\begin{table}%t2
%\centering
\par
\caption{IRAM 30-m observations.\label{radiotab}}
\begin{tabular}{c c  c c }
\hline\hline
Date &  UT $^a$   & $\tau$$^b$ & $F^c~({\rm m}Jy)$ \\  
\hline
2002/12/06 & 20.0 &   0.20 &$0.7\pm1.0$ \\
2002/12/07 & 18.6 &  0.10 &$ 0.8\pm1.0$\\
2003/01/28 & 20.8 &  0.10 &$ 0.3\pm1.7$\\
2003/02/03 & 16.0 &  0.20 &$-1.0\pm1.3$\\
2003/03/11 & 17.1 &  0.20 &$-0.2\pm1.1$\\
\hline
\noalign{\smallskip}
\multicolumn{4}{l}{
$^a$ UT time at the middle of integration.} \\
\multicolumn{4}{l}{
$^b$ Mean zenith atmospheric opacity at 250~GHz.} \\
\multicolumn{4}{l}{ $^c$ Flux from 2002 TX$_{300}$.} \\
\end{tabular} 
\end{table}