\begin{table}%t4
\caption{Selected gas-phase fractional abundances with respect to~H$_2$, determined at the time of maximum HCOOCH$_3$~abundance$^{a}$.}
\label{tab4}
\small%\begin{center}
{
\begin{tabular}{l c c c c}
\hline\hline
Species   &  \multicolumn{4}{c}{Model} \\
              & $t_{\rm f}=5 \times 10^{4}$ yr & $t_{\rm f}=2 \times 10^{5}$ yr & $t_{\rm f}=1 \times 10^{6}$ yr & $t_{\rm f}=2 \times 10^{5}$ yr \\
              & $T=T_2(t)$ & $T=T_2(t)$ & $T=T_2(t)$ & $T=T_1(t)$ \\
\hline
HCOOCH$_3$ (all reactions)       & 1.9$(-9)$  & 7.8$(-9)$  & 1.9$(-8)$  & 5.3$(-9)$ \\
HCOOCH$_3$ (no gas-phase routes) & 1.0$(-10)$ & 5.5$(-9)$  & 1.9$(-8)$  & 3.2$(-9)$ \\
HCOOH                            & 5.5$(-8)$  & 1.1$(-7)$  & 1.2$(-7)$  & 1.2$(-7)$ \\
CH$_3$OCH$_3$                    & 6.1$(-9)$  & 1.4$(-8)$  & 1.1$(-9)$  & 2.2$(-8)$ \\
H$_2$CO                          & 2.3$(-5)$  & 2.3$(-7)$  & 3.9$(-10)$ & 1.5$(-8)$ \\
CH$_3$OH                         & 3.2$(-5)$  & 1.7$(-5)$  & 5.9$(-7)$  & 1.8$(-5)$ \\
\hline 
\end{tabular}}
\medskip
\parbox{13cm}{
$^{a}$ $a(b) = a \times 10^{b}$.}
%\end{center}
\end{table}