\begin{table}%t3
  \caption{\label{radex abundances}Results of the homogeneous {\tt RADEX} model for the narrow PDR~component: column densities and abundances$^{a}$.}
    \small%\centerline
 {
     \begin{tabular}{l c c  }
     \hline\hline
Species & Column Density & Abundance  \\
 $x$ & $N({x})$ & $X({x})$   \\
 & [cm$^{-2}$] &  \\
     \hline
&& \\[-8pt]
 H$_2$O & $2.0\times 10^{14}$ & $ 9.8\times10^{-9}$  \\   
 H$_2^{18}$O & $<$$4.9\times 10^{12}$ & $<$$2.5 \times 10^{-10}$  \\ 
 NH$_3$ & $1.5\times 10^{14}$ & $7.5\times10^{-9}$   \\ 
 $^{12}$CO & $2.0\times 10^{18}$ & $ 1.0\times10^{-4}$   \\
 $^{13}$CO & $2.9\times 10^{16}$ & $ 1.4\times10^{-6}$  \\ 
 CS & $1.5\times 10^{14}$ & $ 7.5\times 10^{-9}$   \\
 C & $2.0\times 10^{17}$ & $ 1.0\times 10^{-5}$  \\
 C$^+$ & $9.0\times 10^{17}$ & $4.5 \times 10^{-5}$   \\
 O & $6.3\times 10^{17}$ & $ 3.2\times 10^{-5}$   \\
      \hline
     \end{tabular}}
\medskip
$^{{a}}$ All results for $T_{\rm K}$~= 55~K, $n({\rm H}_2)$~= 4~\x~10$^5$~cm$^{-3}$, $n({\rm e}^-)$~= 7.7~cm$^{-3}$, $N({\rm H}_2)$~= 2.0~\x~10$^{22}$~cm$^{-2}$.
\end{table}