All Figures

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{H4073F1.PS} \end{figure}$ Figure 1: Upper panel: Dependence of the thermal gas pressure p on the gas temperature $T_{\rm g}$ for various densities in units of $10^{-11}\rm g~cm^{-3}$ (solid and dotted lines as indicated). $T_{\rm rad}\!=\!1500~$ K and $\left\langle\!\frac{{\rm d}v}{{\rm d}l}\!\right\rangle\!=\!10^{-7}~\rm s^{-1}$ are fixed parameters. For comparison, the dashed line shows an ideal mono-atomic gas ( $\mu\!=\!1.3~m_{\rm H}$ ) for $\rho\!=\!10^{-10}\rm g~cm^{-3}$ . Lower panel: Mean molecular weight $\mu$ in atomar mass units over gas temperature for $\rho\!=\!10^{-10}\rm g~cm^{-3}$ . The upper dashed line corresponds to the value of $\mu$ where all hydrogen is bound in H $_{\rm 2}$ , the lower dashed line to atomic hydrogen.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{H4073F2.PS} \end{figure}$ Figure 2: Internal energy e over gas temperature $T_{\rm g}$ (solid line) for constant $\rho\!=\!10^{-10}~\rm g~cm^{-3}$ , $T_{\rm rad}\!=\!1500~$ K and $\left\langle\!\frac{{\rm d}v}{{\rm d}l}\!\right\rangle\!=\!10^{-7}~\rm s^{-1}$ . For comparison, the internal energy of an ideal mono-atomic gas is shown as dashed line.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{H4073F3.PS} \end{figure}$ Figure 3: Radiative cooling rate $-\widehat{Q}_{\rm rad}$ over gas density $\rho$ for $T_{\rm rad}=0$ , $T_{\rm g}=4000~$ K and $\left\langle\!\frac{{\rm d}v}{{\rm d}l}\!\right\rangle=10^{-7}\rm~s^{-1}$ .
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In the text

$\begin{figure} \par\includegraphics[width=8.2cm,clip]{H4073F4.PS} \end{figure}$ Figure 4: Radiative cooling rate $-\widehat{Q}_{\rm rad}$ over gas temperature $T_{\rm g}$ for $T_{\rm rad}\!=\!0$ , $\left\langle\!\frac{{\rm d}v}{{\rm d}l}\!\right\rangle\!=\!10^{-7}~\rm s^{-1}$ and different gas densities ( $\log~\rho$ as indicated). Note that this case is only one example. An increase of the continuous radiation field will, for example, result in more photoionisation and hence to an increase of the electron concentration which usually intensifies the radiative cooling.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{H4073F5.PS}\vspace*{3mm} \par\includegraphics[width=7.8cm,clip]{H4073F6.PS} \end{figure}$ Figure 5: Start models for parameters $M_{\star}\!=\!1.2~M_{\odot}$ , $L_{\star}\!=\!6125~L_{\odot}$ and $T_{\star}\!=\!2600~$ K (derived quantities $R_\star\!=\!2.68\!\times 10^{13}~$ cm, $\log g\!=\!-0.66$ ) compared with two start models calculated with constant $\mu\!=\!1.26~$ amu (H/He mixture, dashed) and $\mu\!=\!2.33~$ amu (H₂/He mixture, dashed-dotted).
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In the text

$\begin{figure} \par\includegraphics[width=8.4cm,clip]{H4073F7.PS} \end{figure}$ Figure 6: A typical radial snapshot of a model with parameters $M_{\star}\!=\!1~M_{\odot}$ , $L_{\star}\!=\!4760~L_{\odot}$ , $T_{\star}\!=\!3000~$ K, $\rm C/O\!=\!1.8$ , $\Delta u\!=\!2~$ km s^-1 and $P\!=\!310~$ d (derived quantities $R_0\!=\!1.78\times 10^{13}~$ cm and $\log g\!=\!-0.38$ ). Upper box: velocity u (full line) and total hydrogen particle density $n_{\rm \langle H\rangle}$ (dashed). Second box: gas temperature $T_{\rm g}$ (full) and grey radiative equilibrium temperature $T_{\rm rad}$ (dashed). Middle double box: radiative (full black), adiabatic (full grey) and viscous (dotted) heating rates (upper part) and cooling rates (lower part) in $[\rm erg/g/s]$ . Fifth box: mean molecular weight $\mu$ . Lower box: degree of condensation of carbon $f_{\rm cond}$ (full) and ratio of radiative acceleration to gravitational deceleration $\alpha$ (dashed).
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In the text

$\begin{figure} \par\includegraphics[width=5.8cm,clip]{H4073F8.PS}\hspace*{2mm} \... ....PS}\hspace*{2mm} \includegraphics[width=5.8cm,clip]{H4073F10.PS} \end{figure}$ Figure 7: Enlarged hydrodynamic structures of a dust-forming, non-dissociative shock wave propagating outwards. Parameters: $M_{\star}\!=\!1~M_{\odot}$ , $L_{\star}\!=\!7500~L_{\odot}$ , $T_{\star}\!=\!2600~$ K, $\rm C/O\!=\!1.8$ , $\Delta u\!=\!2~$ km s^-1 and $P\!=\!650~$ d (derived quantities $R_0\!=\!2.97\times 10^{13}~$ cm and $\log g\!=\!-0.82$ ). The plotted quantities are explained in the caption of Fig. 6. For clarity, the adiabatic work is not depicted here.
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In the text

$\begin{figure} \par\includegraphics[width=5.6cm,clip]{H4073F11.PS}\hspace*{2mm} ... ....PS}\hspace*{2mm} \includegraphics[width=5.6cm,clip]{H4073F13.PS} \end{figure}$ Figure 8: A dissociative shock wave propagating through the circumstellar gas. Parameters: $M_{\star}\!=\!1.2~M_{\odot}$ , $L_{\star}\!=\!5850~L_{\odot}$ , $T_{\star}\!=\!2600~$ K, $\rm C/O\!=\!1.8$ , $\Delta u\!=\!2~$ km s^-1 and $P\!=\!650~$ d (derived quantities $R_0\!=\!2.62\times10^{13}$ cm and $\log g\!=\!-0.64$ ). The plotted quantities are explained in the caption of Fig. 6, however, the velocity is depicted as dashed line and the density as full line here.
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In the text

$\begin{figure} \par\includegraphics[width=6.5cm,clip]{H4073F14.PS} \end{figure}$ Figure 9: Control loop of the radiative instability. $\oplus$ and $\ominus$ mark positive and negative feedbacks, respectively. For example, $\ominus$ indicates that an increase of the causing quantity results in a decrease of the affected quantity and vice versa. All closed loops (including the big circle) have an overall positive feedback, i.e. are self-amplifying.
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{H4073F15.PS} \end{figure}$ Figure A.1: Energy fluxes between the internal energy of the gas contained in its translational degrees of freedom, a molecule subdivided into internal vibrational and rotational excitation energies, and the radiation field.

In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{H4073F2.PS} \end{figure}$	Figure 2: Internal energy e over gas temperature $T_{\rm g}$ (solid line) for constant $\rho\!=\!10^{-10}~\rm g~cm^{-3}$ , $T_{\rm rad}\!=\!1500~$ K and $\left\langle\!\frac{{\rm d}v}{{\rm d}l}\!\right\rangle\!=\!10^{-7}~\rm s^{-1}$ . For comparison, the internal energy of an ideal mono-atomic gas is shown as dashed line.
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$\begin{figure} \par\includegraphics[width=7.8cm,clip]{H4073F3.PS} \end{figure}$	Figure 3: Radiative cooling rate $-\widehat{Q}_{\rm rad}$ over gas density $\rho$ for $T_{\rm rad}=0$ , $T_{\rm g}=4000~$ K and $\left\langle\!\frac{{\rm d}v}{{\rm d}l}\!\right\rangle=10^{-7}\rm~s^{-1}$ .
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$\begin{figure} \par\includegraphics[width=8cm,clip]{H4073F5.PS}\vspace*{3mm} \par\includegraphics[width=7.8cm,clip]{H4073F6.PS} \end{figure}$	Figure 5: Start models for parameters $M_{\star}\!=\!1.2~M_{\odot}$ , $L_{\star}\!=\!6125~L_{\odot}$ and $T_{\star}\!=\!2600~$ K (derived quantities $R_\star\!=\!2.68\!\times 10^{13}~$ cm, $\log g\!=\!-0.66$ ) compared with two start models calculated with constant $\mu\!=\!1.26~$ amu (H/He mixture, dashed) and $\mu\!=\!2.33~$ amu (H₂/He mixture, dashed-dotted).
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$\begin{figure} \par\includegraphics[width=6.5cm,clip]{H4073F14.PS} \end{figure}$	Figure 9: Control loop of the radiative instability. $\oplus$ and $\ominus$ mark positive and negative feedbacks, respectively. For example, $\ominus$ indicates that an increase of the causing quantity results in a decrease of the affected quantity and vice versa. All closed loops (including the big circle) have an overall positive feedback, i.e. are self-amplifying.
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