\begin{table}%t1
%\centering
\small
\caption{Input parameters for the adopted, spherically symmetric model nebula (CLOUDY)}\label{t1}
\begin{tabular}{ll}
\hline\hline
Gas density profile &  The same, standard and  arbitrary, \\
&double-peak+halo distribution at \\
&all four evolutionary steps\\
& (see Figs.~\ref{f2}, \ref{f4}--\ref{f6}; cf. Sect. 4)\\
%&neutral gas = cfr. Sects. 3 and 4\\
\\
Chemical abundances   & He = $-$0.958, C = $-$3.30, N = $-$3.75, \\
\par
 log[{\it N}(X)/{\it N}(H)]& O = $-$3.30, F = $-$6.522, Ne = $-$3.90,  \\
 & Na = $-$5.52, Mg = $-$5.30, Al = $-$6.568, \\
 &Si = $-$5.00, P = $-$6.70, S = $-$5.10,\\
 &Cl = $-$6.66, Ar = $-$5.60,~K = $-$7.11,\\ 
 &Ca = $-$6.64, Fe = $-$6.30,\\
 & other elements = CLOUDY default\\
&\\
Dust                   & CLOUDY default\\
&\\
Local filling factor         & 1.0 \\
\hline
\end{tabular}
\end{table}