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Astron. Astrophys. 344, 263-276 (1999)

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1. Introduction

The first classification of the carbon-rich giants in discrete photometric groups was proposed by Knapik & Bergeat (1997, hereafter Paper I), and Bergeat et al. (1998b, hereafter Paper II). It is independent of any spectral classification. The methods were validated in Paper I for carbon variables through colour-colour and colour-galactic latitude diagrams. They provide for every documented star a CVi-group (intrinsic SEDs from i=1 the earliest one to i=6 the latest one in Paper I) and the amount of the interstellar extinction [FORMULA] in the J-filter. The colour excess is [FORMULA] for the mean extinction law of the diffuse interstellar medium (Mathis 1990) which was shown to be relevant. A good agreement was obtained in most cases when those colour excesses were compared to field values from the maps found in the literature. No gap was actually observed and discrete CVi-groups are adopted here only for convenience. The main features of this new pair method, described in Sect. 2 hereafter, are

  • the simultaneous use of the whole spectral range from UV to IR (up to 17 wavelengths with equal weights a priori ),

  • the derivation of the relevant CVi-group and selective extinction of dust through an accurate linear fit (see Figs. 1, 2, 4, 5, 6 and 7),

  • [FORMULA] as a detection threshold,

  • the derivation of a k-factor (Sect. 2.4), which should be a squared angular diameter on a relative scale (and not an angular diameter as erroneously stated in Sect. 6 of Paper I).

The quantity [FORMULA] showed the correlation with true parallaxes (derived from the HIPPARCOS data: ESA 1997, henceforth called ESA) expected for stars populating a given range in linear diameters (see Fig. 3 and Sect. 6 of Knapik et al. 1998).

The study in Paper I was concentrated on carbon stars with small or moderate amplitudes of variations (namely Lb or SR variables) making use of the six CVi-groups. In Paper II it was extended to the HC stars (i.e. essentially the stars classified as early R on the grounds of their spectra) and related objects like HdC or BaII stars, and RCB variables. The RV Tau variable AC Her was also studied. Here we propose an extension of our pair method (see Sect. 2) to latter SEDs (cooler variables) with a CV7-group (Sect. 3) whose mean effective temperature amounts to nearly [FORMULA] according to a preliminary calibration. It is then shown that the (spectroscopic) CS variable stars can be classified in the CVi framework of carbon stars, while an intrinsic SED denoted as SCV (for SC variables) is introduced to fit the (spectroscopic) SC variables (Sect. 4). A sequence S-SC-CS-C is indicated. The gap between SEDs seems to happen between spectroscopic SC and CS stars. A few carbon stars with IR silicate emission are studied in Sect. 5, as late HC or early CV stars. Finally, Miras are considered as such in Sect. 6 although no specific grouping is required. Variations of CV-group with phase are established for some documented variables of large amplitudes, while opacity variations predominate in other carbon Miras. The case of circumstellar (CS ) extinction and emission is discussed whenever possible. Our method provides reliable results even for extreme objects like IRC +10216 (Sects. 6 and 7), which was not expected.

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© European Southern Observatory (ESO) 1999

Online publication: March 10, 1999