\begin{table}%t1
%\centering
\par
\caption{\label{tab:a}Best-fit parameters for different transport scenarios (see text for details).}
%
\par
\begin{tabular}{c c c c c}
\hline\hline
Scenario & Inner heliosphere & \multicolumn{2}{c}{Back-scatter region}& $\zeta$\\
 & $\lambda_1$ (AU) & $r_{\rm bs}$ (AU) & $\lambda_2$ (AU) &  \\ 
 \hline
A & 1.0 & \multicolumn{2}{c}{no back-scatter region} & 152\\
B & scatter-free & 1.1 & 0.02 & 298\\
C & 3.2 & 1.2 & 0.2 & 27\\
\hline
\end{tabular}
\end{table}