\begin{table}%t2
\caption{\label{TabDiffPar}Physical conditions and elemental abundances of the gas in the TDR~models. }
%\centering
\small
\begin{tabular}{l l l l c}
\hline\hline
&&&& \\[-8pt]
Density                & $n_{\rm H}$  & cm$^{-3}$ & 10--200  &  \\
Radiation field        & $\chi$ &           &  1  &  \\
Extinction             & $A_{\rm V}$  & mag       & 0.1--1   &   \\
Cosmic ray ionization rate & $\zeta$ & s$^{-1}$ & $3 \times 10^{-17}$ & a \\ 
\hline
Elemental abundances  & & & $\left[\textrm{X}\right]/\left[\textrm{H}\right]$ &\\
Helium                 & $\left[\textrm{He}\right]$ & & $1.00 \times 10^{-1}$ & \\
Carbon                 & $\left[\textrm{C}\right]$  & & $1.38 \times 10^{-4}$ & \\
Oxygen                 & $\left[\textrm{O}\right]$  & & $3.02 \times 10^{-4}$ & \\
Nitrogen               & $\left[\textrm{N}\right]$  & & $7.94 \times 10^{-5}$ & \\
Sulfur                 & $\left[\textrm{S}\right]$  & & $1.86 \times 10^{-5}$ & \\
Iron                   & $\left[\textrm{Fe}\right]$ & & $1.50 \times 10^{-8}$ & \\
\hline
a~-- Dalgarno (\cite{Dalg06}).
\end{tabular}
\end{table}