\begin{table}%t3
%\centering
\par
\caption{\label{object_tab} Physical properties and PAH lifetime, $\tau_{0}$, 
          in objects showing PAH emission characteristics.}
        \begin{tabular}{c c c c c c c c c}     % 9 columns 
\hline\hline \noalign{\smallskip}      
           Object  &  $n_{\rm H}$  &  $T$    &  Reference   &   $\tau_{\rm 0}$(ref)  &  $\tau_{\rm 0}$(min)  &  $\tau_{\rm 0}$(max) & $\tau_{\rm object}$   &  Destruction  \\  
                  &  (cm$^{-3}$)  &   K     &             &           (yr)         &       (yr)          &        (yr)         &        (yr)          &    agent       \\
          \hline\\[-3mm]                    
           M~82     &   0.013     &  $5.8\times10^{6}$   &  (a)  &  162                  &  43     &   1302   &    ${\sim}2\times10^{7}$  &   electrons  \\
          M~17     &   0.3       &  $7\times10^{6}$     &  (b)  &  8.2                  &  2      &    71    &    ${\sim}1\times10^{6}$  &   electrons    \\
          N132D   &   10        &  $8\times10^{6}$     &  (c)  &  0.3                  &  0.07   &    2.5   &    ${\sim}3\times10^{3}$  &   electrons         \\
          Orion   &   10$^{4}$  &  $7\times10^{3}$     &  (d)  &  $1.3\times10^{7}$   &  $1.4\times10^{5}$  &  $7.5\times10^{11}$  &  ${\sim}1\times10^{6}$  &  ions    \\ 
           \hline 
        \end{tabular}        
\tablebib{(a) \citet{ran08}; (b): \citet{tow03}; (c): \citet{hwang93}; (d): \citet{tielens05}.}
\end{table}