\begin{table}%t3 
\caption{H$_2$ total column densities (in [$10^{21}$~cm$^{-2}$]) obtained
from $^{13}$CO using a conversion factor of
$N$~(H$_2$)/$N$~($^{13}{\rm CO})=4.8\times10^5$ 
(Dickman~\&~Clemens~\cite{dickmanclemens}) and from C$^{18}$O using the ratio
[C$^{18}$O]/[H$_2$] $=$ 1.7$\times10^{-7}$ (Frerking et~al.~\cite{frerking82}),
and the calculated fractional abundances of the main isotopomers with respect
to [H$_2$] derived from [C$^{18}$O].} 
\label{fa} 
%\centering
$$ 
         \hskip0.15cm \begin{array}{lccccccccccccc} 
\hline\hline 
\noalign{\smallskip} 
\par
 && &&&{\rm H1a} & &{\rm H2a} & &{\rm H2b} 
& &{\rm N1a} & &{\rm T1a}\\  
\hline  
\noalign{\smallskip} 
{\rm [^{13}CO]\Rightarrow [H_2]} && &&& 2.93 && 2.84 && 3.58 && 6.67 && 3.77 \\
{\rm [C^{18}O]\Rightarrow[H_2]} && &&& 6.76 && 11.41 && 9.65 && 16.06 && 7.12 \\
\noalign{\smallskip} 
\hline  
%^\noalign{\smallskip}  
% && [10^{15}] &&&  & &  & &  & &  & & \\  
\noalign{\smallskip}
{\rm CS}     &&   [10^{-9}]    &&& 1.4 && 1.5 && 2.6 && 1.9 && 3.4 \\ 
{\rm HCN}    &&   [10^{-9}]    &&& 2.9 && 3.6 && 2.9 && 1.0 && 7.1 \\ 
{\rm HNC}    &&   [10^{-9}]    &&& 5.0 && 5.2 && 5.0 && 2.6 &&
\hspace{-.2cm}14.9  \\ 
{\rm HCO^+}  &&   [10^{-9}]    &&& 4.6 && 3.9 && 3.0 && 4.1 && 4.9   \\ 
{\rm SO}     &&   [10^{-9}]    &&& 2.8 && 0.7 && 1.0 && 0.5 && 1.3 \\ 
{\rm NH_3}   &&   [10^{-7}]    &&& 2.6 &&  -  && 1.0 && -   && 3.1 \\ 
\noalign{\smallskip} 
\hline  
	\end{array} 
$$  
\end{table}