\begin{table}%t6
 \caption{\label{tau-ratio}Line opacity ratios derived from our PdBI observations.} 
\small%\centering
\par
\begin{tabular}{lcc}
\hline \hline
\multicolumn{1}{c}{Lines}                           & Velocity & Line opacity    \\
		                            & interval &  ratios         \\
\hline\noalign{\smallskip}
HCO$^+$/H$^{13}$CO$^+$ ($2\leftarrow 1$)          & SW~wings & 28 (3)  \\ 
HCN/H$^{13}$CN ($2\leftarrow 1$)                  & SW~wings & 40 ($-$5+7) \\
HNC/HN$^{13}$C ($2\leftarrow 1$)                  & SW~      & 27 (3)  \\
H$^{13}$CN/HC$^{15}$N ($2\leftarrow 1$)           & SW~      & 4.2 (0.3) \\
HN$^{13}$C/H$^{15}$NC ($2\leftarrow 1$)           & SW~      & 6 ($-$2+4) \\
HNC/H$^{15}$NC ($2\leftarrow 1$)                  & SW~      & 166 ($-$58+194) \\
HCO$^+$/HC$^{18}$O$^+$ ($2\leftarrow 1$)          & SW~wings & 53 ($-$10+16) \\
H$^{13}$CO$^+$/HC$^{18}$O$^+$ ($2\leftarrow 1$)   & SW~      & 2.01 (0.07) \\
HC$^{18}$O$^+$/HC$^{17}$O$^+$ ($2\leftarrow 1$)   & SW~      & 12 ($-$2+3) \\
CS/C$^{34}$S ($4\leftarrow 3$))                   & SW~      & 10.4 ($-$0.7+0.8) \\
H$_2$S/H$_2$$^{34}$S ($1_{10} \leftarrow 1_{01}$) & SW~      & 8 (1.5) \\
\hline
\end{tabular}
\end{table}