\begin{table}%t2
\caption{\label{table:2}The column densities of CCH and its $^{13}$C isotopic species.}             %\centering
\par
\small\begin{tabular}{cccc}      
\hline
\hline  \noalign{\smallskip}             
Model&Species&TMC-1&L1527  \\    
& &[cm$^{-2}$]&[cm$^{-2}$] \\
\hline   \noalign{\smallskip}               
LVG\tablefootmark{a}&CCH&$(6.5 \pm 2.7) \times 10^{14}$&$(9.8 \pm 0.2) \times 10^{14}$ \\
&C$^{13}$CH&$(3.9 \pm 0.4) \times 10^{12}$&$(1.2 \pm 0.1) \times 10^{13}$ \\
&$^{13}$CCH&$(2.6 \pm 0.4) \times 10^{12}$&$(7.3 \pm 0.8) \times 10^{12}$ \\
LTE\tablefootmark{b}&CCH&$(5.4 \pm 0.1) \times 10^{14}$&$(8.1^{+0.1}_{-0.2}) \times 10^{14}$ \\
$\quad$&C$^{13}$CH&$(3.4^{+0.5}_{-0.7}) \times 10^{12}$&$(9.3 \pm 0.7) \times 10^{12}$ \\
$\quad$&$^{13}$CCH&$(2.1 \pm 0.5) \times 10^{12}$&$(5.7 \pm 0.3) \times 10^{12}$ \\
\hline
\end{tabular}
\tablefoot{The errors quoted represent three times the standard deviation.
\tablefoottext{a}{The H$_2$ densities and the gas kinetic temperatures are assumed to be $3 \times 10^4$ and 10~K, $1 \times 10^6$ cm$^{-3}$ and 15~K, for TMC-1 and L1527, respectively.}
\tablefoottext{b}{The excitation temperatures are assumed to be 6.7~K and 12.3~K for TMC-1 and L1527, respectively.}}
\end{table}