\begin{table}%t1
\caption{\label{tab1}Distribution of the sample stars into four Str\"omgren groups (before parallax selection).}
%\centering
\par
\begin{tabular}{cccccc}
\hline \hline \noalign{\smallskip}
Group& Spectral type& Luminosity class&$\beta$&Number of stars&Calibration\\
\hline
  1&    B0 to A0&  III to V&   $2.59<\beta<2.88$&   993&
Crawford (\cite{Craw78})\\
  2&    A0 to A3&  III to V&   $2.87<\beta<2.93$&    626&
Hilditch et~al. (\cite{Hilditch})\\
  3&    A3 to F0&  III to V&   $2.72<\beta<2.88$&   1517&
Crawford (\cite{Craw79})\\
  4&    F0 to G2&  III to V&   $2.56<\beta<2.72$&   3857&
Crawford (\cite{Craw75})\\
\hline
 TOTAL&           &          &                 &   6993&
                 \\
\hline
\end{tabular}
\end{table}