\begin{table} %t10
   \caption{Beam-averaged molecular abundances with respect to H$_{\rm 2}$
 for the adopted disk sizes.}\label{tab:mol_abundances}
\par
 %\centerline
{
 \begin{tabular}{llllllll}
 \hline
\hline
  &  \multicolumn{7}{c}{$X=N/ N({\rm H_{\rm 2}})$}\\
 \noalign{\smallskip}
 \cline{2-8}
 \noalign{\smallskip}
 \multicolumn{1}{c}{Species} &
  \multicolumn{1}{c}{LkCa15}&\multicolumn{1}{c}{TW
  Hya}&\multicolumn{1}{c}{\object{HD~163296}}&\multicolumn{1}{c}{MWC
  480}&\multicolumn{2}{c}{DM~Tau}&\multicolumn{1}{c}{\object{IRAS 16293-2422}$^b$}\\ 
 \cline{6-7}
 & & & & & This work$^a$ & Dutrey et~al. & \\
 \hline
 CO               &   \phantom{$<$}3.4($-$07)     & \phantom{$<$}5.7($-$08) & \phantom{$<$}3.1($-$07)  & \phantom{$<$}6.9($-$07) &  \phantom{$<$}9.6($-$06)  & \phantom{$<$}1.5($-$05) &\phantom{$<$}4.0($-$05)\\
 HCO$^{+}$        &   \phantom{$<$}5.6($-$12)     & \phantom{$<$}2.2($-$11)$^c$  & \phantom{$<$}7.8($-$12) & \phantom{$<$}1.0($-$10) &\phantom{$<$}7.4($-$10)&\phantom{$<$}7.4($-$10)  &\phantom{$<$}1.4($-$09)\\ 
 H$^{13}$CO$^{+}$ &    $<$2.6($-$12) & \phantom{$<$}3.6($-$13)  & \multicolumn{1}{c}{...}  &\multicolumn{1}{c}{...} &$<$3.6($-$11) &$<$3.6($-$11) &\phantom{$<$}2.4($-$11)\\
 DCO$^{+}$        &    $<$2.31($-$12)& \phantom{$<$}7.8($-$13)  & \multicolumn{1}{c}{...} & \multicolumn{1}{c}{...}&\multicolumn{1}{c}{...} &\multicolumn{1}{c}{...} &\phantom{$<$}1.3($-$11)\\ 
 CN               &   \phantom{$<$}2.4($-$10)     & \phantom{$<$}1.2($-$10)  & \phantom{$<$}1.3($-$10)   &\phantom{$<$}1.4($-$10) &\phantom{$<$}9.0($-$09) &\phantom{$<$}3.2($-$09) &\phantom{$<$}8.0($-$11)\\ 
 HCN              &   \phantom{$<$}3.1($-$11)     & \phantom{$<$}1.6($-$11)  & $<$9.1($-$12) & $<$1.1($-$11) &\phantom{$<$}4.9($-$10) &\phantom{$<$}5.5($-$10) &\phantom{$<$}1.1($-$09)\\ 
 H$^{13}$CN       &    \multicolumn{1}{c}{...}     & $<$8.4($-$13)  & \multicolumn{1}{c}{...} &\multicolumn{1}{c}{...}   & \multicolumn{1}{c}{...} &\multicolumn{1}{c}{...} & \phantom{$<$}1.8($-$11)\\
 HNC              &    \multicolumn{1}{c}{...}     & $<$2.6($-$12) & \multicolumn{1}{c}{...}      & \multicolumn{1}{c}{...}&\phantom{$<$}1.5($-$10) &\phantom{$<$}2.4($-$10)&\phantom{$<$}6.9($-$11)\\  
 DCN              &    \multicolumn{1}{c}{...}     &  $<$7.1($-$14) & \multicolumn{1}{c}{...}      & \multicolumn{1}{c}{...}&\multicolumn{1}{c}{...} &\multicolumn{1}{c}{...} &\phantom{$<$}1.3($-$11)\\ 
 CS               &   $<$8.5($-$11)     &  \multicolumn{1}{c}{...}     & \multicolumn{1}{c}{...}      & \multicolumn{1}{c}{...}&\phantom{$<$}2.4($-$10)&\phantom{$<$}3.3($-$10)&\phantom{$<$}3.0($-$09)\\ 
 H$_{\rm 2}$CO    &  \phantom{$<$}4.1($-$11)      & $<$1.4($-$12)  &  $<$1.0($-$11) & $<$1.4($-$11) &\phantom{$<$}2.4($-$10)&\phantom{$<$}5.0($-$10)& \phantom{$<$}7.0($-$10)\\ 
 CH$_{\rm 3}$OH   &    $<$3.7($-$10) & $<$1.9($-$11)  &  $<$1.5($-$10)  & $<$2.0($-$09) &\multicolumn{1}{c}{...} &\multicolumn{1}{c}{...}          &\phantom{$<$}3.5($-$10)\\ 
 N$_{\rm 2}$H$^+$ &    $<$2.3($-$11) & $<$1.8($-$11)  &  \multicolumn{1}{c}{...}     &$<$1.5($-$11)&$<$5.0($-$09) &$<$2.0($-$10)& \multicolumn{1}{c}{...}\\ 
 H$_{\rm 2}$D$^+$ &    $<$1.5($-$11) & $<$7.8($-$12)  &  \multicolumn{1}{c}{...}     & $<$1.0($-$11)& \multicolumn{1}{c}{...} &\multicolumn{1}{c}{...} & \multicolumn{1}{c}{...}\\ 
 SO               &\multicolumn{1}{c}{...} &$<$4.1($-$11)&\multicolumn{1}{c}{...}&\multicolumn{1}{c}{...}&\multicolumn{1}{c}{...}&\multicolumn{1}{c}{...} & \phantom{$<$}4.4($-$09)\\
 \hline
 \end{tabular}}
 \smallskip
   $^a$ Re-analysis of data from Dutrey et~al.\ (\cite{Dutrey97}), see text.\\ 
   $^b$ Outer envelope abundances from Sch\"oier et~al.\ (\cite{Schoier02}).\\
   $^c$ Value inferred from H$^{13}$CO$^+$. 
   \end{table}