\begin{table}%t2
\caption{\label{tab:vib}Excitation properties of HC$_3$N bending modes from \citet{wyr99}.}              
%\centerline
{                          % used for centering table
\begin{tabular}{c c c c c}        % centered columns (4 columns)
\hline\hline  \noalign{\smallskip}               % inserts double horizontal lines
Transition & $\lambda_{\rm ex}$ & $A_{u\ell}$ & $n_{\rm cr}$ & Vibration \\    % table heading 
& [$\mu$m] & [s$^{-1}$] & [cm$^{-3}$] &  \\
\hline  \noalign{\smallskip}                        % inserts single horizontal line
$v_6=1$--0 & 20 & 0.15 &  3 $\times$ 10$^{11}$ & CCN bend\\
$v_7=1$--0 & 45 & 6 $\times$ 10$^{-4}$ & 4 $\times$ 10$^{8}$ & CCC bend\\
\hline                                   %inserts single line
\end{tabular}}
\medskip
Notice the large critical densities, see discussion
in the text.
\end{table}