\begin{table}%t3
\caption{\label{fit_tab}PAH destruction analytical fit parameters and survival timescales          for electron and ion collisions.}
%\centering
\small
\begin{tabular}{l c c c c c c}     
\hline\hline\noalign{\smallskip}
   &  \multicolumn{2}{c}{Electron}  &  \multicolumn{2}{c}{Ion}  &  \multicolumn{2}{c}{Fit}\\
   &   $N_{\rm C}$ = 50   &  $N_{\rm C}$ = 200  &  $N_{\rm C}$ = 50   &  $N_{\rm C}$ = 200   &  \multicolumn{2}{c}{parameters} \\
\hline
$\varepsilon(v_{\rm S7}) = m_{1}x+q_{1}$ & $0.50 \leq v_{\rm S7}\leq 0.75$   &  ---  &  ---  &  ---  &  $m_{1}$ = 0.464  &  $q_{1}$ = --0.232  \\
$\varepsilon(v_{\rm S7}) = m_{2}x+q_{2}$ & $0.75 \leq v_{\rm S7}\leq 1.00$   &  ---  &  ---  &  ---  &  $m_{2}$ = 3.536  &  $q_{2}$ = --2.536  \\
$\varepsilon(v_{\rm S7}) = m_{3}x+q_{3}$ &  --- &  --- &  ---  &  $0.50 \leq v_{\rm S7}\leq 1.50$    &  $m_{3}$ = 0.879  &  $q_{3}$ = --0.319  \\
$\varepsilon(v_{\rm S7}) = a x^{b}$     &   --- &  ---  &  $0.50 \leq v_{\rm S7}\leq 1.50$  &  ---   &  $\;\;a$ = 0.405 &  $b$ = 2.232  \\
$\varepsilon(v_{\rm S7}) = c~\ln~(x^{d})~/x$  &   --- &  $0.50 \leq v_{\rm S7}\leq 1.50$  &   ---  &   ---    & $\;\;c$ = 5.366 &  $d$ = 0.7    \\
$\varepsilon(v_{\rm S7}) =  k$  & $1.00 \leq v_{\rm S7}\leq 2.00$  & $1.50 \leq v_{\rm S7}\leq 2.00$  &  
$1.50 \leq v_{\rm S7}\leq 2.00$  &  $1.50 \leq v_{\rm S7}\leq 2.00$  &  \multicolumn{2}{c}{$k$ = 1}    \\
\hline\noalign{\smallskip}
\multicolumn{1}{c}{$t_{\rm SNR}$  (yr)}  &  1.6 $\times$10$^{8}$  &  4.0 $\times$10$^{8}$  &  1.8 $\times$10$^{8}$  &  1.4 $\times$10$^{8}$  &    &   \\ 
\hline
\end{tabular}
\tablefoot{Fit parameters and survival timescales calculated assuming $T_0$~= 7.5~eV and $E_0$~=~4.6~eV.}
\end{table}