\begin{table}%t3
\caption{\label{tab3}LTE column densities.}
\small%\centering
\par
\begin{tabular}{lcccclccc}
  \hline \hline\noalign{\smallskip}
\multicolumn{1}{l}{Source} &
\multicolumn{1}{c}{$T_{\rm r}$} &
\multicolumn{1}{c}{$N$(C$^{18}$O)} &
\multicolumn{1}{c}{$N$(C$^{17}$O)} &
\multicolumn{1}{c}{$R_1$\tablefootmark{*}} &
\multicolumn{1}{c}{$T_{\rm r}$ (K)} &
\multicolumn{1}{c}{$N$(N$_2$H$^+$)} &
\multicolumn{1}{c}{$N$(N$_2$D$^+$)} &
\multicolumn{1}{c}{$R_2$\tablefootmark{*}} \\ 
\multicolumn{1}{l}{} &
\multicolumn{1}{c}{(K)} &
\multicolumn{1}{c}{(cm$^{-2}$)} &
\multicolumn{1}{c}{(cm$^{-2}$)} &
\multicolumn{1}{c}{} &
\multicolumn{1}{c}{(K)} &
\multicolumn{1}{c}{(cm$^{-2}$)} &
\multicolumn{1}{c}{(cm$^{-2}$)} &
\multicolumn{1}{c}{} \\ 
\hline\noalign{\smallskip} 
Serpens-FIRS 1    & 11 & 8.2~$\times$~10$^{15}$  &  3.2~$\times$~10$^{15}$ & 2.6       &       11\tablefootmark{a} &  7.7~$\times$~10$^{13}$ & 3.4~$\times$~10$^{11}$ & 0.004 \\  
Cep E-mm            & 15 &  2.5~$\times$~10$^{15}$  & 7.6~$\times$~10$^{14}$ & 3.3     &       9\tablefootmark{b} & 1.4~$\times$~10$^{13}$ & 5.8~$\times$~10$^{11}$ & 0.04 \\ % &  11 \\
L1641 S3 MMS 1      & 10 &  3.2~$\times$~10$^{15}$  & 1.3~$\times$~10$^{15}$ & 2.5     &       8\tablefootmark{b} & 3.6~$\times$~10$^{13}$ & $<$2.1~$\times$~10$^{11}$ & $<$0.006 \\ % &   \\
IC1396N                &  25  &   1.1~$\times$~10$^{16}$ & 3.7~$\times$~10$^{15}$ & 3.0 &      10\tablefootmark{b}  & 4.1~$\times$~10$^{13}$ & $<$4.1~$\times$~10$^{11}$ & $<$0.01 \\ % & 20 \\     
CB3                      & 14  &  5.2~$\times$~10$^{15}$ & 1.6~$\times$~10$^{15}$ & 3.3 &      6\tablefootmark{a} & 1.2~$\times$~10$^{13}$ & 1.0~$\times$~10$^{12}$ & 0.08 \\ % &  14 \\
OMC2-FIR4          &  22 & 4.8~$\times$~10$^{15}$ & 1.2~$\times$~10$^ {15}$ & 3.4 &              22\tablefootmark{a} & 2.0~$\times$~10$^{14}$ &  8.7~$\times$~10$^{11}$ & 0.004 \\ % & 20 \\ 
NGC 7129$-$FIRS 2       &  23 & 1.9~$\times$~10$^{15}$  & 6.9~$\times$~10$^{14}$ & 2.8 &        13\tablefootmark{c} &  3.8~$\times$~10$^{13}$ & 5.4~$\times$~10$^{11}$ & 0.014 \\ % &  21 \\
S140                     &  50\tablefootmark{c} & 1.0~$\times$~10$^{16}$  & 2.7~$\times$~10$^{15}$ & 3.7 &  50\tablefootmark{d} & 1.2~$\times$~10$^{14}$ & $<$4.5~$\times$~10$^{11}$ &  $<$0.004 \\  
LkH$\alpha$ 234\tablefootmark{b}  &    &                         &                          &        &  22       &  1.0~$\times$~10$^{13}$ & $<$2.3~$\times$~10$^{11}$ &  $<$0.02 \\  \hline
\end{tabular}
\tablefoot{\tablefoottext{*}{$R_1=N$(C$^{18}$O)/$N$(C$^{17}$O), $R_2=N$(N$_2$D$^+$)/$N$(N$_2$H$^+$).}\\
\tablefoottext{a}{Rotation temperature derived from our N$_2$D$^+$ observations.}\\
\tablefoottext{b}{Rotation temperature derived from our N$_2$H$^+$ observations.}\\
\tablefoottext{c}{From Fuente et~al. (\cite{Fuente2005a}).}\\
\tablefoottext{d}{From Minchin et~al. (\cite{Minchin1995}).}}
\end{table}