\begin{table}%t5
%\centering
\par
\caption{\label{tab-pdr_std_par}Standard conditions and gas-phase elemental  abundances.
Molecular, atomic and electron abundances, noted [x], refer to H.}
\small \begin{tabular}{l c} 
\hline \hline  
Parameter  & Value    \\
\hline  
Radiation field $\chi$  & 60 (Draine units) \\
Density $n_{\rm H}(r)=n(\emr{H})+2n(\emr{H_2})$ & $\propto r^{-3}$, up to $\sim$$2 \times 10^5$~cm$^{-3}$ \\
Line of sight depth $l_{\rm depth}$ &  0.1~pc  \\
\emr{[He]}~=~$n(\emr{He})/n_\emr{H}$  &  $1.00 \times 10^{-1}$\\
\emr{[O]}  &  $3.02 \times 10^{-4}$  \\
\emr{[^{12}C]}  &  $1.38 \times 10^{-4}$  \\
\emr{[N]}   &  $7.95 \times 10^{-5}$ \\
\emr{[D]}   &  $1.60 \times 10^{-5}$ \\
\emr{[S]}   &  $3.50 \times 10^{-6}$ \\
\emr{[^{13}C]}=\emr{[^{12}C]}/60  &  $2.30 \times 10^{-6}$\\
\emr{[PAH]}   &  variable: 0--$10^{-7}$ \\
\emr{[metals] \equiv [M] \equiv [Fe+Mg+...]}  &  variable: $10^{-11}$--$10^{-5}$ \\
Cosmic-ray ionization rate $\zeta$  &  variable: $10^{-18}$--$10^{-15}$~s$^{-1}$ \\ 
\hline
\end{tabular}
\end{table}