\begin{table}%t3
%\centering
\par
\caption{\label{abundances}Standard initial chemical abundances relative to the total hydrogen number density.}
\begin{tabular}{lclclc}
\hline\hline \noalign{\smallskip}
 & $Y({\rm X})$ &  & $Y({\rm X})$
&  & $Y({\rm X})$ \\
\hline
H$_{2}$ & 0.5 & H & $1.00\times 10^{-6}$ & He & 0.1 \\
C & $1.32\times 10^{-4}$ & N & $7.50\times 10^{-5}$ & O & $3.19\times 10^{-4}$\\
S & $1.86\times 10^{-5}$ & Si$^+$ & $2.46\times 10^{-7}$ & Fe$^+$ & $1.50\times 10^{-8}$ \\
\hline
\end{tabular}
\tablefoot{$Y({\rm X})=n({\rm X})/n_{\rm H}$.}
\end{table}