\begin{table}%t2
\caption{Fit parameters according to fitting formulas~\req{fit_form_1}
  and~\req{fit_form_2} with corresponding asymptotic standard errors.\label{fit_tab}}
\par
 %\centering
\par
\begin{tabular}{rrrr}
\hline\hline
\mbox{$\rho_u$ [$~{\rm g}~{\rm cm}^{-3}$]} & \mbox{$ a $} & \mbox{$b$} & \mbox{$c$} \\
\hline
\mbox{$1.25 \times 10^7$} & \mbox{$1.25712 \pm 0.0215$}  & \mbox{$ 0.177654 \pm 0.05236$} &
\mbox{$ 1.28103 \pm 0.0.04581$}\\
\mbox{$2.5 \times 10^7$}  & \mbox{$1.29816 \pm 0.0797$}  & \mbox{$ 0.180935 \pm 0.01718$} & \mbox{$0.779895 \pm 0.01982$}\\
\mbox{$5  \times 10^7$}  & \mbox{$1.05533 \pm 0.04931$} & \mbox{$
0.141428 \pm 0.01171$} & \mbox{$0.558411 \pm 0.01883$}\\
\mbox{$7.5 \times 10^7$}  & \mbox{$1.07245 \pm 0.05131$} & \mbox{$ 0.150079 \pm 0.2911$} &  \mbox{$0.466975 \pm 0.03134$}\\
\hline
\end{tabular}
\end{table}