\begin{table}%t4
%%%\centering

\par
\caption{\label{tab:criticdens}Critical densities ($n_{\rm critical}$) at 10~K and 20~K for 
C$^{18}$O.}
%%\centerline

{\begin{tabular}{c c c c }
		\hline 
		\hline
		C$^{18}$O 	& $A_{{\rm u}}$ 	 & $K_{{\rm u}}$				 & $n_{{\rm critical}}$  \\
		transition  & (s$^{-1}$) & (10$^{-11}$~cm$^3$s$^{-1}$)  & (10$^{3}$~cm$^{-3}$) \\
								&						 & 10~K--20~K											&	10~K--20~K \\	
		\hline\noalign{\smallskip}
		$J=1\rightarrow0$	& 6.266 $\times$ 10$^{-8}$ & 3.3--3.3  & 1.9--1.9  \\      
		$J=2\rightarrow1$	& 6.011 $\times$ 10$^{-7}$ & 7.2--6.5  & 8.3--9.3  \\    
		$J=3\rightarrow2$	& 2.172 $\times$ 10$^{-6}$ & 7.9--7.1  & 27--30  \\        
		\hline
		\end{tabular}
}
\medskip		
These values were derived using the information provided by the
Leiden Atomic and Molecular Database \citep{2005yCat..34320369S}: 
$A_{{\rm u}}$ being the Einstein Coefficient for the upper level, and 
$K_{{\rm u}}$ the respective collision rate at both 10~K and 20~K. 
\end{table}