\begin{table}%t1
%\centering
\par
\caption{Models.}
\label{tab2}
\small
\begin{tabular}{ll}
\hline
\hline
%\noalign{\smallskip}
Model  & $a$ \\
%\noalign{\smallskip}
\hline
%\noalign{\smallskip}
M0 & 0 \\
%\noalign{\smallskip}
M1 & 0.01 \\
%\noalign{\smallskip}
M2 & 0.03 \\
%\noalign{\smallskip}
M3 & 0.1 \\
\hline
\end{tabular}
\end{table}