\begin{table}%t4 
\caption{Observed and modeled column densities $N$ in AB Aur and DM Tau.}
\label{chi2_fit}
%\begin{center}
{
\begin{tabular}{ccccccccc}
\hline\hline 
Molecule & & \multicolumn{3}{c}{$\chi^2$-minimization  method} & &  \multicolumn{2}{c}{Chemical model} & DM Tau \\
 && $N$ & 1 $\sigma$ & $N$/$N$($^{13}$CO)$^{\rm 1}$ &&  $N$  &  $N$/$N$($^{13}$CO)$^{\rm 2}$ & $N$/$N$($^{13}$CO)$^{\rm 1}$\\
 & & [\cmm] & error & && [\cmm] & & \\
\hline%\noalign{\smallskip}
H$_2$ && 6$\times$10$^{22}$ & $\rm 1\times10^{22}$ & $1.5\times10^{6}$ && $5\times10^{22}$ & $1.3\times10^{6}$ & $1\times10^{7}$  \\
$^{13}$CO$^{(*3)}$&& $4\times10^{16}$  &  $5\times10^{15}$ & 1 && $4\times10^{16}$ & 1                & 1\\
\hco && $6\times10^{12}$  &  $3\times10^{11}$ & $1.5\times10^{-4}$  && $1.5\times10^{13}$ &  $4\times10^{-4}$ & $2\times10^{-3}$\\
HCN && $5\times10^{11}$  &  $3\times10^{11}$ & $1.3\times10^{-5}$  && $4\times10^{11}$ &  $10^{-5}$ & $7\times10^{-4}$\\
CS && $3\times10^{12}$ &  $3\times10^{12}$ & $<$$8\times10^{-5}$  && $2\times10^{11}$ &  $5\times10^{-6}$ & $3\times10^{-4}$\\
\cch && $2\times10^{13}$  &  $2\times10^{13}$ & $<$$5\times10^{-4}$  && $10^{10}$ &  $2.5\times10^{-7}$    & $10^{-3}$\\
\choh && 0 &  $7\times10^{15}$ & $<$$2\times10^{-1}$ && 0 & 0 &  0 \\
\hline
\end{tabular}}
{
$^{\rm 1}$  Relative to the $^{13}$CO column density at 250~AU obtained by the $\chi^2$-minimization method.\\
$^{\rm 2}$  Relative to the $^{13}$CO column density at 250~AU obtained by the chemical modeling.\\
$^{\rm 3}$  See results reported by Pietu et~al.\ (2005).}
%\end{center}
\par
\end{table}