All Figures

$\begin{figure} \par\includegraphics[width=8.5cm,clip]{h4643_f1} \end{figure}$ Figure 1: The ISO/SWS spectrum of R Scl. An example of an optically bright carbon star that exhibits the "30'' $\mu$ m feature attributed to cool MgS grains. The "30'' $\mu$ m feature is shaded in grey. At shorter wavelengths there are several prominent photospheric, molecular absorption bands. The emission band at 11.3 $\mu$ m is attributed to silicon-carbide grains.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{h4643_f2} \end{figure}$ Figure 2: IRAS colour-colour diagram of galactic carbon stars (diamonds). Stars that exhibit a single temperature SED are shown in black. Stars with a far-IR excess are grey. We also show the tracks that the models follow in the diagram. The tracks are for a mass-loss burst lasting for 200 years. The dust mass-loss rate increases from the bottom to the top curves ( $2\times 10^{-8}{-}2\times 10^{-7}~M_{\odot}$ /yr). The time elapsed since the burst is indicated in the figure. The difference between the dashed and the full lines is the relative amount of MgS. At the onset of the burst, when the dust is warmest, the contribution of MgS makes the tracks differ substantially.
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In the text

$\begin{figure} \par\includegraphics[width=17cm,clip]{h4643_f3} \end{figure}$ Figure 3: Synthetic SEDs from the expanding dust shell containing amorphous carbon and MgS grains. In each panel, the curves from top to bottom represent the system 300, 600, 900, 1200, 1500, 2000, 3000, 5000 and 10 000 year after ejection of the shell. The top panels show the effect of varying the percentage of MgS grains. In the bottom six panels, the mass-loss rate increases from top to bottom and the duration of the burst increases from left to right. Also indicated are the wavelength regions that contribute to the different IRAS broadband filters. The MgS grains are the cause of the "30'' $\mu$ m feature, the broad extra emission band between $\sim$ 23 and $\sim$ 45 $\mu$ m.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{h4643_f4} \end{figure}$ Figure 4: IRAS colour-magnitude diagram of galactic carbon stars. The thick black line shows the mean of the [25]-[60] colour as a function of 12 $\mu$ m flux for the carbon stars (shown in grey). The sources with the largest 60 $\mu$ m excess are all very faint at 12 $\mu$ m (to the left of the dashed line). The photometry of these sources is probably affected by cirrus contamination or confusion due to the poor angular resolution of IRAS.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{h4643_f5} \end{figure}$ Figure 5: Illustration of the definition of the centroid position of the "30'' $\mu$ m feature. We shows two model spectra of a detached shell containing 0 and 10 percent MgS. The difference is the "30'' $\mu$ m feature due to the presence of the MgS grains. On this scale equal surface corresponds to equal energy. The centroid position is indicated by the dashed line, which divides the "30'' $\mu$ m feature into equal surface.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{h4643_f6} \end{figure}$ Figure 6: MgS temperature and the centroid position of the "30'' $\mu$ m feature as a function of distance to the star. The diamonds show the MgS temperature at the inner edges of the detached dust shell. The squares represent the centroid position of the "30'' $\mu$ m emission feature versus the typical distance of the shell, which is defined as the inner radius plus one quarter of the shell thickness. The grey dashed line shows a simple analytic representation of the relation between the distance and the centroid position (see Eq. (2)).
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In the text

$\begin{figure} \par\includegraphics[width=12cm,clip]{h4643_f7} \end{figure}$ Figure 7: The detached-shell model compared to the observations of R Scl in black. The optical photometry is shown in triangles (Johnson et al. 1966). Between 2 and 45 $\mu$ m we show the SWS spectrum (Hony et al. 2002); between 45 and 200 $\mu$ m we show the LWS spectrum. The diamonds represent the IRAS measurements and the submillimetre points from Bagnulo et al. (1998). The dashed gray line is the model SED, taking identical parameters for the dust shell as derived for the CO shell by Schöier & Olofsson (2001). The full gray line shows the model spectrum after correction for the limited size of the apertures of the SWS spectrograph. The dotted line represents the photospheric continuum. We also show the synthetic IRAS fluxes as derived from the model SED. The 25 and 60 $\mu$ m IRAS fluxes are in excellent agreement. The predicted flux level at longer wavelengths is slightly too low. The model explains the observations very well. In the inset, we show the fit to the "30'' $\mu$ m feature on a linear scale. We also show the IRAS/LRS spectrum, which starts to deviate from the SWS spectrum at the wavelength, where the model predicts a difference, due to the different beam-size of IRAS and SWS.
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{h4643_f8} \end{figure}$ Figure 8: The angular dimensions of the circumstellar shell of R Scl compared with the regions probed by ISO/SWS and IRAS. The black rectangles indicate the dimensions of the two most relevant SWS apertures. The dashed circles give an indication of the regions corresponding to IRAS 12, 25 and 60 $\mu$ m point source measurements. The circle for the IRAS 100 $\mu$ m filter lies outside the figure. The shell extends from 13 $^{\prime \prime }$ to 26 $^{\prime \prime }$ , which corresponds to $7\times 10^{16}{-}14\times 10^{16}$ cm at a distance of 360 pc. The complete shell is probed by the IRAS measurements. The SWS instrument detects only a part of the shell radiation. Of course, the back and front side of the spherical shell do contribute to the SWS flux.
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In the text

$\begin{figure} \par\includegraphics[width=12cm,clip]{h4643_f9} \end{figure}$ Figure 9: The single detached-shell model applied to U Cam. Symbols are the same as in Fig. 7. We use the inner and outer radius and the gas mass of the CO shell and a 4% MgS fraction. The model that fits the SWS observations best does not reproduce the IRAS flux levels. Clearly, the single dust shell model is not applicable and the IRAS excess is predominantly due to more extended emission than the CO shell.
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In the text

$\begin{figure} \par\includegraphics[width=8.5cm,clip]{h4643_f1} \end{figure}$	Figure 1: The ISO/SWS spectrum of R Scl. An example of an optically bright carbon star that exhibits the "30'' $\mu$ m feature attributed to cool MgS grains. The "30'' $\mu$ m feature is shaded in grey. At shorter wavelengths there are several prominent photospheric, molecular absorption bands. The emission band at 11.3 $\mu$ m is attributed to silicon-carbide grains.
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