\begin{table}%t3
\caption{Column densities for the various molecules from statistical
equilibrium calculations.}\label{tab3}
%\centerline
{
\renewcommand{\arraystretch}{1.2}
\begin{tabular}{lll}\hline\hline
Molecule & $N({\rm core)}$$^{a}$ & $N({\rm wing})$$^{a}$ \\
                & [cm$^{-2}$]  & [cm$^{-2}$] \\ \hline
CO$^{b}$        & 5.1$~\times~ 10^{17}$  & 1.2$~\times~ 10^{17}$ \\ 
CH$_3$OH        & 3.4$~\times~ 10^{15}$  & 5.8$~\times~ 10^{15}$ \\ 
CS              & 5.4$~\times~ 10^{13}$  & 3.2$~\times~ 10^{13}$ \\
HCN             & 1.1$~\times~ 10^{13}$  & 5.7$~\times~ 10^{12}$ \\ 
HCO$^+$         & 6.6$~\times~ 10^{12}$  & 3.5$~\times~ 10^{12}$ \\ 
H$_2$CO         & 8.9$~\times~ 10^{13}$  & 1.8$~\times~ 10^{13}$ \\ 
N$_2$H$^+$      & 9.0$~\times~ 10^{12}$  & $\ldots$    \\
SiO             & $\ldots$     & 1.4$~\times~ 10^{14}$ \\
SO              & 6.2$~\times~ 10^{13}$  & 1.3$~\times~ 10^{14}$ \\ \hline
\end{tabular}}
\medskip
Notes: $^{a}$Calculated for $n_{\rm H_2} = 1\times 10^6~{\rm cm}^{-3}$
and $T=20$~K (core) and $T=80$~K (wing). $^{b}$Based on $^{12}$CO
measurements for the column densities in the wings and C$^{18}$O
measurements for the core (assuming a $^{16}$O/$^{18}$O isotope ratio
of 540).
\end{table}