\begin{table}%t4
%\centering
\par
\caption {\label{R}Isotopic abundance ratios for the local ISM taken from \cite{Wilson99}.}
\begin{tabular}{cc}
\hline\hline
\noalign{\smallskip}
 Species &  Ratio ($R$) \\ \hline 
$\displaystyle\frac{{\rm HC}^{18}{\rm O}^+}{{\rm H}^{13}{\rm CO}^+} =  \frac{[^{12}{\rm C}]}{[^{13}{\rm C}]} \times  \frac{[^{18}{\rm O}]}{[^{16}{\rm O}]}  $ & $\displaystyle\frac{69}{577}$\\
$\displaystyle\frac{{\rm H}^{13}{\rm C}^{18}{\rm O}^+}{{\rm H}^{13}{\rm CO}^+} =  \frac{[^{18}{\rm O}]}{[^{16}{\rm O}]}$ & $\displaystyle\frac{1}{577}$ \\
$\displaystyle\frac{{\rm H}^{15}{\rm NC}}{{\rm HN}^{13}{\rm C}} = \frac{[^{15}{\rm N}]}{[^{14}{\rm N}]} \times  \frac{[^{12}{\rm C}]}{[^{13}{\rm C}]}  $ & $\displaystyle\frac{69}{388}$ \\
$\displaystyle\frac{{\rm H}^{15}{\rm N}^{13}{\rm C}}{{\rm HN}^{13}{\rm C}} = \frac{[^{15}{\rm N}]}{[^{14}{\rm N}]}$ & $\displaystyle\frac{1}{388} $ \\ 
\hline
\end{tabular}
\end{table}