\begin{table}%T6
\caption{Beam-averaged results.}
\label{m51beamav}
%\centerline
 {
\begin{tabular}{lccccccc}
\hline\hline
\noalign{\smallskip} 
Model & \multicolumn{3}{c}{Beam-averaged column densities} & \multicolumn{2}{c}{Face-on mass density}       & Relative mass & Ratio $N(\hdeux)/I({\rm CO})$ \\
   & $N$(CO) & $N$(C)    & $N(\hdeux)$ & $\sigma(\hdeux)$ & $\sigma_{\rm gas}^{a}$ & components 1:2 & $X$ \\
   & \multicolumn{2}{c}{($10^{18} \cmdeux$)} & ($10^{21} \cmdeux$) & \multicolumn{2}{c}{($\Msun$/pc$^{-2}$)} & & ($10^{20}$~mol$\cmdeux/\kkms$)\\
\noalign{\smallskip}
\hline
\noalign{\smallskip}
\multicolumn{8}{c}{M~51; $N_{\rm H}/N_{\rm C}^{b}$ = 1250; $N$(HI)$^{c} = 2\times 10^{20} \cmdeux$} \\
\noalign{\smallskip}
\hline
\noalign{\smallskip}
1  & 0.36  & 4.4  & 2.9  & 43  & 60  & 0.6 : 0.4  & 0.74 \\
2  & 0.36  & 1.2  & 1.0  & 15  & 20  & 0.4 : 0.6  & 0.25 \\
3  & 0.42  & 3.3  & 2.2  & 33  & 47  & 0.8 : 0.2  & 0.41 \\
\hline
\end{tabular}}
\smallskip
Note: $^a$ Sum of $\hdeux$ and HI multiplied by 1.35 to take into account
the contribution by helium; $^b$ See text, Sect. 4.2; $^c$ HI column
density valid for both Center and (nearby) Arm; Tilanus \& Allen
(\cite{Tilanus91}).
\end{table}