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$\begin{figure} \par\includegraphics[width=7.9cm,clip]{6173fig1.eps} \end{figure}$ Figure 1: Two-dimensional slices for an exemplary time step at t = 70 390 s: a) logarithmic gas temperature $\log T$ , b) fraction of carbon atoms bound in CO $n_{\rm CO} / n_{\rm C, total}$ , c) net radiative heating rate $Q_{\rm rad, R}$ for the simulation without CO line cooling, and d) the relative difference $\Delta Q_{\rm rad}$ between the runs with and without CO cooling. Note that the colour range has been limited in the last panel in order to to make smaller variations visible. The full range is $\Delta Q_{\rm rad} = [-2.54, 0.20]\times10^9~{\rm erg~g}^{-1}~{\rm s}^{-1}$ . The lines represent contours for T = 5000 K (white solid) and Rosseland optical depth unity (long-dashed). The short-dashed line in panels c and d marks $Q_{\rm rad, R} = 0$ and $\Delta Q_{\rm rad} = 0$ , respectively.
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In the text

$\begin{figure} \par {\includegraphics[width=7.6cm,clip]{6173fig2.eps} } \end{figure}$ Figure 2: Horizontally and temporally averaged temperature stratification a), temperature fluctuation b), and CO number density c) for the simulations with (solid line) and without (dashed line) radiative CO cooling. The dotted lines represent the difference between both cases, e.g. $\Delta T = \langle T_{\rm CO}\rangle_{x,t} - \langle T_{\rm R}\rangle_{x,t}$ .
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In the text

$\begin{figure} \par {\includegraphics[width=8cm,clip]{6173fig3.eps} } \end{figure}$ Figure 3: Time evolution towards radiative equilibrium (RE) for simulations without (solid) and with CO line cooling for different initial chromospheric temperatures of 2000 K (dot-dashed) and 7000 K (triple-dot-dashed): a) horizontally averaged temperature stratification for initial time step (thin) and after 10 000 s (thick) and b) time evolution of the horizontally averaged temperature at z = 650 km. The shaded area marks the transition layer for artificial damping of velocity fluctuations.
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In the text

$\begin{figure} \par {\includegraphics[width=7.8cm,clip]{6173fig4.eps} } \end{figure}$ Figure 4: Cooling experiment with prescribed (mechanical) heating. Gas temperature after 4 h averaged on column mass scale for different amounts of prescribed (mechanical) heating with $\log q_0 = -15.5$ and -16.0 (thin solid) and without additional heating (thick solid). The thick dot-dashed line represents a simulation with grey radiative transfer alone (i.e. without CO). The results of AA89 (see their Fig. 1) are plotted for comparison (dotted lines). The corresponding mechanical heating ( $\log q_0$ ) is noted on the left. The curve annotated with "RE'' is the non-LTE RE stratification by Anderson (1989). The shaded area marks the transition layer for artificial damping of velocity fluctuations.
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In the text

$\begin{figure} \par {\includegraphics[width=7.6cm,clip]{6173fig2.eps} } \end{figure}$	Figure 2: Horizontally and temporally averaged temperature stratification a), temperature fluctuation b), and CO number density c) for the simulations with (solid line) and without (dashed line) radiative CO cooling. The dotted lines represent the difference between both cases, e.g. $\Delta T = \langle T_{\rm CO}\rangle_{x,t} - \langle T_{\rm R}\rangle_{x,t}$ .
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