\begin{table}%t4
\par
\caption{\label{tab_mod_param}{Constraints on model parameters.}}
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                          % used for centering table
\begin{tabular}{ll}
\hline\hline
Parameter  & Constraint \\
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$R_{{\rm in}}$~ \ \ \ \ \ free & standard 1$\times$10$^{17}$~cm \\
$N_{{\rm in}}$(H) free & $P_{{\rm in}}$ $<$ $P_{{\rm out}}$; stdd 1200~\cmt; 
``\fariv\ $\lambda$4740/$\lambda$4711+'' \\
\tbb~ \ \ \ \ free & $\tau_{{\rm c}}$/$\tau_{1}$ $>$ 1.2; ``upper limit \foi\ $\lambda$6300'' \\
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$\tau_{1}$(thin)& absolute flux $I$(\hb) \\
$f_{2}^{{\rm cov}}$(thick) & \foii\ $\lambda$3727; ($f_{1}^{{\rm cov}}$ + $f_{2}^{{\rm cov}}$ $=$ 1) \\
$P_{{\rm out}}$  & ratio \fsii\ $\lambda$6731/$\lambda$6716 \\
$\tau_{{\rm c}}$/$\tau_{1}$ & \hei\ $\lambda$5876 (see text) \\
$L$     & ratio \fariv\ $\lambda$4740 / \fariii\ $\lambda$7135 \\
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He      & \heii\   $\lambda$4686 \\
C  & ratio \foiii\ $\lambda$4363/($\lambda$4959+$\lambda$5007) \\
N       & \fnii\   $\lambda$6584 \\
O       & \foiii\  $\lambda$5007+$\lambda$4959 \\
Ne      & \fneiii\ $\lambda$3869 \\
S       & S/Ar = 4.37 by number (solar); ``\fsiii\ $\lambda$6312'' \\
Ar      & \fariii\ $\lambda$7135 + \fariv\ $\lambda$4740 \\
Fe & Fe/H = 4$\times$10$^{-7}$ by number; ``upper limit \ffevi'' \\
\noalign{\vskip3pt} \noalign{\hrule} \noalign{\vskip3pt}  & \\
\end{tabular}
\end{table}