\begin{table}%t2
\caption{\label{param}Model parameters.} 
%\centerline
 {\small 
\par\begin{tabular}{lll}
\hline \hline
\multicolumn{3}{c}{\bf Fixed parameters} \\ 
\hline 
 heliocentric distance & $r_{\rm h}$ &  0.989~AU\\ 
 geocentric distance & $\Delta$ &  1.368~AU\\ 
 gas expansion velocity$^{ {(a)}}$ & ${v}_{\rm exp}$ & 
 1.05~km~s$^{-1}$ \\ 
 total CO production rate$^{{\rm (a)}}$ & 
$Q_{\rm CO}$ & $ 2.0 \times 10^{30}$~s$^{-1}$ \\ 
 CO photodissociation rate$^{ {(b)}}$  & $\beta_{\rm CO}$ & 
 $7.50 \times 10^{-7}~r_{\rm h}^{-2}$ \\ 
 nucleus rotation period & $P$ & 11.35~h \\ 
\hline 
\multicolumn{3}{c}{\bf Free parameters} \\ 
\hline 
fraction of CO in the coma outgassed in the jet && \fco \\ 
jet latitude && $\ell$  \\ 
jet half-width && $\Psi$ \\ 
 rotation axis aspect angle && \thw \\ 
 rotation axis position angle && \paw \\ 
\hline
\end{tabular}} 
\medskip
$^{(a)}$ \citet{biver99}; $^{(b)}$ \citet{huebner92}.  
\end{table}