-water. The labels on the contours correspond to the numbering in Table 2.
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