\begin{table}%t6
\caption{\label{tab:abundances}Inferred abundances [x]=$N$(x)/$N_\emr{H}$ where $N_\emr{H}$=$N$(H)+2$N$(H$_2$).} 
%\centerline
 {\small   \begin{tabular}{lcc} 
 \hline \hline
 Species          &  Shielded core          &   PDR \\
 &  $A_\emr{V} \geq 6$   &   $A_\emr{V}=0$--2\\
  &  $\delta x \simeq 45''$  &   $\delta x \simeq 15''$\\
\hline \noalign{\smallskip}
$N_\emr{H}$ (cm$^{-2}$)  & $5.8 \times 10^{22}$    &  $3.1 \times 10^{22}$\\ 
\emr{[H^{13}CO^+]}       & $6.5 \times 10^{-11}$   &  $1.5 \times 10^{-11}$\\
\emr{[H^{12}CO^+]}       & $3.9 \times 10^{-9}$    &  $9.0 \times 10^{-10}$\\
\emr{[DCO^+]}            & $8.0 \times 10^{-11}$   &  (--)\\
\emr{[HOC^+]}            & (--)  &  $0.4 \times 10^{-11}$~$^{\dagger}$\\
\emr{[CO^+]}            & (--)  &   $\leq$$5.0 \times 10^{-13}$\\ 
\emr{[e^-]}              & $(1{-}8)\times 10^{-9}$   &  $10^{-6}{-}10^{-4}$\\
\hline
\end{tabular}}
\medskip
$^{\dagger}$ Assuming extended emission. It would be $1.2 \times 10^{-11}$ if HOC$^+$ arises from a 12$''$--width filament as HCO (\cite{ger09}).
\end{table}