\begin{table}%t12
%\centering
\par
\caption{Estimates of the X-ray luminosity in low-mass objects from the 
observed CN abundances.}
\label{tlmx}
\small
\begin{tabular}{lccc} \hline\hline
Source & $r_{\rm beam}$ & $n_{\rm H}({r_{\rm beam}})$ & X-ray luminosity $L_{\rm X}$ \\ 
       & AU             & cm$^{-3}$                   & erg~s$^{-1}$ \\ \hline\noalign{\smallskip}
IRAS 16293 & 1120 & $10^7$ & $\gtrsim$$4\times 10^{29}$ \\
N1333-I2   & 1540 & $10^6$ & $\gtrsim$$7\times 10^{29}$ \\
N1333-I4A  & 1540 & $10^7$ & $\gtrsim$$7\times 10^{29}$ \\
L1448-C    & 1540 & $10^6$ & $\gtrsim$$7\times 10^{30}$ \\
L483       & 1400 & $10^6$ & $\gtrsim$$6\times 10^{30}$ \\
L723       & 2100 & $10^6$ & $\gtrsim$$1\times 10^{31}$ \\
SMM4       & 1750 & $10^6$ & $\gtrsim$$1\times 10^{30}$ \\
V1057 Cyg  & 4550 & $10^5$ & $\gtrsim$$6\times 10^{30}$ \\
L1489      & 980  & $10^5$ & $\gtrsim$$8\times 10^{29}$ \\ \hline
\end{tabular}
\end{table}