\begin{table}%t2
  %\centering
\par
      \caption {\label{tab:model}Predicted steady-state fractional abundances of several
      nitrogen-bearing species in three different models.}
   { \scriptsize
    \begin{tabular}{l l l l c}
      \hline
      \hline
      \noalign{\smallskip}
      Species   & I & II & III & Observations \\
      \hline
      N$_2$H$^+$\tfm{a} & 100:0            & 90:10            & 90:10\\
      NH$_4^+$\tfm{b} & 85:2:13          & 85:2:13          & 95:2:3\\
      N$_2$   & $1.0( -5)$ & $1.0( -5)$ & $1.0( -5)$ & $-$ \\
      N    & $2.4( -7)$ & $2.5( -7)$ & $2.9( -7)$ & $-$ \\
      NH   & $3.5(-11)$ & $9.5(-11)$ & $8.8(-11)$ & $2.0\pm1.0(-9)$ \\
      {NH$_2$}  & $1.9(-10)$ & $1.9(-10)$ & $1.3(-10)$ & $4.0\pm2.0(-10)$ \\
%      \ce{NH3}  & $3.7( -8)$ & $3.7( -8)$ & $8.3( -8)$ & $0.6-4(-8)$ \\
      NH$_3$  & $3.7( -8)$ & $3.7( -8)$ & $8.3( -8)$ & $\rm 0.3^{+0.1}_{-0.1}-2^{+0.8}_{-0.8}(-8)$ \\
%      \ce{N2}   & $1.0 \tdix{ -5}$ & $1.0 \tdix{ -5}$ & $1.0 \tdix{ -5}$ & $-$ \\
%      \ce{N}    & $2.4 \tdix{ -7}$ & $2.5 \tdix{ -7}$ & $2.9 \tdix{ -7}$ & $-$ \\
%      NH   & $3.5 \tdix{-11}$ & $9.5 \tdix{-11}$ & $8.8 \tdix{-11}$ & $4\pm2\tdix{-9}$ \\
%      {NH$_2$}  & $1.9 \tdix{-10}$ & $1.9 \tdix{-10}$ & $1.3 \tdix{-10}$ & $7.5\pm3.6\tdix{-10}$ \\
%      \ce{NH3}  & $3.7 \tdix{ -8}$ & $3.7 \tdix{ -8}$ & $8.3 \tdix{ -8}$ & $0.6-4\tdix{-8}$ \\
%      \ce{NH3}:NH:{NH$_2$} & 190:0.19:1.0 & 190:0.5:1.0 & 640:0.68:1.0 \\  
      NH:{NH$_2$}:NH$_3$ & 0.2:1:190 & 0.5:1:190 & 0.7:1:640 & 5:1:300\\  
      \hline
    \end{tabular}}
  \tfoot {Numbers in parenthesis are powers of ten. {Branching
      ratios (BR) for the dissociative recombination (DR) of {NH$_{2}^+$}
      are N + H$_2$ (4\%), {NH + H} (39\%) and {N + 2H} (57\%)
      \citep{thomas2005}.}  \tft{a} {BR for the DR of N$_2$H$^+$ into
      {N$_2$ + H} and {NH + N}, respectively
      \citep{molek2007,adams2009}.}  \tft{b} {BR for the DR of
      NH$_4^+$ into {NH$_3$ + H}, {NH$_2$ + H$_2$}, and 
      {NH$_2$ + 2H},
      respectively \citep{ojekull2004}.  See additional details in
      Table~\ref{tab:dr}.}}
  \end{table}