\begin{table}%t2
\caption{\label{tab2}Description of the observations.}
\small%\centering
\par
\begin{tabular}{lr ccc} 
 \hline \hline\noalign{\smallskip}
\multicolumn{1}{c}{Line} &       
\multicolumn{1}{c}{Freq. (MHz)} &
\multicolumn{1}{c}{{\it HPBW}} &
\multicolumn{1}{c}{$\eta_{\rm MB}$} &
\multicolumn{1}{c}{Tel} 
  \\ \hline\noalign{\smallskip}
C$^{18}$O~   ~1$\rightarrow$0   &  109~782.2   &   22$''$      &  0.75 & IRAM  \\
C$^{18}$O~   ~2$\rightarrow$1   &  219~560.3   &   11$''$      &  0.55 & IRAM  \\
C$^{17}$O~   ~1$\rightarrow$0   &  112~359.3   &   22$''$      &  0.74 & IRAM  \\
C$^{17}$O~   ~2$\rightarrow$1   &  224~714.4   &   11$''$      &  0.54 & IRAM  \\
N$_2$H$^+$   ~1$\rightarrow$0   &   93~173.2   &   26$''$      &  0.77 & IRAM  \\
N$_2$H$^+$   ~4$\rightarrow$3   &  372~672.5   &   13$''$      &  0.63 & JCMT  \\ 
N$_2$D$^+$   ~2$\rightarrow$1   &  154~217.1   &   16$''$      &  0.67 & IRAM  \\ 
N$_2$D$^+$   ~3$\rightarrow$2   &  231~321.7   &   11$''$      &  0.52 & IRAM  \\\hline
\end{tabular}
\end{table}