\begin{table}%t10
%\centering
\par
\caption{Observed fractional abundances from literature.}
\label{tfl}
\small
%\begin{center}
{\begin{tabular}{lrrr} \hline\hline
Source & HCN & HCO$^+$ & SO \\
       & $\times 10^{-9}$ & $\times 10^{-9}$ & $\times 10^{-9}$ \\ \hline
AFGL 2591  & $20.0^a$ & $10.0^a$ & $10.0^b$ \\
W3 IRS 5   & $2.0^c$ & $0.73^c$ & $10.0^b$ \\
IRAS 16293 & $1.1^d$ & $1.4^d$ & $4.4^d$ \\
N1333-I2   & $2.0^e$ & $3.3^e$ & $3.4^e$ \\
N1333-I4A  & $0.36^e$ & $0.43^e$ & $4.6^e$ \\
L1448-C    & $5.4^e$ & $9.1^e$ & $1.4^e$ \\
L483       & $2.0^e$ & $2.0^e$ & $0.29^e$ \\
L723       & $1.0^e$ & $4.1^e$ & $2.4^e$ \\
SMM4       & $2.0^f$ & $1.0^f$ & ... \\
L1489      & $0.65^e$ & $18.0^e$ & $2.0^e$ \\ \hline
\end{tabular}}
{
$^a$ van der Tak et~al. (\cite{ve00}); $^b$ van der Tak et~al. 
(\cite{vb03}) $^c$ Helmich et~al. (\cite{hv97}); $^d$ Sch\"oier et~al. 
(\cite{sj02}); $^e$ J{\o}rgensen et~al. (\cite{js04}); $^f$ Hogerheijde et~al. 
(\cite{hv99}).}
%\end{center}
\end{table}