\begin{table}%t1
\caption{\label{tobs}CO$^+$ column densities $N$ derived from observations toward PDRs.}            
%\centerline
{\begin{tabular}{l c c c c}        
\hline\hline\noalign{\smallskip}
                Object  & $n({\rm H_2})$ & $G_0$ & $N$  & $N$/$2\times 10^{21}$ \\   
        & [cm$^{-3}$] & [Habing] & [$10^{12}$~cm$^{-2}$] &  [$10^{-9}$] \\  
\hline    \noalign{\smallskip}                    
M$17$SW & $5 \times 10^5$ & $5 \times 10^4$ & $1$--$1.8$ & $0.5$--$0.9$ \\
Orion Bar & $2.5 \times10^5$ & $5 \times 10^4$ & $1$--$2.7$ & $0.5$--$1.35$ \\
S$140$ & $10^5$ & $150$ & $0.03$ & $0.015$  \\
NGC~$7023$ & $10^4$--$10^5$ & $2 \times 10^3$ & $0.3$ & $0.15$  \\
Mon~R$2$ & $1.5\times 10^6$ & $4.9 \times 10^5$ & $0.53$ & $0.27$  \\
G$29.96$--$0.02$& $7\times 10^5$ & $1.5 \times 10^5$ & $0.47$ & $0.24$  \\
\hline  
\end{tabular}}
\medskip
\par
$^a$\citet{LaWM93,StSS95}; $^b$\citet{StSS95,FuRG03,SaZi04}; $^c$\citet{PaMi95,StSS95,SaZi04}; $^d$\citet{FuMa97,FuRG03}; $^e$\citet{RiFR03}. 
\end{table}