\begin{table}%t1
\caption{Model parameters  \label{tab:tab1}} %\centerline
{
\begin{tabular}{ll}
\hline \hline
\noalign{\medskip}
{\em Maser and continuum ring properties}: &                    \\
\sp Maser ring inner radius \Rin$^{a}$          	&22~pc             \\
\sp Maser ring radial thickness \DR$^{a}$       	&3~pc             \\
\sp Maser ring height \DH$^{a}$                 	&6~pc              \\
\sp Maser ring Rotation velocity $v\sub{rot}^{b}$	&57 \kms           \\
\sp Continuum peak emissivity radius \Rc$^{b}$  	&37~pc             \\
\sp Inclination angle $i^{b}$      	&60\deg            \\
\hline \noalign{\medskip}
{\em Free-free absorption bicone properties}\/:    &                \\
\sp Opacity $\tau\sub{ff}$    		& 0.7      \\
\sp Opening angle ({\it FWHM})                & $60^{\circ}$      \\
\hline \noalign{\medskip}
{\em Cloud properties}\/:    &                \\
\sp Cloud size $a^{c}$                   		& $<$0.7~pc       \\
\sp Internal velocity width \dv$^{c,d}$      	& 20 \kms    \\
\sp Random velocity dispersion \Dv$^{c,d}$		& 60 \kms    \\
\sp Outflow velocity $v_{z}$  				& 60 \kms    \\
\sp Outflow velocity dispersion $\Delta v_{z}^{d}$	& 30 \kms    \\
\sp Number of clouds along radius \No   & 1.8          \\
\sp Cloud volume filling factor $f$		& $<$0.08	\\
\sp Cloud maser optical depth $\tau\sub{c}$   & 1.5          \\
%
\hline 
\\\end{tabular}}
Refs: $^{a}$T97; $^{b}$P01;  $^{c}$D99;                    \\
$^{d}$ The dispersions represent the {\it FWHM} of a
Gaussian probability\\ distribution.
\label{ta:table1}
\end{table}