\begin{table}%t2
\caption{\label{table:Willacy}The fractional abundance (relative to H$_2$) taken for the parent species by \citet{Willacy1997AandA...324..237W}.}
%\centering
\par
\begin{tabular}{llll}
\hline \hline \noalign{\smallskip}
Parent & Fractional & Parent & Fractional \\
species & abundance & species & abundance \\
\hline \noalign{\smallskip}
CO     & $4.00 \times 10^{-4}$  & N$_2$ & $5.00 \times10^{-5}$ \\
H$_2$O & $3.00 \times 10^{-4}$  & H$_2$S & $1.27 \times 10^{-5}$ \\
CH$_4$ & $3.00 \times 10^{-5}$  & SiS & $3.50 \times 10^{-6}$ \\
NH$_3$ & $1.00 \times 10^{-6}$  & SiO & $3.15 \times 10^{-5}$ \\
He     & $1.00 \times 10^{-1}$  & PH$_3$& $3.00 \times 10^{-8}$\\
HCl    & $4.40 \times 10^{-7}$  &       &          \\
\hline
\end{tabular}
\end{table}