\begin{table} %t11
\caption{Relative molecular abundances.} \label{tab:abun_rel}
\par
%\centerline
{
\begin{tabular}{llll}
\hline
\hline
\noalign{\smallskip}
\multicolumn{1}{c}{Source} & \multicolumn{1}{c}{$N$(HCO$^+$)} & \multicolumn{1}{c}{$N$(CN)}&\multicolumn{1}{c}{$N$(DCO$^+$)}\\
\cline{2-2}
\cline{3-3}
\cline{4-4}
\noalign{\smallskip}
                            & \multicolumn{1}{c}{$N$(CO)}      & \multicolumn{1}{c}{$N$(HCN)}&\multicolumn{1}{c}{$N$(HCO$^+$)}\\
\hline
\object{LkCa~15}           & \phantom{$>$}1.6($-$5) & \phantom{$>$}\ 7.9 & $<$0.411\\
\object{TW~Hya}            & \phantom{$>$}3.8($-$4)$^1$ & \phantom{$>$}\ 7.1& \phantom{$>$}0.035\\
                           & \phantom{$>$}{1.4($-$4)$^2$} &  &\\
DM~Tau$^a$        & \phantom{$>$}5.3($-$5) & \phantom{$>$}\ 5.8&\\
DM~Tau$^b$        & \phantom{$>$}7.6($-$5) & \phantom{$>$}18.4&\\
\hline
\object{HD~163296}         & \phantom{$>$}2.7($-$5) & $>$12.4&\\
\object{MWC~480}           & \phantom{$>$}1.5($-$5) & $>$11.7&\\
\hline
\object{IRAS 16293-2422}$^c$   & \phantom{$>$}3.6($-$5) & \phantom{$>$}\ 0.07& \\
TMC-1$^d$             & \phantom{$>$}1.0($-$4) & \phantom{$>$}\ 1.5&\\
Orion Bar$^e$         & \phantom{$>$}2.0($-$5) & \phantom{$>$}\ 3.8&\\
IC~63$^f$             & \phantom{$>$}2.7($-$5) & \phantom{$>$}\ 0.7&\\
\hline
\end{tabular}}
\smallskip
  References.\ $^a$ Dutrey et~al.\ (\cite{Dutrey97}); $^b$ from Table 7;
  $^c$ Sch\"oier et~al.\ (2002); $^d$ Ohishi et~al.\ (\cite{Ohishi92});
  $^e$ Hogerheijde et~al.\ (\cite{Hogerheijde95}); $^f$ Jansen et~al.\   (\cite{Jansen95}). \\
{\it Notes.}\ $^1$ Value derived from
  H$^{13}$CO$^+$, assuming [$^{12}$C]/[$^{13}$C] = 60.  
$^2$~Value derived from HCO$^+$.
\end{table}