\begin{table}%t4
%\centering
\par
\caption{\label{tab4}Temperature regimes for the onset of some physical
processes relevant to comets, from Meech \& Svoren (\cite{meech1}).}
\begin{tabular}{l c c}
\hline \hline
   & $T$ (K) &~AU\\
\hline
   $I_{\rm h}$ (crystalline water ice) sublimation & 180 & 3.5--4  \\
   $I_{\rm a}$ (amorphous water ice) phase transition & 90--160 & 11  \\
   $\rm CO_2$ sublimation & 80 & 13  \\
   $\rm CO$ sublimation & 25 & 120    \\
\hline
\end{tabular}
\end{table}