Table I A summary of the spectral -features observed in the LRS spectra of the three groups o-f carbon stars. The de-finition o-f the groups is given in the text. wavelength Xmax identification Group I B - 12 urn E1 9.7 M™ Silicate 12 - 23 jim E IB ^m Silicate Group II < 8.5 M"i A C2H2 CS? 12 - 16 f-i/n A 13.7 - 14 Mm C2H2 HCN? 8 - 10 Mm E 8.6 M"i Unknown 10 - 13 Mm E 11.3 - 11 .7 M«> SiC Group III 10 - 13 MJn E 11.3 - 11 .7 tun SiC B - 23 Htn C Amorphous carbon 1 The letter in this column indicates the nature o-f the -feature: A = absorption; E = emission; C indicates the presence of continuum opacity. II. LRS SPECTRA The sample consists of 304 carbon stars with entries in the LRS catalog (Papers I-III). The LRS spectra have been divided into three groups. Group I consists of nine stars with 9.7 and 18 tun silicate features in their LRS spectra pointing to oxygen-rich dust in the circumstellar shell. These sources are discussed in Paper I. The remaining stars all have spectra with carbon-rich dust features. Using NIR photometry we have shown that in the group II spectra the stellar photosphere is the dominant continuum. The NIR color temperature is of the order of 25OO K. Paper II contains a discussion of sources with this class of spectra. The continuum in the group III spectra is probably due to amorphous carbon dust. The circumstellar dust shell modifier the NIR photospheric spectral distribution significantly leading to NIR color temperatures lower than 2000 K (Paper III). The 82 spectral features observed in the three groups ai stars are summarized in Table I. Silicon carbide (SiC) dust is observed in both classes o-f carbon-rich spectra. Following Paper II we de-fine an SiC index as the natural logarithm o-f the ratio between the integrated observed -flux and the integrated continuum between 10 and 13 ^m. The continuum is obtained by a linear -fit in a log(S,)-log(X) diagram. The SiC index of the 295 stars in groups II and III has been calculated and their distribution over SiC index is given in figure 1. Almost all stars have an SiC index between 0.0 and 0.3. SiC index distribution 0.25 SiC index Fig. 1 The distribution of the 295 stars belonging to groups II and III over the SiC index. 83 Teleskoop in de 1612 MHz OH maser-lijn. De verkregen KQH/S35 verhoudingen bevestigen dat de 1612 MHz maser verzadigd ie en wordt gepompt door 35 ^jn fotonen met da teoretiach voorspelde doelmatigheid van 0.25. Meer 35 jun -fotonen zijn nodig per 1612 MHz -foton wanneer V ï 15 km s . Dit hangt waarschijnlijk samen met de grotere snelheidsgradient in ds circumstellaire schil. 173 -i r -0.30 -0.10 0-10 0.30 O.SO 0.10 -0.30 -0.10 0.10 0.30 O.SO 0.70 L0G112/25) (JY) L0G(12/25) (JY) °-0.30 ' -0.10 ' O'.IO ' o'.3O ' o'.SO ' ~0 70 '-i'™"1 °S° ' ","•« ' ,°''° ' . G'*° °'-7° L0G(12/25) (JY) L0G(12/25) (JY1 Fig. 2 The log(S12/S-5) versus log<S25/S60> color-color plot for carbon stars detected at 12, 25 and 60 tun by IRAS. The uncertainties quoted in the IRAS point source catalog are indicated by error bars. (a) The nine group I stars, (b) The 69 group II stars, (c) The 15 group III stars and (d) All 277 stars in the sample. Model tracks are given for shell masses AM = 1 10* 2.5 10 \ 5 10 *, 1 10 , 3 Z 2.5 10~ , 5 1O~ , 1 1O" and 2.5 10~^Mo. The mass increases -from the inner to the outer track. A black body line is also drawn. The results for the values o-f the time parameter:40, 100, 200, 400, 1000, 2OO0, 4000 and 10000 yrs are connected by a line. The time increases counter-clock-wise. B4 III. THE COLOR-COLOR DIAGRAM The log(S12/S25> versus log(S25/S60) color-color diagram for 277 of the 304 stars, detected by IRAS in the 12, 25 and 60 ^m band, is given in figure 2. The broad-band flux densities are color- corrected using Table VI.C.6 from the IRAS Explanatory Supplement (Joint IRAS Science Working Group 1985). The black body temperature is estimated from the ratio of the 25 and 60 fun flux densities. Using the classification of the LRS spectra introduced in the previous section we see that the three classes populate different loci in the color-color diagram. The stars with silicate shells are situated in the upper left part of the diagram (fig. 2.a). The large spread in the S25/S60 ratio ls caused by the stars of group II, which are mainly irregular variables (fig. 2.b). The group III stars, predominantly Mira variables, are located around the BOO-L000 K black body points (fig. 2.c). Besides the nine stars of group I two other stars are clearly located above the black body line. The LRS spectra combined with IRAS broad-band data for both stars are plotted in figure 3. The prominent SiC feature leaves no doubt that both circumstellar dust shells are carbon rich. No NIR photometry is available for these two stars. The IRAS broad-band fluxes should be used with care. Chester (1986) has shown that the flux densities of a large fraction of sources fainter than 0.8, 1.0 and 1.2 Jy in the 12, 25 and 60 u/n bands respectively, have been overestimated. Our sample is an LRS selection of an optically chosen set of stars, so that the 12 and 25 fi/n flux densities are well above those values and only the 60 jun -flux density may be influenced by this effect. Figure 4 gives the color—color plot of the 177 sources having 85 1Z88 8.43581 -27.3447 (, l.BE»B2 "A, i.BE.ee » 28 1M 14Z7 9.1bZ76 -53.4944 2 l.BE*B2 Fig. 3 IRAS broad-band and LRS data -for C1288 and C1427. '-0.B0 -O.S0 -0.20 0.10 0-40 0.70 L0G(12/25) (JY) Fig. 4 The log(S12/S25> versus log(S25/Sy0) color-color plot for the 177 stars being detected at l2, 25 and 60 urn by IRAS, with S60 > 1.2 Jy. The uncertainties quoted in the IRAS point source catalog are indicated by error bars. ^60 ^ *"^ ^v" '^ con-firms that the 60 j±m excess observed in -Figure 2 is real and not due to a systematic overestimate of the -Flux density. IV. A SIMPLE DUST SHELL MODEL The majority of sources in the sample is situated under the black body line in -figure 2. The extended vertical spread o-f the cloud of points is caused by variations in the magnitude of the 60 pm excess in stars of group II. The stellar photosphere in these sources dominates the spectrum out to 20 jam, while the 60 pin excess is due to a cold dust shell, possibly a remnant of an earlier phase in the evolution of the source (Paper II). To explain the observed scatter in the color—color diagram the following shell model is proposed. At the end of its oxygen- rich phase the star is surrounded by an oxygen-rich circumstellar dust shell. The event in the interior r.»f the star which causes the transition from oxygen-rich to carbon-rich in the outer layers also causes a large decrease of the mass loss rate. As almost no new dust is formed inside the shell a distinct dust shell moves away from the star with constant speed. Due to its expansion it gradually cools and dilutes. The contribution of the expanding shell to the energy distribution of the source will shift to longer wavelengths. First in the NIR and later in the LRS wavelength region <7.7 to 23 pm) the carbon-rich photosphere becomes visible. The expanding shell gives rise to a 60 urn excess. This model is suggested by the following observational facts. 1/ The three stars belonging to group I for which variability types are known are irregular pulsators. This implies low mass 87 loss rates, if we assume that the mass loss is driven by stellar pulsations. BM Gem (=C716), the only star o-f group I for which NIR photometry is available (Noguchi et al. 19B1; NKKOSO), has a NIR color temperature close to 2500 K. This indicates the absence of observable amounts o-f warm dust, which con-firms the present low mass loss rate (Paper I). 2/ Those stars from group II which are irregular pulsators and for which reliable mass loss rates are determined by Knapp and Morris (1985) and Knapp (1986) have mass loss rates of ~ 10 Mo yr~ a factor 10 to 1000 lower than the mass loss rates obtained for other AGB stars observed by Knapp and Morris (1985). The main purpose of our simple model is to explain the distribution of stars in the color—color diagram. The sources of group III, which cluster around the black body line, are not considered in this model. These stars lose mass rather steadily, presumably driven by their regular pulsations. They populate a well-defined part of the color—color diagram with logCS^/^s* between 0.4 and 0.6 and log(S25/B60) between 0.55 and 0.8 (see fig.