1999A&A...342..773B

The present work is an extension of a recent study by Knapik & Bergeat (1997A&A...321..236K, henceforth called Paper I) of the spectral energy distributions (SEDs) of about 300 cool carbon-rich variables and of the interstellar extinction observed on their line of sights. The methods were originally developed for Semi-Regular (SR) and Irregular (L)-variables. Shortly, this is a kind of a pair method making use simultaneously of the whole SED from UV to IR. Our approach is applied here to the galactic carbon-rich giants with bluer SEDs, namely the hot carbon (HC) stars, including many ``constant'' stars and a minority of variables: AC Her a RV Tau star, the R Coronae Borealis (RCB) stars and others. Some HdC (i.e. carbon-rich hydrogen deficient stars) and Ba II stars are also considered. The total number of studied HC stars amounts to about 140. With few exceptions, the colour excesses for interstellar extinction are found in good agreement with the field values from maps published in the literature, taking into account the approximate distances to our stars from HIPPARCOS data (1997HIP...C......0E, henceforth called ESA) or binarity. We propose a classification scheme with six photometric groups (or boxes: HC0 to HC5) from the bluest to the reddest SEDs. Oxygen-rich SEDs earlier than HC0, are attributed to the hottest stars (AC Her, most RCB-variables and a few others). Previous findings are confirmed of a junction between oxygen-rich and carbon-rich SEDs at spectral type G. The latest (HC5) group is immediately close to the earliest one in Paper I, namely CV1. The sequence of groups then goes regularly from HC0 to CV6. Substantial infrared excesses with respect to our solutions are found in HD 100764 a HC1 carbon star, AC Her a G0g RV Tau star, and the RCB stars classified in either HC or oxygen-groups. The colour excesses at maximum light can usually be attributed to interstellar reddening, with neutral circumstellar (CS) reddening (large grains) or no CS extinction at all on the line of sight (non spherical geometry) as possible explanations. The latter model (disc or patchy distribution through successive puffs) is favoured. Two RCB variables for which we exploit SEDs on a rising branch (V CrA) or minimum light (RS Tel), show CS laws, respectively a selective extinction compatible with small grains and an extinction partly neutral indicative of large grains on the line of sight.