\begin{table}%t5
  \caption{\label{tab:dusterr}Range of reliability of the physical parameters of the grains
             (from errors in the fitting and variations in the assumed 
              geometry).}
  %\centering
\par
    \begin{tabular}{llll}
      \hline
      \hline
        \multicolumn{2}{c}{} & \multicolumn{1}{l}{ Lower limit}
                             &   Upper limit  \\
      \hline
        PAH  & $Y$       & $0$              & $1.0\times 10^{-6}$  \\
      \hline
          VSG  & $\alpha$  & $2.6$            & $5.2$            \\
                 & $a_+$     & $35$ \AA         & $120$ \AA        \\
                 & $Y$       & $1.3\times 10^{-5}$  & $2.3\times 10^{-5}$  \\
      \hline
          BG   & $\alpha$  & $5.8$            & $35$             \\
                 & $a_-$     & $21$ \AA         & $29$ \AA         \\
                 & $Y$       & $3.5\times 10^{-4}$  & $4.7\times 10^{-4}$  \\
      \hline
    \end{tabular}
    \end{table}