\begin{table}%t2
\caption{\label{Line intensities Radex}Observations and model calculations for the narrow PDR component in the {\tt RADEX} homogeneous model$^a$.}
\small%\centerline
 {
 \begin{tabular}{l l r r c c r c c r c}
 \hline\hline
Species   &  Transition & Frequency  & \multicolumn{3}{c}{Observations}
 & Ref.  &\multicolumn{4}{c}{Model calculations} \\
 & & $\nu$ & $T_{\rm b}$ & $4\pi I$ & $\Delta \upsilon$ & & $T_{\rm ex}$ &
 $\tau$ & $T_{\rm b}$ & $4\pi I$ \\
 & & [GHz] & [K] & [erg s$^{-1}$ cm$^{-2}$] & [km s$^{-1}$] & & [K] &
 & [K] & [erg s$^{-1}$ cm$^{-2}$] \\
\hline
&&&&&&&&&& \\[-8pt]
 H$_2$O & \trans & 556.936 & 0.92 & \dots & 3.1 & $c$ &
  8.2 & 6.6 & 0.92 & $6.8\times 10^{-6}$ \\   
 H$_2^{18}$O & \trans & 547.676 & $<$$0.051$ & \dots & (3.1)$^b$ & $c$&
  6.3 & 0.2 & $<$$0.05$ & $<$$3.5\times 10^{-7}$ \\ 
 NH$_3$ &  1$_0$--0$_{~0}$ & 572.498 & 1.1 & \dots & 3.3 &$c$ & 
  9.0 & 6.1 & 1.1 & $9.2\times 10^{-6}$ \\
  & (1, 1) & 23.695 & 2.9 & \dots & \dots &$d$ &
  54 & 0.02 & 0.87 & $2.5\times 10^{-10}$ \\
  & (2, 2) & 23.723 & 2.0 & \dots & \dots & $d$ &
  47 & 0.01 & 0.50 & $1.7\times 10^{-10}$ \\
   & (3, 3) & 23.870 & 1.4 & \dots & \dots &$d$ &
  $-5.1$ & $-0.25$ & 2.2 & $9.0\times 10^{-10}$ \\
 & (4, 4) & 24.139 & 0.15 & \dots & \dots & $d$ &  46 & 0.001 & 0.06 & $3.8\times 10^{-11}$ \\ 
 $^{13}$CO & 2--1 & 220.399 & 12 & \dots & 3.3 & $e$&
  55 & 0.2 & 9.1 & $4.3\times 10^{-6}$ \\
  &  5--4 & 550.926 & 14.7 & \dots & 3.2 &$c$ &
  51 & 0.5 & 14.7 & $1.1\times 10^{-4}$ \\
 CS & 2--1 & 97.981 & 8.6 & \dots & 3.0 & $f$ &
  47 & 0.2  & 9.1 & $3.5\times 10^{-7}$ \\
 & 3--2 & 146.969 & 10.2 & \dots & 3.0 & $f$ &
  20 & 1.0 & 10.0 & $1.3\times 10^{-6}$ \\
 &  5--4 & 244.936 & 5.4 & \dots & 3.0 & $f$ &
  13 & 1.3 & 5.5 & $3.3\times 10^{-6}$ \\
  &  6--5 & 293.912 & 3.3 & \dots & 2.2 & $f$ &
  12  & 0.7 & 3.1 & $3.3\times 10^{-6}$ \\
 $[$C I$]$ &  1--0 & 492.161 & 3.7 & \dots & 3.5 & $g$ & 
  54 & 0.1 & 3.7 & $2.1\times 10^{-5}$ \\
 $[$C II$]$ & 3/2--1/2 & 1900.537 & \dots & $4.4\times 10^{-3}$ & \dots
   & $h$ & 55 & 1.0 & 13.4 & $4.4\times 10^{-3}$ \\
 $[$O I$]$ & 1--2 & 4744.778 & \dots & $4.1\times 10^{-3}$ & \dots &
   $h$ & 46 & 0.9 & 0.88 & $4.5\times 10^{-3}$ \\
\hline
\end{tabular}}
\medskip
$^a$ All results for $T_{\rm K}$ = 55~K, $n({\rm H}_2)$~= 4~\x~10$^5$~cm$^{-3}$. $T_{\rm b}$~= Rayleigh-Jeans brightness temperature,  $4\pi I$~= flux, $\Delta \upsilon$~= line width, $T_{\rm ex}$~= excitation temperature,  $\tau$~= line-centre peak opacity; $^b$~from H$_2$O; $^c$~this work; $^d$~\citet{1986AetA.157.207.Ungerechts.etal}; $^e$~\citet{1993AetA...277..595M.Minchin.etal};   $^f$~\citet{1984ApJ...276..625S}; $^g$~\citet{1994ApJ...425L..49P}; $^h$~\citet{1996AetA...315L.285E}.
\end{table}