\begin{table}%t1
\caption{Line parameters.}
\small%\centerline
{
\begin{tabular}{lrrr}
\hline \hline
Line & Freq. & $E_{\rm{u}}/k$ & $n_{\rm{crit}}^a$ \\
     & [GHz] & [K]          & [$10^5$~cm$^{-3}$]\\
\hline
HCN(1$-$0), $F=1{-}1$    & 88.630 &   4 & 32 \\
HCN(1$-$0), $F=2{-}1$    & 88.632 &   4 & 32 \\
HCN(1$-$0), $F=0{-}1$    & 88.634 &   4 & 32 \\
CH$_3$CN$(5_4-4_4)$ & 91.959 & 128 & 7\\ 
CH$_3$CN$(5_3-4_3)$ & 91.971 &  78 & 7 \\ 
CH$_3$CN$(5_2-4_2)$ & 91.980 &  42 & 7 \\ 
CH$_3$CN$(5_1-4_1)$ & 91.985 &  20 & 6 \\ 
CH$_3$CN$(5_0-4_0)$ & 91.987 &  13 & 6 \\ 
\hline 
\end{tabular}}
\smallskip
 For all observed lines the columns list (left to right) the species/quantum numbers, frequencies, upper level energy states $E_{\rm{u}}/k$ ($k$ is the Boltzmann constant) and critical densities $n_{\rm{crit}}$. \\
 $^a$ The critical densities $n_{\rm{crit}}=A/\gamma$ are calculated at 60~K (Einstein coefficient $A$ and collisional rate $\gamma$). For HCN, $A$ and $\gamma$ are taken from LAMBDA \citep{schoeier2005}. For CH$_3$CN, $A$ is calculated from $A=0.3\lambda_{100}^{-3}\mu ^2$ ($\lambda_{100}$ in units of 
  100~$\mu$m and $\mu=3.9$~debye), and the corresponding $\gamma$-values are from \citet{pei1995a,pei1995b}. For a line optical depth $\tau >1$, $n_{\rm{crit}}$ has to be multiplied by $1/\tau$ \citep{tielens2005}. 
\label{lines}
\end{table}