\begin{table}%t2
\caption{Column densities of selected molecules in IRC +10216.}
\label{table-columndensities} 
\small%\centerline
 {
\begin{tabular}{llc@{ }cc}
\hline \hline
%&&&& \\[-8pt]
\multicolumn{1}{l}{Molecule} & \multicolumn{1}{c}{$T_{\rm rot}$ (K)} & \multicolumn{3}{c}{$N_{\rm tot}$ (cm$^{-2}$)}    \\
\hline
                    &                     & \multicolumn{1}{c}{observed} & \multicolumn{2}{c}{calculated} \\
\cline{3-5}
                    &                     &   & \multicolumn{1}{c}{This work} & \multicolumn{1}{c}{[1]} \\
\cline{4-5}
HC$_3$N    & 15     &  2.2(15)$^\textnormal{\scriptsize{[2]}}$ & 2.8(15) & 3.6(15) \\
\textbf{CH$_2$CHCN} & 46     &  5.5(12) &       1.4(13) & 2.2(11) \\
\textbf{CH$_2$CN}   & 50     &  8.6(12) &       4.6(12) & 1.4(13) \\
CH$_3$CN            & 40     &  3.0(13)$^\textnormal{\scriptsize{[2]}}$       & 6.3(12) & 6.8(12) \\
CH$_3$C$_3$N        & 30$^a$ & $<$1.3(12)$^\textnormal{\scriptsize{[2]}}$    & 1.2(11) & 1.4(12) \\
\textbf{CH$_3$CCH}  & 30$^a$ & 1.8(13) &      1.1(12) & 8.0(12) \\
CH$_3$C$_4$H        & 30$^a$ & $<$9.7(12)$^\textnormal{\scriptsize{[2]}}$    & 8.2(12) & 9.0(12) \\
H$_2$CO             & 28     &  5.4(12)$^\textnormal{\scriptsize{[3]}}$ & 2.8(12) & --      \\
\textbf{H$_2$CS}    & 30$^a$ &  1.0(13) &       1.3(12) & 4.4(11) \\
\hline
\end{tabular}}
\medskip
Note. $a(b)$ refers to $a$~$\times$ 10$^b$. A superscript~``$a$''
indicates an assumed rotational temperature. \\
References. [1]: \citet{mil00}; [2]: from unpublished IRAM 30-m data; [3]: from the
observations by \citet{for04}.
\end{table}