\begin{table}%t3
\caption{\label{TabTex}Excitation temperatures inferred from the $\tau_{{\rm HCO}^{+}
    (1-2)}/\tau_{{\rm HCO}^{+} (0-1)}$ ratio.}
  %\centerline
{\small  
\begin{tabular}{l c c c}
\hline\hline 
Source      & \multicolumn{1}{c}{$T_{\rm ex}$ (K)} &
\multicolumn{2}{c}{errors\tablefootmark{a} (K)}\\ 
\hline
            &       &   +  &   --    \\
G10.62-0.38 &  3.0  & 1.4  &  0.5  \\
G34.3+0.1   &  3.0  & 8.4  &  0.8  \\
W49N        &  2.7  & 0.8  &  0.4  \\
W51         &  2.9  & 1.1  &  0.5  \\
\hline
\end{tabular}}
\tablefoot{\tablefoottext{a}{The errors are derived from the results of numerical calculation
  which do not take into account the lower limit of 2.73 K set by the cosmic
  microwave background.}}
\end{table}