Abstract. We evaluate effective temperatures of 390 carbon-rich stars. The interstellar extinction on their lines of sights was determined and circumstellar contributions derived. The intrinsic (dereddened) spectral energy distributions (SEDs) are classified into 14 photometric groups (HCi, CVj and SCV with i = 0,5 and j = 1,7). The new scale of effective temperatures proposed here is calibrated on the 54 angular diameters (measured on 52 stars) available at present from lunar occultations and interferometry. The brightness distribution on stellar discs and its influence on diameter evaluations are discussed. The effective temperatures directly deduced from those diameters correlate with the classification into photometric groups, despite the large error bars on diameters. The main parameter of our photometric classification is thus effective temperature. Our photometric k 1/2 coefficients are shown to be angular diameters on a relative scale for a given photometric group, (more precisely for a given effective temperature). The angular diameters are consistent with the photometric data previously shown to be consistent with the true parallaxes from HIPPARCOS observations (Knapik, et al. 1998, Sect. 6). Provisional effective temperatures, as constrained by a successful comparison of dereddened SEDs from observations to model atmosphere predictions, are in good agreement with the values directly calculated from the observed angular diameters and with those deduced from five selected intrinsic color indices. These three approaches were used to calibrate a reference angular diameter Φ0 and the associated coefficient CT eff . The effective temperature proposed for each star is the arithmetic mean of two estimates, one ("bolometric") from a reference integrated flux F0, the other ("spectral") from calibrated color indices which are representative of SED shapes. Effective temperatures for about 390 carbon stars are provided on this new homogeneous scale, together with values for some stars classified with oxygen-type SEDs with a total of 438 SEDs (410 stars) studied. Apparent bolometric magnitudes are given. Objects with strong infrared excesses and optically thick circumstellar dust shells are discussed separately. The new effective temperature scale is shown to be compatible and (statistically) consistent with the sample of direct values from the observed angular diameters. The effective temperatures are confirmed to be higher than the mean color temperatures (from 140 to 440 K). They are in good agreement with the published estimates from the infrared flux method for T eff ≥ 3170 K, while an increasing discrepancy is observed toward lower temperatures. As an illustration of the efficiency of the photometric classification and effective temperature scale, the C/O ratios and the Merrill-Sanford (M-S) band intensities are investigated. It is shown that the maximum value, mean value and dispersion of C/O increase along the photometric CV-sequence, i.e. with decreasing effective temperature. The M-S bands of SiC2 are sho...

Abstract. The luminosity function (LF) of nearly 300 Galactic carbon giants is derived. Adding BaII giants and various related objects, about 370 objects are located in the RGB and AGB portions of the theoretical HR diagram. As intermediate steps, (1) bolometric corrections are calibrated against selected intrinsic color indices; (2) the diagram of photometric coefficients k 1/2 vs. astrometric true parallaxes are interpreted in terms of ranges of photospheric radii for every photometric group; (3) coefficients C R and C L for bias-free evaluation of mean photospheric radii and mean luminosities are computed. The LF of Galactic carbon giants exhibits two maxima corresponding to the HC-stars of the thick disk and to the CV-stars of the old thin disk respectively. It is discussed and compared to those of carbon stars in the Magellanic Clouds and Galactic bulge. The HC-part is similar to the LF of the Galactic bulge, reinforcing the idea that the Bulge and the thick disk are part of the same dynamical component. The CV-part looks similar to the LF of the Large Magellanic Cloud (LMC), but the former is wider due to the substantial errors on HIPPARCOS parallaxes. The obtained mean luminosities increase with increasing radii and decreasing effective temperatures, along the HC-CV sequence of photometric groups, except for HC0, the earliest one. This trend illustrates the RGB-and AGB-tracks of low-and intermediate-mass stars for a range in metallicities. From a comparison with theoretical tracks in the HR diagram, the initial masses M i range from about 0.8 to 4.0 M for carbon giants, with possibly larger masses for a few extreme objects. A large range of metallicities is likely, from metal-poor HC-stars classified as CH stars on the grounds of their spectra (a spheroidal component), to near-solar compositions of many CV-stars. Technetium-rich carbon giants are brighter than the lower limit M bol −3.6 ± 0.4 and centered at M bol −4.7 3) on average. Most RCB variables and HdC stars range from M bol −1 to −4 against −0.2 to −2.4 for those of the three population II Cepheids in the sample. The former stars show the largest luminosities ( M bol < ∼ −4 at the highest effective temperatures (6500-7500 K), close to the M bol −5 value for the hot LMC RCB-stars (W Men and HV 5637). A full discussion of the results is postponed to a companion paper on pulsation modes and pulsation masses of carbon-rich long period variables (LPVs; Paper IV, present issue).