\begin{table}%t6
\caption{Initial abundances. The total oxygen budget is allowed to vary to explore the effect of different C/O elemental ratios. Water is assumed to be absent at $t=0$. \label{table_initial_abundances}}
%\centering
\par
\begin{tabular}{ll}
\hline
\hline
\multicolumn{1}{c}{Species} & \multicolumn{1}{c}{$n$(species)/$n$(H)}\\
\hline
 H       & 1.0 \\
 He      & $7.1 \times 10^{-2}$\\
 C       & $1.0 \times 10^{-4}$\\
 N       & $2.0 \times 10^{-5}$\\
 O       & $1.7 \times 10^{-4}$\\
 C$^{+}$ & $4.0 \times 10^{-6}$\\
 CO~     & $8.5 \times 10^{-5}$\\
 H$_2$O  & $1.0 \times 10^{-30}$\\
\hline   
\end{tabular}
\end{table}