\begin{table} %t8
\caption{Optical depths estimates from line ratios between isotopologues.}
\label{moldisks:optical_depth}
%\centerline
{
\begin{tabular}{lllll}
\hline 
\hline
\noalign{\smallskip}
 &  \multicolumn{1}{c}{LkCa15}&\multicolumn{1}{c}{\object{TW~Hya}}&\multicolumn{1}{c}{\object{HD~163296}}&\multicolumn{1}{c}{MWC 480}\\
\noalign{\smallskip}
\hline
$^{12}$CO $J\!=\!3\!\rightarrow\!2$         & \phantom{$<$}26.8  & \phantom{$<$0}8.3   & \phantom{$<$}18.9       & \phantom{$<$}14.9\\
$^{13}$CO $J\!=\!3\!\rightarrow\!2$         & \phantom{$<$0}0.44 & \phantom{$<$0}0.14  & \phantom{$<$0}0.31      & \phantom{$<$0}0.25\\
HCO$^+$ $J\!=\!4\!\rightarrow\!3$           & \phantom{$<$}19.4  & \phantom{$<$0}{3.8}   & \multicolumn{1}{c}{...} & \multicolumn{1}{c}{...} \\
H$^{13}$CO$^+$ $J\!=\!4\!\rightarrow\!3$    & $<$\ 0.32 & \phantom{$<$0}0.06  & \multicolumn{1}{c}{...} & \multicolumn{1}{c}{...} \\
HCN $J\!=\!4\!\rightarrow\!3$               & \phantom{$<$0}0.75 & $<$\phantom{0}5.1   & \multicolumn{1}{c}{...} & \multicolumn{1}{c}{...}\\
H$^{13}$CN $J\!=\!4\!\rightarrow\!3$        & \phantom{$<$0}0.012& $<$\phantom{0}0.085 & \multicolumn{1}{c}{...} & \multicolumn{1}{c}{...}\\
\hline
\end{tabular}}
\smallskip
  {\it Note.} We assume that the
isotopologues have the same excitation temperature and that 
[$^{12}$C]/[$^{13}$C$]=60$.\\
\end{table}