\begin{table}%t3
%\centering
\par
\caption{Values of exponents $m,n,k$, the scale distance~$r_{\rm{0}}$, and coefficient~$\alpha$ ($g(r)$-like function,
Eq.~(2a)) estimated from observed water sublimation rates for
C/1995~O1~Hale-Bopp and C/1996~B2~Hyakutake (Szutowicz et~al.
\cite{Sz02a},b).\label{tab3}} 
\begin{tabular}{ccc}
  \hline\hline
  % after \\: \hline or \cline{col1-col2} \cline{col3-col4} ...
  Coefficient  & Hale-Bopp & Hyakutake \\
               & [function $g^*_{\rm{HB}}$($r$)]  & [function $g^*_{\rm{Hya}}$($r$)] \\
  \hline
  $\alpha$      & 0.008860          & 3.31564             \\
  $m$           & $2.828       $    & $0.537         $    \\
  $n$           & $3.76        $    & 1.0                 \\
  $k$           & $3.7        $     & $5.76        $      \\
  $r_{\rm {0}}$ (AU)    & $5.3        $     & 2.808               \\
  $Q_m$ (mol./s)& $8.9\times 10^{30}$ & $1.9\times 10^{29}$  \\
  \hline
\end{tabular}
\end{table}