\begin{table}%t1
\caption{\label{sensors}Electron temperature indicators and symbols.}
\small%\centering
\par
\begin{tabular}{ll}
\hline
\hline
Spectroscopic index & Temperature symbols \\
\hline
$ (I_{\lambda} (3646^-) - I_{\lambda} (3646^+))$/H$\beta$ & 
$T_{\rm e}$(Bac) \\
 $[$\ion{O}{iii}]~$(\lambda 4959 + \lambda 5007)/\lambda 4363$ & 
$T_{\rm e}$(\ion{O}{iii}) \\
$[$\ion{N}{ii}]~$(\lambda 6548 + \lambda 6584)/\lambda 5755$ &   $T_{\rm e}$(\ion{N}{ii}) \\ 
$[$\ion{O}{i}]~$(\lambda 6300 + \lambda 6364)/\lambda 5577$ & 
$T_{\rm e}$(\ion{O}{i}) \\
$[$\ion{Ar}{iii}]~$(\lambda 7136 + \lambda 7751)/\lambda 5192$ &
$T_{\rm e}$(\ion{Ar}{iii}) \\
$[$\ion{C}{i}]~$(\lambda 9823 + \lambda 9849)/\lambda 8728$ & 
$T_{\rm e}$(\ion{C}{i}) \\
\hline
\end{tabular}
\end{table}