\begin{table}%t1
 \caption{\label{table1}The critical velocity that the downflow must reach in order to drive an enthalpy flux sufficient to sustain the radiative emission.}
 \small%\centering
\par
\begin{tabular}{@{\extracolsep{\fill}}c c c c c c}
\hline\hline
&$T_{\rm a}=1$~MK&3&5&10&20\\
\hline\noalign{\smallskip}
$n=10^9$~cm$^{-3}$&$|v_{\rm critical}|=1.5$~km~s$^{-1}$&1.0&0.9&0.9&0.9\\
$5\times10^9$&7.6&4.9&4.7&4.5&4.4\\
$10^{10}$&15.3&9.9&9.4&9.0&8.9\\
$5\times10^{10}$&76.0&49.0&47.0&45.0&44.0\\
$10^{11}$&153.0&99.0&94.0&90.0&89.0\\
\hline
\end{tabular}
\end{table}