\begin{table}%t1
%\centering
\par
\caption{\label{tab_observation}Summary of HIFI observations.}
{\scriptsize
\begin{tabular}{llcccc}
\hline \hline \noalign{\smallskip} 
\multicolumn{1}{l}{Line} & \multicolumn{1}{c}{Freq.} & 
\multicolumn{1}{c}{$T_a \times \tau$} & \multicolumn{1}{c}{$v_{\rm lsr}$} &
\multicolumn{1}{c}{$\Delta v$} & \multicolumn{1}{c}{$\tau$} \\ 
\multicolumn{1}{l}{} & \multicolumn{1}{c}{(GHz)} & 
\multicolumn{1}{c}{(K)} & \multicolumn{1}{c}{(km~s$^{-1}$)} &
\multicolumn{1}{c}{(km~s$^{-1}$)} & \multicolumn{1}{c}{} \\ 
\hline
CH  &   536.761 & 1.81(0.21)  &  10.9(0.1)  &  4.9(0.3) &  3.6(0.6) \\
NH  &   974.478 & --0.24(0.07) &   9.9(0.4)  &  4.9(0.8) &  0.1(0.7) \\ \hline 
\multicolumn{1}{l}{Line} & \multicolumn{1}{c}{Freq.} &
\multicolumn{1}{c}{Area} & \multicolumn{1}{c}{$v_{\rm lsr}$} &
\multicolumn{1}{c}{$\Delta v_{lsr}$} & \multicolumn{1}{c}{$T_{\rm MB}$} \\ 
\multicolumn{1}{l}{} & \multicolumn{1}{c}{(GHz)} &
\multicolumn{1}{c}{($K \times$~km~s$^{-1}$)} & \multicolumn{1}{c}{(km~s$^{-1}$)} &
\multicolumn{1}{c}{(km~s$^{-1}$)} & \multicolumn{1}{c}{(K)}\\ 
\hline
HCO$^+$ & 535.061  &  19.4(4.0)  & 10.4(0.3) &  3.2(0.8) &  5.7 \\
CS  & 538.688  &  1.09(0.04) & 10.5(0.1) &  2.2(0.1)  & 0.4 \\
o$-$H$_2^{18}$O & 547.676  &  0.22(0.03) & 10.7(0.1) & 2.4(0.3) & 0.08 \\
$^{13}$CO & 550.926  &  51.1(2.0)  & 10.6(0.1) &  3.4(0.2)  &  14.0 \\
C$^{18}$O & 548.830. &  14.0(0.4)  & 10.5(0.1) &  2.6(0.1)  &  5.0 \\
\hline
\end{tabular}}
\end{table}