\begin{table}%tb1
%\centering
\par
\caption{Assumed initial abundances and cosmic-ray ionization rate for the 
chemical models (Figs.~\ref{fltncn}--\ref{fgtncnhcn}).}
\label{tinit}
\small
\begin{tabular}{lll} \hline\hline
Species & Initial abundance & Ref. \\ \hline
Initial abundances $T > 100$~K \\ 
H$_2$      & $1$                  &   \\
CO         & $2.0\times 10^{-4}$  & a \\ 
CO$_2$     & $3.0\times 10^{-5}$  & b \\ 
H$_2$O     & $1.5\times 10^{-4}$  & c \\
H$_2$S     & $10^{-8}$  & d \\
H$_2$CO    & $8.0\times 10^{-8}$  & d \\
N$_2$      & $7.0\times 10^{-5}$  & e \\
CH$_4$     & $10^{-7}$  & e \\
C$_2$H$_4$ & $8.0\times 10^{-8}$  & e \\
C$_2$H$_6$ & $10^{-8}$  & e \\
CH$_3$OH   & $1.5\times 10^{-7}$  & d \\
O          & 0.0      & e \\
S          & 0.0      & e \\
Initial abundances $T < 100$~K \\ 
H$_2$      & $1$            &   \\
CO  & $2.0\times 10^{-4}$  & d \\ 
CO$_2$     & 0.0      & f \\ 
H$_2$O     & 0.0      & f \\
H$_2$S     & 0.0      & f \\
N$_2$      & $7.0\times 10^{-5}$  & e \\
CH$_4$     & $10^{-7}$  & e \\
C$_2$H$_4$ & $8.0\times 10^{-8}$  & e \\
C$_2$H$_6$ & $10^{-8}$  & e \\
H$_2$CO $(60 < T($K$) < 100)$ & $8.0\times 10^{-8}$ & d \\
H$_2$CO $(T($K$) < 60)$ & 0.0 & d \\
CH$_3$OH $(60 < T($K$) < 100)$ & $1.5\times 10^{-7}$ & d \\
CH$_3$OH $(T($K$) < 60)$ & 0.0 & d \\
O          & $1.0^{-4}$  & d \\
S          & $6.0\times 10^{-9}$  & g \\
Cosmic-ray ionization rate $\zeta_{{\rm cr}}$ (s$^{-1}$) & $0.8\times 10^{-17}$ & h \\ 
\hline
\end{tabular}
\begin{list}{}{}
\item[] All abundances are relative to molecular hydrogen. 
$^a$ J{\o}rgensen et~al. (\cite{js02}), $^b$ Boonman et~al. (\cite{be03}), 
$^c$ Boonman \& van Dishoeck (\cite{bv03}), $^d$ Doty et~al. (\cite{ds04}), 
$^e$ Charnley (\cite{ch97}), $^f$ assumed to be frozen-out or absent in cold 
gas-phase, $^g$ Doty et~al. (\cite{dv02}), $^h$ St\"auber et~al. (\cite{sj06}).
\end{list}
\end{table}