\begin{table}%T4
\caption{Two-phase photoionization model for cloud at $z=2.517032$.}
\label{tab_photo}      
%\centerline
 {\small                          
\begin{tabular}{l  r r r }        
\hline\hline                 
 & Observed& \multicolumn{2}{c}{Model}\\
        \cline{3-4}
   &      & Low & High\\    
\hline               
[C/H]         &      &     $-2.20$ &   $\la $$-2.10$  \\
$\log U$      &      &     $-1.15$ &   $ -0.65$     \\
$\log n$ [cm$^{-3}$]&&    $-3.75$  &   $ -4.3$     \\
Size [kpc]    &      &   20        &   $\la $$250$ \\
{[C/O]}       &      &      0$^a$      &   $\approx $$-0.8$    \\
{[C/N]}       &      &      ...    &   $\ga $$0.2$    \\
$\log N(\ho)$ &14.88 &     14.58   &   14.58    \\
$\log N(\ct)$ &12.50 &     12.50   &   $\approx $$12.00$    \\
$\log N(\cf)$ &12.73 &     12.43   &    12.43     \\
$\log N(\os)$ &13.92 &     12.21   &    13.91     \\
$\log N(\nf)$ &$<$12.00 &     11.60   &   $\la $$12.00$      \\
\hline                                   %inserts single line
\end{tabular}}
\smallskip
$^a$ Assumed.
\end{table}