\begin{table}%t6
\caption{\label{tab6}Model results.}
\small%\centering
\par
\begin{tabular}{llllccc} 
 \hline \hline\noalign{\smallskip}
\multicolumn{1}{c}{Source} &
\multicolumn{1}{c}{Model} &       
\multicolumn{1}{c}{CO~Binding energy} &       
\multicolumn{1}{c}{$\chi$$_{\rm C^{18}O}$\tablefootmark{a}} &
\multicolumn{1}{c}{$\chi$$_{\rm N_2H^+}$} &
\multicolumn{1}{c}{$\chi$$_{\rm N_2D^+}$} &
\multicolumn{1}{c}{}\\ 
 \noalign{\smallskip} \hline
Serpens--FIRS 1   &  Model 3 & 1100   &  4.37  &  3.89  &  4.52  &  only 10\% of CO~evaporated \\
                  & {\bf Model 4} & {\bf 1100}   &  {\bf 2.55}  &  {\bf 3.77}  &  {\bf 4.46}  &  \boldmath{\bf only 10\% survives for $T>100$~K} \\
                   \noalign{\smallskip}
%
Cep E-mm    &  Model 1 & 1100   & 1.48  & 4.12  & 0.71  &  \\ % 9.6
            &  Model 2 & 5000   & 1.51  & 0.86  & 0.45  &  \\ % 10.7
            &  {\bf Model 3} & {\bf 1100}   & {\bf 0.91}  & {\bf 2.26}  & {\bf 1.19}  &  {\bf only 10\% of CO~evaporated} \\
            &  Model 4 & 1100   & 1.42  & 2.19  & 1.12  &  only 10\% survives for $T>100$~K \\ \noalign{\smallskip}
IC~1396~N   &  Model 3 & 1100   & 2.95  & 4.45  & $>$4.3    &  only 10\% of CO~evaporated \\
            &  {\bf Model 4} & {\bf 1100}   & {\bf 1.79}  & {\bf 4.35}  & {\bf $>$4.0}    &   \boldmath{\bf only 10\% survives for $T>100$~K} \\  \noalign{\smallskip}
%
CB3         &  {\bf Model 3} & {\bf 1100}   & {\bf 1.63}  & {\bf 0.76}  & {\bf 0.31}  &  {\bf only 10\% of CO~evaporated} \\
            &  Model 4 & 1100   & 1.24  & 0.86  & 0.51  &  only 10\% survives for $T>100$~K \\ 
            \noalign{\smallskip}
NGC~7129$-$FIRS~2   &  {\bf Model 3} & {\bf 1100}   &  {\bf 0.97} &  {\bf 1.29}  &  {\bf 2.05}   &  {\bf only 10\% of CO~evaporated} \\
                   &  Model 4 & 1100   &  1.39 &  1.48  &  1.73   &  only 10\% survives for $T>100$~K \\ 
\hline  
\end{tabular}
\tablefoot{\tablefoottext{a}{$\chi$ is defined as $\chi = \sqrt[]{[\Sigma (I_{\rm model}-I_{\rm obs})^2]/N} $. Note that this is an absolute error and the comparison
among values in different sources is not\\
straightforward. See Figs.~\ref{fig11} to~\ref{fig18}.}}
\end{table}