\begin{table}%t5
\caption{Mass and $H_2$ column- and  volume-density estimates from 
lines. Molecular 
abundances relative to $\rm H_2$ have been assumed equal to 1.2~$\times$ $10^{-7}$, 3.4~$\times$
$10^{-8}$ and $2 \times 10^{-9}$ for \CII , \CIII\ and \ace , respectively.
$N_{\rm H_2}$ and $\Delta v_{1/2}$ are the ${\rm H_2}$ column density and the FWHM of the lines,
respectively.}
\label{tmass}
%\centering
\par
\begin{tabular}{cccccccc}
\hline \hline
 Tracer  & $\theta_{\rm s}$  & $D$ & $N_{\rm H_2}$& $\Delta v_{1/2}$  & \mvir\ & 
$M_{{\rm line}}$ & $n_{\rm H_{2}}$ \\
   & (\asec\ )   & (pc) & (\cmq\ ) & (\kms\ )  &   ($M_\odot$) &  ($M_\odot$) & (\cmc\ ) \\
\hline
\CIII\ (2--1) (PdBI) & 1.3 & 0.03 & 5.4 $\times$ $10^{24}$ & 2.9 & 30 & 42 & 6.0 $\times$ 10$^7$ \\
\ace\ (13--12) (PdBI) & 1.4 & 0.033 & 3.3 $\times$ $10^{24}$ & 3.0 & 32 & 15 & 1.7 $\times$ 10$^{7}$ \\
\CII\ (1--0) (PdBI) & 1.9 & 0.045 & 4.3 $\times$ $10^{24}$ & 3.4 & 55 & 69 & 3.2 $\times$ 10$^7$ \\
HCO$^{+}$(1--0)($^{\dagger}$) & 7 & 0.16 & 1.66 $\times$ $10^{24}$ & 3.3  & 200 & 370 & 3.4 $\times$ $10^6$ \\ 
\CIII\ (2--1) (30 m) & 15 & 0.35 & 1.1 $\times$ $10^{23}$ & 2.9 & 314 & 110 & 1.0 $\times$ 10$^{5}$ \\
\CII\ (1--0) (30 m) & 18 & 0.43 & 1.0 $\times$ $10^{23}$ & 2.4 & 259 & 200 & 1.1 $\times$ 10$^{5}$ \\
\hline
\noalign{\smallskip}
\multicolumn{8}{l}{\hspace*{-4mm}($^{\dagger}$) From Molinari et~al. (\cite{mol02}).}
\end{tabular}
\end{table}