7. Discussion and conclusion
First of all we summarize our analysis of carbon variables as revised and extended (the CV7 and SCV-groups) in Sects. 3 and 4. Small corrections were operated on the data of Tables 1 and 2 of Paper I to which we substitute Tables 6 and 7. We emphasize again the fact that there is no evidence for gaps between the groups. A continuous classification might be useful later on when a much larger body of data will be available. The reader is referred to Paper I for a full discussion which is not repeated here.
Table 6. The seven photometric groups (G) of unreddened carbon variables and the SCV-group of SC stars intermediary between CS and S stars (see Sect. 4). A representative star is mentioned for each group (name in GCVS). Three mean colour indices are given with their standard deviations (see text for details). Replaces able 1 of Paper I.
Table 7. A list of the fifty-one carbon variables found unreddened (i.e. with ) in the present study. The stars C-entries in Stephenson (1989) are given; (a): a possible 0.02-0.03 excess is guessed for C833 = R Lep at maximum light (bright epoch); (b): T Cnc is found unreddened at phase 0.1 while CS reddening appears at some phases; (c): Miras at maximum light. Replaces Table 2 of Paper I.
The SCV-group is found intermediary between the carbon stars (CV-groups) and S stars, as discussed in Sect. 4.4. The HC stars were classified into six groups (HC0 to HC5) in Paper II. Some hot carbon-rich related objects were also studied. They can be found either in a HC-group or alternatively in an oxygen-type group. This was the case of part of the RCB variables and of the RV Tau variable AC Her (see Paper II). On the contrary, the carbon stars with IR silicate emission are classified CV1 or CV2 (with the exception of HD 189605 which is HC5, the earlier group just next to CV1), i.e. they exhibit SEDs for carbon variables of early type (see Sect. 5 and Table 4). Finally the carbon and CS Miras can be classified in the above-described scheme, like the SR and Lb variables in Paper I. Many Miras are however very cool objects (CV6 or CV7) with evidences of strong opacities (including CS grains whose IR emission is prominent).
While of less accuracy when compared to Paper I studies, the evaluation of the interstellar extinction on HC and SCV stars is quite acceptable (Paper II and Sect. 4.3). Finally, solutions for 585 stars were derived on a homogeneous scale while achieving the studies of 687 carbon SEDs in the three papers. When the SEDs of oxygen-rich stars of Papers II and III are added, this is more than 600 stars analysed so far.
We have been able to disentangle the circumstellar and interstellar extinctions in some cases and large quantities of CS grains are inferred from the available data. The comparison between the derived colour excesses and those obtained from maps in the literature proves satisfactory. The difficult point remains the cases of Miras and IRAS C (Sect. 6) including cool CV7 stars (Sect. 3) where extinction and emission from CS grains combine with interstellar extinction. The selective CS extinction studied at is shown to follow, at least approximately, the law for the diffuse interstellar medium as quoted by Mathis (1990). Scattering from corresponding small grains is shown to have a negligible contribution to the light observed from CW Leo = IRC+10216. Large carbonaceous scatterers are indicated instead. Clearly those extreme objects deserve further studies with additional (preferably simultaneous) data constituting large databases. The authors are aware that many values for Miras in Table 5 are only preliminary.
The next step in our approach will be a calibration of our photometric groups in terms of effective temperatures (Bergeat & Knapik 1999).
© European Southern Observatory (ESO) 1999
Online publication: March 10, 1999