\begin{table}%t2
      \caption{Maximum outer limit $R_{\rm out}$ of the photosynthesis-sustaining habitable zone of Gl~581 for $L=0.013~L_\odot$
                 as a function of the maximum atmospheric CO$_2$ pressure $P_{\rm max}$ for three different
                 climate models.}
         \label{rout}
  $$
         \begin{array}{llll}
            \hline\hline
            \noalign{\smallskip}
             P_{\rm max} ({\rm bar}) &  R_{\rm out}^{a} {\rm (AU)}
                                           &  R_{\rm out}^{b} {\rm (AU)}
                                           &  R_{\rm out}^{c} {\rm (AU)} \\
                    \noalign{\smallskip}  \hline
                      4   & 0.207 & 0.253 & 0.220 \\
            5   & 0.210 & 0.257 & 0.228 \\    
            6   & 0.213 & 0.260 & 0.236 \\
            7   & 0.216 & 0.263 & 0.242 \\
            8   & 0.219 & 0.265 & 0.247 \\
            9   & 0.221 & 0.267 & 0.252 \\
            10  & 0.223 & 0.269 & 0.261 \\
                        \hline
         \end{array}
     $$
\smallskip $^{{a}}$ \cite{chamberlain80}.
 $^{{b}}$  \cite{budyko82}.
 $^{{c}}$ \cite{williams98}.
   \end{table}