4. Discussion and conclusion
The HC stars (Sect. 2) have been classified into six groups (HC0 to HC5) and for those stars, Tables 1 and 2 correspond to their counterparts for CV stars in Paper I. Many of them (about 75%) are non variable stars to the accuracy of presently available measurements (including the HIPPARCOS Hp-data from ESA). They parallel the G0 to M3 sequence of oxygen-rich giants (see Table 2). The difference between "carbon" and "oxygen" SEDs continuously diminishes when dealing with earlier groups or spectral types, and it finally vanishes at type G. The SEDs of RCB stars and others (AC Her as a RV Tau star and a few BaII or HdC stars) were found to belong to the oxygen-rich category (see Sect. 3 and Table 5). The stars described in this paper span the 3300-7500 K range in effective temperatures.
While of less accuracy when compared to Paper I study, the evaluation of the interstellar extinction on HC stars is quite acceptable (Sect. 2.3.) It proves excellent for oxygen-types SEDs (Sect. 3.2.). The results for about 140 stars are provided in Tables 3 and 4 on a single homogeneous scale which is found compatible with those from other methods. The comparison between the derived excesses and those obtained from maps in the literature has proved satisfactory.
We have been able to disentangle the circumstellar and interstellar extinctions in a few cases and some conclusions are reached about the CS grains and their spatial distribution. A disc model with carbon grains is favoured for the exceptional HC1 star HD 100764, as proposed by Skinner (1994). It appears however as a very thick one subtending at least a total angle of with 43% of the observed luminosity being from the shell. The interstellar extinction found for the carbon-rich RV Tau-a star AC Her is found to be E(B-V)=0.17 with a G0g classification. No CS extinction on the line of sight is detected. It could be a common feature of the "constant mean light" RV Tau-a stars. Variable CS extinction on the line of sight to RV Tau-b stars would be the rule as suggested by redder colours and fluctuating mean light.
Finally, we have determined the groups and extinctions for a sample of RCB stars. The interstellar component is given in Table 5, as derived from observations at maximum light. Two stars (V CrA and RS Tel) were studied while obscured. Large (neutral) carbonaceous grains substantially contribute to dense puffs during deep minimas while distributions peaking towards smaller (selective) grains are predominant after some clearing, before maximum light. Those features can be explained in terms of destructive collisions in expanding puffs. Such collisions are the mechanism suggested by Biermann & Harwit (1980) to generate the radii distribution (MRN) used in the classical models of the interstellar extinction law.
© European Southern Observatory (ESO) 1999
Online publication: February 23, 1999