\begin{table}%t1
\caption{Infrared pumping approximations.}
\label{tab:mod}
%\centering
\small
\begin{tabular}{l l}
\hline
\hline
& \\[-8pt]
Model& $J_{\rm int}^{\rm D}$ \\
& \\[-8pt]
\hline
PDR-a   & 0 \\
PDR-b1 & $\beta_{ij}\;\tau^{\rm D}_1\;I^{\rm D}$ \\
PDR-b2 & $\beta_{ij}\;\tau^{\rm D}_2\;I^{\rm D}$ \\
PDR-c   & Eq.~(\ref{eq:J_D}) \\
\hline
\end{tabular}
\end{table}